• NDT Resource Center: Resources at Education at Material Premier – http://www.ndt-ed.org
• Multimedia Group, Department of Engineering, Cambridge University, UK: Teach Yourself Phase Diagrams – http://www-g.eng.cam.ac.uk/mmg/teaching
• Steel Matter: http://www.matter.org.uk/steelmatter
• University of Bolton, UK: Basic principles of materials – http://www.ami.ac.uk/courses/topics
• Key to Metals: Resource Center at Articles – http://www.keytometals.com
• SubsTech (Substances & Technologies): http://www.substech.com
• AISI (American Iron and Steel Institute): Learning Center – http://www.steel.org
• Corus: Internet Teaching Resources – http://www.corusgroup.com/en/responsibility/education/resources/internet
• Tata Steel International (Australasia) Ltd: Products – http://tatasteelnz.com
• BRITISH STAINLESS STEEL ASSOCIATION: Technical Help – http://www.bssa.org.uk
• The Nickel Institute: Nickel & Its Uses at Technical Support – http://nickelinstitute.org
• The International Stainless Steel Forum (ISSF): http://www.worldstainless.org
• EverBright(China) St. St. Pipe Co., Ltd: Reference – http://www.eb-stainless.com
• Ductile Iron Society: Ductile Iron Data – http://www.ductile.org
• Steel Founders Society of America: Publications – http://sfsa.org

Strength of a Metal/Alloy

The strength of a metal/alloy can be described as the resistance against the onset of plastic deformation under an external load. Plastic deformation occurs by the movement of dislocations through the metal (in this article, generally metal word will be used for metal or an alloy) lattice which enables single lattice planes to slip consecutively over one another. If this motion is hindered by lattice defects, a higher external load must be applied so that the dislocations can overcome the obstacles. It is for this reason that means of increasing the strength of steels always aim at hindering dislocation movement. Various obstacles to dislocation motion are as per the table given below.

Solid Solution Strengthening

Solid solution strengthening occurs when the atoms of the new element (solute) form a solid solution with the original element (solvent), but there is still only one phase.

The increase in strength is produced by solute atoms which are dissolved in the metal matrix (solvent). Since solute atoms differ in size as compared to the atoms of metal matrix they introduce tensile or compressive lattice strains (lattice distortion) that hinders the movement of dislocations. The increase of yield stress depends on the kind, amount and distribution of the solute atoms. Solute atoms, those dissolve interstitially between the atoms of the matrix results in a high lattice distortion (e.g. C and N) as compared to others which dissolve substitutionally and occupy regular lattice positions (e.g. Cr).

Increasing strength by solid solution strengthening leads to a decrease in toughness (Toughness is the ability of a material to avoid brittle fracture. Toughness = Strength + Ductility).

Strain Strengthening

Linear lattice defects are the dislocations themselves. The lattice distortion surrounding the dislocation disturbs the movement of other dislocations. A dislocation in the path of other dislocation can act as an obstacle to the motion of the latter. This interaction increases with increasing dislocation density.

When cold forming steel, e.g. cold rolling, dislocations are continuously produced because they permanently block each other. The dislocation density rises and increases the strength of the steel. Such strengthening is accompanied by a pronounced reduction in toughness.

Grain Refinement

Grain refinement is the most important strengthening mechanism because it is the only method of strengthening which is accompanied by an increase in resistance to brittle fracture. The toughness of the alloy also increases as grain size decreases.

As shown in the figure given below, slip planes do not cross from one grain to another but are confined by the grain boundaries. Due to this, in large grains there is more slippage and greater plastic deformation. In small grains, slip process is confined and results in small slippage.

During plastic deformation, slip or dislocation motion must take place across the common grain boundary, which acts as a barrier to dislocation due to two major reasons.

1. Crystallographic misorientation of the grains
2. Atomic disorder within a grain boundary resulting in discontinuity of slip planes.

Thus the grain boundaries are barriers to dislocation motion. Consequently as the grain size is decreased, the number of barriers increases and this is reflected in increased yield strength.

It may however be noted that fine grain size is not desirable for creep resistance as grain boundary sliding can cause creep elongation/cavitation.

Precipitation/dispersion Strengthening

Precipitates are obstacles to the motion of dislocation. There are two mechanisms for strengthening by this method – Multiphase Metals and Precipitation Strengthening.

Multiphase Metals

In this method, strengthening the metal is carried out by adding elements that have no or partial solubility in the parent metal. This will result in the appearance of a second phase distributed throughout the crystal or between crystals (e.g. Fe₃C, called as Cementite). This secondary phase can raise or reduce the strength of an alloy.

The properties of a polyphase (two or more phase) material depend on the nature, amount, size, shape, distribution, and orientation of the phases.

For example, in case of hypoeutectoid steels, carbon as cementite in pearlite increases strength of steel where as in case of steel beyond the eutectoid composition (hypereutectoid steels), the strength levels off or even reduces due to presence of proeutectoid cementite network. The hardness, however continues to increase due to the greater proportion of hard cementite

Precipitation Hardening

In designing alloys for strength, an approach often taken is to develop an alloy with a structure that consists of particles (which impede dislocation movement) dispersed in a ductile matrix. Such a dispersion can be obtained by choosing an alloy that is a single phase at elevated temperature but on cooling will precipitate another phase in the matrix. A thermal process is then developed to produce the desired distribution of precipitate in the matrix. When the alloy is strengthened by this thermal treatment, it is called precipitation strengthening or hardening.

Precipitation hardening consists of three main steps: solution treatment, quenching, and aging. Solution treatment involves heating the alloy to a temperature that allows the alloying atoms (called the solute) to dissolve into the solution. This results in a homogeneous solid solution of one phase. Quenching rapidly cools the solution and freezes the atoms in solution. In more technical terms, the quenching cools the material so fast that the atoms of the alloying elements do not have time to diffuse out of the solution. In the as-quenched condition, the solute is supersaturated meaning that the lattice is overly stressed by the alloying atoms. Aging is the process where the solute particles diffuse out of solution and into clusters that distort and strengthen the material.

Solute atoms in a solid solution and precipitates both are obstacles to the motion of dislocation. However precipitates are having pronounced effect on strengthening. The strengthening by the two methods can be compared by a real life example – difficulty with pebbles and boulders on a road.

Solute atoms are like pebbles
Precipitates are like boulders

Generally the ductility of cast iron is very low, and it cannot be rolled, drawn or worked at room temperature. However, they melt readily and can be cast into complicated shapes which are usually machined to final dimensions. They are known as cast irons because casting is the only suitable process applied to these alloys. Though the common cast irons are brittle and have lower strength, they can be cast more readily than steels and are cheap.

“Cast iron is brittle.” is an outdated but widely held truism which mistakenly implies that all cast irons are the same, and none are ductile. In fact, they can be made ductile. Malleable cast iron made from white cast iron and nodular iron are ductile. Ductile iron offers the design engineer a unique combination of a wide range of high strength, wear resistance, fatigue resistance, toughness and ductility in addition to the well-known advantages of cast iron – castability, machinability, damping properties, and economy of production. Unfortunately, these positive attributes of ductile iron are not as widely known as the mistaken impression of brittleness is well known.

Information about various types of cast iron is given in this article.

Classification of Cast Irons

The best method of classifying cast iron is according to metallographic structure.

There are four variables as under to be considered which lead to the different types of cast irons.

1. Carbon content
2. The alloy and impurity content
3. The cooling rate during and after freezing
4. The heat treatment after casting.

Above variables control the condition of the carbon and also its physical form. The carbon may be combined as iron carbide in cementite, or it may exist as free carbon in graphite. The shape and distribution of the free carbon particles will greatly influence the physical properties of the cast iron. Various types of cast irons are as under.

White cast irons

In this type of cast irons, all the carbon is in the combined form as cementite.

Malleable cast irons

In this type of cast irons, most or all of the carbon is uncombined in the form of irregular round particles known as temper carbon. This is obtained by heat treatment of white cast iron.

Gray cast irons

In this type of cast irons, most or all of the carbon is uncombined in the form of graphite flakes.

Chilled cast irons

In this type of cast irons, a white cast iron layer at the surface is combined with a grey cast iron interior.

Nodular cast irons

By special alloy additions, in this type of cast irons, the carbon is largely uncombined in the form of compact spheroids. This structure differs from malleable iron in that it is obtained directly from solidification and the round carbon particles are more regular in shape.

Alloy cast irons

In this type of cast irons, the properties or the structure of any of the above types are modified by the addition of alloying elements.

Detail information on above cast irons is given below.

White Cast Irons

All white cast irons are hypoeutectic alloys. Their typical microstructure on fast cooling from the liquid state consists of dendrites of transformed austenite (pearlite) in a white interdendritic network of cementite. Since white cast iron contains a relatively large amount of cementite as a continuous interdendritic network, it makes it hard and wear resistant but extremely brittle and difficult to machine. Completely white cast irons are limited in engineering applications due to this brittleness and lack of machinability. They are used where resistance to wear is most important and the service does not require ductility, such as liners for cement mixer and extrusion nozzles. A large quantity of white cast iron is used as a starting material for the manufacture of malleable cast iron. The range of mechanical properties for unalloyed white cast iron is as under.

Tensile strength: 20000 to 70000 psi
Hardness, Brinell: 375 to 600

Malleable Cast Irons

Cementite (iron carbide) is a metastable phase. There is a tendency for cementite to decompose into iron and carbon, but under normal conditions it tends to persist indefinitely in its original form. This tendency to form free carbon is the basis for the manufacture of malleable cast iron. Decomposition of cementite into iron and carbon is favored by elevated temperature, the existence of solid nonmetallic impurities, higher carbon contents and the presence of elements that aid the decomposition of cementite.

The purpose of malleabilization is to convert all the combined carbon in white iron into irregular nodules of temper carbon (graphite) and ferrite. Commercially, this process is carried out in two steps known as the first and second stages of the anneal.

White irons suitable for conversion to malleable iron are of the following range of composition.

Carbon: 2.00-2.65 percent
Silicon: 0.90-1.40 percent
Manganese: 0.25-0.55 percent
Phosphorus: Less than 0.18 percent
Sulfur: 0.05 percent

In the first stage of annealing, the white iron casting is slowly reheated to a temperature between 1650 and 1750°F. They are held at this temperature until all massive carbides have been decomposed. Since graphitization is a relatively slow process, the casting must be soaked at temperature for at least 20 hours and large loads may require as much as 72 hours. The structure at completion of first stage graphitization consists of temper carbon nodules distributed throughout the matrix of saturated austenite.

After first stage of annealing, castings are cooled as rapidly as practical to about 1400°F in preparation for the second stage of the annealing heat treatment. The fast cooling cycle usually requires 2 to 6 hours depending on the equipment used.

In the second stage annealing, the castings are cooled slowly at a rate of 5 to 15°F/h through the critical range at which the eutectoid reaction will take place. During the slow cooling, the carbon dissolved in the austenite is converted to graphite on the existing temper carbon particles, and the remaining austenite transforms to ferrite. Once graphitization is complete, no further structural changes take place during cooling to room temperature, and the structure consists of temper carbon nodules in a ferrite matrix. This type is known as standard or ferritic malleable iron.

In the form of compact nodules, the temper carbon does not break up the continuity of the tough ferritic matrix. This results in a higher strength and ductility than exhibited by gray cast iron. The graphite nodules also serve to lubricate cutting tools, which account for the very high machinability of malleable iron. Ferritic malleable iron has been widely used for automotive, agricultural and railroad equipment, railing casting on bridges, chain hoist assemblies, pipe fittings and many applications in general hardware.

Alloyed malleable irons are those whose properties result from the addition of alloying elements. Since alloyed malleable irons are completely malleableized, their influence is largely on the ferritic matrix. Two principal kinds are copper alloyed and copper-molybdenum alloyed malleable iron. The effect of copper is to increase corrosion resistance, tensile strength and yield point at very slight reduction in ductility. Hardness is also increased. The addition of copper and molybdenum in combination produces a malleable iron of superior corrosion resistance and mechanical properties.

The strength and hardness of pearlitic malleable iron is more than ferritic malleable iron. In this type of iron, the structure consists of temper carbon nodules in a pearlite matrix. For making pearlitic malleable iron, a controlled quantity of carbon, in the order of 0.3 to 0.9 percent is retained as finely distributed iron carbide during first stage of annealing.

If manganese is added, the regular cycle (of second annealing) can be maintained to retain combined carbon throughout the matrix, or the second stage annealing of the normal process be replaced by a quench, usually air, which cools the castings through the eutectoid range fast enough to retain combined carbon throughout the matrix. The amount of pearlite formed depends upon the temperature at which the quench starts and the rate of cooling. If the air quench produces a fast enough cooling rate through the eutectoid range, the matrix will be completely pearlitic. It is a common practice to temper most pearlitic malleable iron after air cooling to spheroidize the pearlite, lower hardness, and improve machinability and toughness.

Pearlitic malleable irons are used for axle and differential housings, camshafts and crankshafts in automobiles, gears, elevator buckets in conveyor equipment, pumps, nozzles, hammers, etc.

For comparison, tensile properties of different types of malleable iron are given in the following table.

 

Type	Tensile strength, 1000 psi	Yield Strength, 1000 psi	Elongation, % in 2 in.	BHN
Ferritic	50-60	32-39	20-10	110-145
Cu-Mo Alloyed Ferritic	58-65	40-45	20-15	135-155
Pearlitic	65-120	45-100	16-2	163-269

Gray Cast Iron

Cast irons are alloys of iron, carbon, and silicon in which more carbon is present than can be retained in solid solution in austenite at the eutectic temperature. In gray cast iron, the carbon that exceeds the solubility in austenite precipitates as flake graphite. Gray irons usually contain 2.5 to 4% C, 1 to 3% Si, and additions of manganese, depending on the desired microstructure (as low as 0.1% Mn in ferritic gray irons and as high as 1.2% in pearlitics). Sulphur and phosphorus are also present in small amounts as residual impurities.

Gray cast iron is by far the oldest and most common form of cast iron. As a result, it is assumed by many to be the only form of cast iron and the terms “cast iron” and “gray iron” are used interchangeably. Unfortunately the only commonly known property of gray iron – brittleness – is also assigned to “cast iron” and hence to all cast irons. Gray iron is named so because its fracture has a gray appearance. It contains carbon in the form of flake graphite in a matrix which consists of ferrite, pearlite or a mixture of the two. In the manufacture of gray cast irons, the tendency of cementite to separate into graphite (graphitization) and austenite or ferrite/pearlite is aided by high carbon content and the proper amount of graphitizing elements, notably silicon. With proper control, the alloy will form austenite and graphite at the eutectic temperature (of the stable iron-graphite equilibrium diagram) of 2075°F. At any rate, any cementite which is formed will graphitize rapidly. The graphite appears as many irregular, generally elongated and curved plates which give gray cast iron its characteristic grayish or blackish fracture.

The strength of gray cast iron depends almost entirely on the matrix in which the graphite is embedded. This matrix is largely determined by the condition of the eutectoid cementite. If the composition and cooling rate are such that the eutectoid cementite also graphitizes, then the matrix will be entirely ferritic. On the other hand, if graphitization of the eutectoid cementite is prevented, the matrix will be entirely pearlitic. The constitution of the matrix may be varied from pearlitic, through mixture of pearlitic and ferritic in different proportions, down to practically pure ferritic. Moderate cooling rate results in a pearlitic gray iron where as slow cooling rate results in a ferritic gray iron. The graphite-ferrite mixture is the softest and weakest gray iron. The strength and hardness increases with the increase in combined carbon, reaching a maximum with the pearlitic gray iron.

Silicon is very important element in the metallurgy of gray iron. It is a graphitizer and also increases fluidity. Therefore, during solidification in the presence of silicon, carbon is precipitated as primary graphite in the form of flakes. Once primary graphite has formed, its shape cannot be altered by any method. It is these weak graphite flakes that break up the continuity of the matrix and the notch effect at the end of these flakes that accounts for the low strength and low ductility of gray iron.

Following figure shows percentage of carbon and silicon in cast irons and steels.

Heat Treatment of Gray Iron

Stress relieving is probably the most frequently applied heat treatment for gray irons. Gray iron in the as-cast condition usually contains residual stresses because cooling proceeds at different rates throughout various sections of a casting. The resultant residual stresses may reduce strength, cause distortion, and in some cases even result in cracking.

Annealing of gray iron improves its machinability.

Gray iron is normalized to enhance its mechanical properties, such as hardness and tensile strength, or to restore as-cast properties that have been modified by another heat treating process, such as preheating and post heating associated with repair welding.

Gray iron, like steel, can be hardened and tempered. The high matrix hardness and graphite as lubricant results in a surface with good wear resistance for some applications such as farm implement gears, sprockets, diesel cylinder liners, etc.

Thus, heat treatment extends the field of application of gray iron as an engineering material.

Size and Distribution of Graphite Flakes

Large graphite flakes seriously interrupt the continuity of pearlitic matrix, thereby reducing the strength and ductility of gray iron. Small graphite flakes are less damaging and are therefore generally preferred.

Graphite-flak sizes are usually determined by comparison with standard sizes prepared jointly by the AFS (American Foundrymen’s Society) and the ASTM (American Society for Testing Materials). The procedure for preparation and measurement of flake size is given in ASTM standards specification A247. The measurement is made of the lengths of the largest graphite flakes in an unetched section of the gray iron at 100x. Numbers are assigned as indicated in the table given below.

 

AFS-ASTM Flake Size Number	1	2	3	4	5	6	7	8
Length of Longest Flake at 100x, MM	128	64	32	16	8	4	2	1
Length of Longest Flake at 100x, inch	≥4	2-4	1-2	1/2-1	1/4-1/2	1/8-1/4	1/16-1/8	≤ 1/16

The way in which the graphite lakes are arranged in the microstructure of gray iron is usually indicated as one or more types that have been jointly prepared by the AFS and the ASTM. The five flake types are as under.

Type A: Uniform distribution, random orientation
Type B: Rosette groupings, random orientation
Type C: Superimposed flake sizes, random orientation
Type D: Interdendritic segregation, random orientation
Type E: Interdendritic segregation, preferred orientation

Mechanical Properties of Gray Cast Iron

The most important classification of gray cast iron is given in the ASTM Specification A48. The gray iron castings are classed in seven classes (Nos. 20, 25, 30, 35, 40, 50 and 60) which give the minimum tensile strength of test bars in thousands of pounds per square inch. For example, class 20 gray iron would have a minimum tensile strength of 20000 psi.

Gray irons do not exhibit a well defined yield point as do mild steels. The percent elongation is small for all cast irons, rarely exceeding 3 to 4 percent, and the reduction of area is too small to be appreciable.

Many grades of gray iron have higher torsional shear strength than some grades of steel. This characteristic, along with low notch sensitivity, makes gray iron a suitable material for various types of shafting.

Applications of Gray Cast Iron

The fluidity of liquid gray iron, and its expansion during solidification due to the formation of graphite, has made this metal ideal for the economical production of shrinkage-free, intricate castings.

Because gray iron is the least expensive type of casting, it should always be considered first when a cast metal is being selected. Another metal should be chosen only when the mechanical and physical properties of gray iron are inadequate.

Examples of applications requiring bare minimum casting properties and lowest possible cost are counterweights for elevators and industrial furnace doors. Gray iron castings are widely used for gear housings, steam turbine housings, enclosures for electrical equipment, pump housings, motor frames and sewer covers

Chilled Cast Iron

Chilled-iron castings are made by casting the molten metal against a metal chiller, resulting in a surface of white cast iron. This hard, abrasion-resistant white-iron surface or case is backed up by a softer gray iron core. This case-core structure is obtained by careful control of the overall alloy composition and adjustment of the cooling rate.

Freezing starts first, and cooling rate is most rapid where the molten metal is in contact with the mold walls. The cooling rate decreases as the center of the casting is approached.

If only selected surfaces are to be white iron, it is common practice to use a composition which would normally solidify as gray iron and employ metal liner (chills) to accelerate the cooling rate of the selected areas. The depth of the white iron layer is controlled by using thin metal plates whenever a thin white iron layer is desired and heavier metal plates where a deeper chill is necessary.

Chilled iron casting is used for railway-car wheels, crushing rolls, sprockets, and many other heavy-duty machinery parts.

Nodular Cast Iron

Nodular cast iron, also known as ductile iron (in UAS), spheroidal graphite iron (in UK and Europe) and spherulitic iron is cast iron in which the graphite is present as tiny balls or spheroids. The compact spheroids interrupt the continuity of the matrix much less than graphite flakes, resulting in higher strength and toughness compared with a similar structure of gray cast iron. Nodular cast iron differs from malleable iron in that it is usually obtained as a result of solidification and does not require heat treatment. The spheroids are more rounded than the irregular aggregates of temper carbon found in malleable iron.

The total carbon content of nodular iron is the same as in gray iron. Spheroidal graphite particles form during solidification because of the presence of a small amount of certain alloying elements. The nodule forming addition, usually magnesium or cerium is made to the ladle just before casting. Since these elements have a strong affinity for sulfur, in the base iron-alloy, sulfur content must be below 0.015 percent for the treatment to be effective, and the alloys are described as desulfurized.

ASTM A536 – Standard Specification for Ductile Iron Castings is the most frequently used specification, covering the general engineering grades of ductile iron.

Standard specifications for ductile iron are normally based on mechanical properties, except for those defining austenitic ductile iron, which are based on composition. Mechanical requirements for various grades of ductile irons as per ASTM A536 are as under.

 

Property	Grade 60/40/18	Grade 65/45/12	Grade 80/55/06	Grade 100/70/03	Grade 120/90/02
Tensile strength, min, psi	60 000	65 000	80 000	100 000	120 000
Tensile strength, min, MPa	414	448	552	689	827
Yield strength, min, psi	40 000	45 000	55 000	70 000	90 000
Yield strength, min, MPa	276	310	379	483	621
Elongation in 2 in. min, %	18	12	6.0	3.0	2.0

The amount of ferrite in the as-cast matrix depends on composition and rate of cooling. Nodular iron with a matrix having a maximum of 10 percent pearlite is known as ferritic irons. This structure gives maximum ductility, toughness and machinability.

A matrix structure which is largely pearlitic can be produced as cast or by normalizing. Pearlitic ductile irons are stronger but less ductile than ferritic iron.

A martensitic matrix may be obtained by quenching in oil or water. The quenched structures are usually tempered after hardening to desired strength and hardness.

Austenitic ductile irons are highly alloyed types which retain their austenitic structure down to at least minus 75°F. They have relatively high corrosion resistance and good creep properties at elevated temperatures. More information of them is given in this article under alloy cast irons.

Tensile mechanical properties of basic types of nodular iron are given in the following table.

 

Type	Alloy Content	Tensile Strength, psi	Yield Strength, psi	Elongation, % in 2 in.	BHN
Ferritic	Low	55000	35000	25	130
	High	90000	70000	12	210
Pearlitic	Low	80000	60000	10	200
	Low (Normalized)	130000	90000	7	275
	High	130000	110000	2	275
Quenched	-	100000	80000	10	215
	-	150000	130000	2	320
Austenitic	*	60000	30000	40	130
	**	60000	40000	10	160

* 3.00 % C, 2.50 % Si, 20.00 % Ni, 2.00 % Mn
** 3.00 % C, 2.00 % Si, 20.00 % Ni, 1.00 % Mn, 1.50 % Cr

Nodular iron is used in agricultural – tractor and implement parts, automotive and diesel – crankshafts, pistons and cylinder heads, electrical fittings, motor frames, circuit breaker parts, Mining – hoist drums, drive pulleys, flywheels and elevator buckets, steel mills, tools and dies.

Alloy Cast Irons

An alloy cast iron is one which contains a specially added element or elements in sufficient amount to produce a measurable modification in the physical or mechanical properties.

Alloying elements are added to cast iron for special properties such as resistance to corrosion, heat or wear and to improve mechanical properties. The most common alloying elements are nickel, chromium, molybdenum and copper. For various industrial applications requiring resistance against wear, corrosion and heat, two types of alloy cast irons, High-alloy white cast irons and Ni-Resist cast irons are widely used. Information about both these varieties is given in the sections that follow.

High-alloy White Cast Irons

The high-alloy white irons are primarily used for abrasion-resistant applications. The chromium content of high-alloy white irons also enhances their corrosion-resistant properties. The large volume fraction of primary and/or eutectic carbides in their microstructures provides the high hardness needed for crushing and grinding other materials. The metallic matrix supporting the carbide phase in these irons can be adjusted by alloy content and heat treatment to develop the proper balance between the resistance to abrasion and the toughness needed to withstand repeated impact.

While low-alloy white iron castings, which have alloy content below 4%, develop hardness in the range of 350 to 550 HB, the high-alloy irons range in hardness from 450 to 800 HB.

Specification ASTM A532 covers the composition and hardness of the abrasion-resistant white iron grades. This specification deals with abrasion-resistant cast irons used for mining, milling, earth-handling, and manufacturing industries. It may be noted that simple and low-alloy white cast irons that consist essentially of iron carbides and pearlite are specifically excluded from this specification.

The high-alloy white cast irons fall into two major groups

Nickel-chromium white irons

They are low-chromium alloys containing 3 to 5% Ni and 1 to 4% Cr, with one alloy modification that contains 7 to 11% Cr. The nickel-chromium irons are also commonly identified as Ni-Hard types 1 to 4.

Chromium-molybdenum irons

They contain 11 to 23% Cr, up to 3.5% Mo and often additionally alloyed with nickel or copper.

A third group comprises the 25% or 28% Cr white irons, which may contain other alloying additions of molybdenum and/or nickel up to 1.5%.

The chemical composition for various class and type of high-alloy white irons as per ASTM A532 in % is as under.

 

Specification	C	Mn	Si	Cr	Ni	Mo	Cu	S	P
ASTM A532 I-A	3.0-3.6	1.3	0.8	1.4 -4.0	3.3-5.0	1.0	-	0.15	0.30
ASTM A532 I-B	2.5-3.0	1.3	0.8	1.4 -4.0	3.3-5.0	1.0	-	0.15	0.30
ASTM A532 I-C	2.9-3.7	1.3	0.8	1.1 -1.5	2.7 -4.0	1.0	-	0.15	0.30
ASTM A532 I-D	2.5-3.6	1.3	1.0 -2.2	7.0 –11.0	4.5-7.0	1.0	-	0.15	0.10
ASTM A532 II-A	2.4-2.8	0.5-1.5	1.0	11.0-14.0	0.5	0.5-1.0	1.2	0.06	0.10
ASTM A532 II-B	2.4-2.8	0.5-1.5	1.0	14.0-18.0	0.5	1.0-3.0	1.2	0.06	0.10
ASTM A532 II-C	2.8-3.6	0.5-1.5	1.0	14.0-18.0	0.5	2.3-3.5	1.2	0.06	0.10
ASTM A532 II-D	2.0-2.6	0.5-1.5	1.0	18.0-23.0	1.5	1.5	1.2	0.06	0.10
ASTM A532 II-E	2.6-3.2	0.5-1.5	1.0	18.0-23.0	1.5	1.0-2.0	1.2	0.06	0.10
ASTM A532 III-A	2.3-3.0	0.5-1.5	1.0	23.0-28.0	1.5	1.5	1.2	0.06	0.10

Nickel-Chromium White Irons

In these martensitic white irons, nickel is the primary alloying element because at levels of 3 to 5% it is effective in suppressing the transformation of the austenite matrix to pearlite, thus ensuring that a hard martensitic structure (usually containing significant amounts of retained austenite) will develop upon cooling in the mold. Chromium is included in these alloys, at levels from 1.4 to 4%, to ensure that the irons solidify carbidic, that is, to counteract the graphitizing effect of nickel. These types of white irons are used for crushing and grinding applications.

The optimum composition of a nickel-chromium white iron alloy depends on the properties required for the service conditions and the dimensions and weight of the casting. Abrasion resistance is generally a function of the bulk hardness and the volume of carbide in the microstructure. When abrasion resistance is the principal requirement and resistance to impact loading is secondary, alloys having high carbon contents, ASTM A532 class I type A (Ni-Hard 1), are recommended. When conditions of repeated impact are anticipated, the lower carbon alloys, ASTM A532 class I type B (Ni-Hard 2) are recommended because they have less carbide and, therefore, greater toughness. A special grade, class I type C, has been developed for producing grinding balls and slugs. Here, the nickel-chromium alloy composition has been adapted for chill casting and specialized sand casting processes.

The ASTM A532 Class I type D (Ni-Hard 4) alloy is a modified nickel-chromium iron that contains higher levels of chromium, ranging from 7 to 11%, and increased levels of nickel, ranging from 5 to 7%. Carbon is varied according to the properties needed for the intended service. Carbon contents in the range of 3.2 to 3.6% are prescribed when maximum abrasion resistance is desired. When impact loading is expected, carbon content should be held in the range of 2.7 to 3.2%.

Nickel content increases with section size or cooling time of the casting to inhibit pearlitic transformation. For castings of 38 to 50 mm thick, 3.4 to 4.2% Ni is sufficient to suppress pearlite formation upon mold cooling. Heavier sections may require nickel levels up to 5.5% to avoid the formation of pearlite. It is important to limit nickel content to the level needed for control of pearlite; excess nickel increases the amount of retained austenite and lowers hardness.

Silicon is needed for two reasons. A minimum amount of silicon is necessary to improve fluidity of the melt and to produce a fluid slag, but of equal importance is its effect on as-cast hardness. Increased levels of silicon, in the range of 1 to 1.5%, have been found to increase the amount of martensite and the resulting hardness. It is important to note that higher silicon contents can promote pearlite and may increase the nickel requirement.

Chromium is primarily added to offset the graphitizing effects of nickel and silicon in types A, B, and C alloys, ranges from 1.4 to 3.5%. Chromium content must be increased with increasing section size. In type D alloy, chromium levels range from 7 to 11% (typically 9%) for the purpose of producing eutectic carbides of the M7C3 chromium carbide type, which are harder and less deleterious to toughness.

Manganese is typically held to a maximum of 0.8% even though 1.3% maximum is allowed according to ASTM A532 specification. While it provides increased hardenability to avoid pearlite formation, it is a more potent austenite stabilizer than nickel, and promotes increased amounts of retained austenite and lower as-cast hardness. For this reason, higher manganese levels are undesirable. When considering the nickel content required to avoid pearlite in a given casting, the level of manganese present should be a factor.

Copper increases both hardenability and the retention of austenite and therefore must be controlled for the same reason that manganese must be limited. Copper should be treated as a nickel substitute and, when properly included in the calculation of the amount of nickel required to inhibit pearlite, it reduces the nickel requirement.

Molybdenum is a potent hardenability agent in these alloys and is used in heavy-section castings to augment hardenability and inhibit pearlite.

High-Chromium White Irons

The high-chromium white irons have excellent abrasion resistance and are used effectively in slurry pumps, coal-grinding mills, shot-blasting equipment, and components for quarrying, hard-rock mining, and milling. In some applications they must also be able to withstand heavy impact loading. These alloyed white irons are recognized as providing the best combination of toughness and abrasion resistance attainable among the white cast irons.

In the high-chromium irons, as with most abrasion-resistant materials, there is a trade-off between wear resistance and toughness. By varying composition and heat treatment, these properties can be adjusted to meet the needs of most abrasive applications.

Specification ASTM A532 covers the compositions and hardnesses of two general classes of the high-chromium irons. The chromium-molybdenum irons (Class II of ASTM A532) contain 11 to 23% Cr and up to 3.5% Mo and can be supplied either as-cast with an austenitic or austenitic-martensitic matrix, or heat-treated with a martensitic matrix microstructure for maximum abrasion resistance and toughness. They are usually considered the hardest of all grades of white cast irons. Compared to the lower-alloy nickel-chromium white irons, the eutectic carbides are harder and can be heat-treated to achieve castings of higher hardness. Molybdenum, as well as nickel and copper when needed, is added to prevent pearlite and to ensure maximum hardness.

The high-chromium irons (class III of ASTM A532) represent the oldest grade of high-chromium irons. These general-purpose irons, also called 25% Cr and 28% Cr irons, contain 23 to 28% Cr with up to 1.5% Mo. To prevent pearlite and attain maximum hardness, molybdenum is added in all but the lightest-cast sections. Alloying with nickel and copper up to 1% is also practiced. Although the maximum attainable hardness is not as high as in the class II chromium-molybdenum white irons, these alloys are selected when resistance to corrosion is also desired.

Ni-Resist Cast Irons

Austenitic gray iron castings known as Ni-Resist irons (also called standard Ni-Resist irons) and austenitic ductile iron castings known as ductile Ni-Resist irons are used primarily for their resistance to heat, corrosion, and wear. Both families of alloys have similar general characteristics as described below.

General characteristics of the Ni-Resist irons

Corrosion Resistance:

They have been specified to solve corrosion problems involving the handling of sour well oils, salts, salt water, acids and alkalies. They are intermediate in corrosion resistance between gray cast irons and austenitic chromium-nickel stainless steels. The excellent erosion-corrosion resistance of the Ni-Resist alloys has resulted in extensive applications for pump and valve components in sea water handling systems.

Wear Resistance:

Engine, pump and their parts like pistons, wearing rings, sleeves, glands, etc. and other metal-to-metal rubbing parts have been cast in the Ni-Resist alloys.

Erosion Resistance:

Slurries, wet steam and gases with entrained particles are among the elements extremely erosive to most metals. The Ni-Resist alloys offer a combination of corrosion-erosion-resistant properties that provide superior service under many erosive conditions.

Toughness and Low-Temperature Stability:

Ni-Resist castings are considerably superior to gray iron under these service conditions.

Controlled Expansion:

Expansivities from 2.2 to 10.6 x 10-6 in./in. per degree F are available with different types of Ni-Resist irons. This affords a conventional cast metal with low expansivity for precision machines or parts.

Heat Resistance:

For unusual high-temperature service, 1600°F and above, the ductile grade of Ni-Resist should be considered. The standard grades (Ni-Resist irons) exhibit a high order of heat resistance to 1300°F. The alloys have a relatively low rate of oxidation, and what oxidation does occur adheres tenaciously to the base metal. This property is especially helpful in gas turbine, manifold and turbocharger applications.

Castability:

The Castability of Ni-Resist permits casting complicated and intricate designs often difficult to produce with some other high-alloy cast metals. Both the castability and machinability of the Ni-Resist alloys aid in producing finished parts economically.

Ni-Resist Irons (Austenitic Gray Iron Castings)

Basically, the family is a series of cast irons to which has been added sufficient nickel to produce an austenitic structure similar to that of austenitic stainless steel. Austenitic gray iron is characterized by uniformly distributed graphite flakes, some carbides, and an austenitic structure. This structure provides the alloy family’s heat and corrosion resistant properties.

The several types of Ni-Resist are produced by varying the nickel content and, to some extent, the chromium content and are covered by ASTM A436 – Standard Specification for Austenitic Gray Iron Castings. The types of castings covered are Type 1, Type 1b, Type 2, Type 2b, Type 3, Type 4, Type 5, and Type 6. Their chemical composition in % is as under.

 

Specification	C	Mn	Si	Cr	Ni	Mo	Cu	S
ASTM A436 TYPE1	3.00	0.5-1.5	1.00-2.80	1.50-2.50	13.50-17.50	-	5.50-7.50	0.12
ASTM A436 TYPE 1b	3.00	0.5-1.5	1.00-2.80	2.50-3.50	13.50-17.50	-	5.50-7.50	0.12
ASTM A436 TYPE 2	3.00	0.5-1.5	1.00-2.80	1.5 -2.5	18.00-22.00	-	0.50	0.12
ASTM A436 TYPE 2b	3.00	0.5-1.5	1.00-2.80	3.00-6.00	18.00-22.00	-	0.50	0.12
ASTM A436 TYPE 3	2.60	0.5-1.5	1.00-2.00	2.50-3.50	28.00-32.00	-	0.50	0.12
ASTM A436 TYPE 4	2.60	0.5-1.5	5.00-6.00	4.50-5.50	29.00-32.00	-	0.50	0.12
ASTM A436 TYPE 5	2.40	0.5-1.5	1.00-2.00	0.10	34.00-36.00	-	0.50	0.12
ASTM A436 TYPE 6	3.00	0.5-1.5	1.50-2.50	1.00-2.00	18.00-22.00	1.00	3.5 -5.5	0.12

The low nickel content of Types 1 and 1b is accompanied by copper to provide the corrosion-resisting characteristics. (Note: Types 1 and 1b are not responsive to the magnesium treatment process, and have no ductile counterparts.)

Mechanical requirements for various types of Ni-Resist irons covered by ASTM A436 are as under.

 

Property	Type
	1	1b	2	2b	3	4	5
Tensile strength, min, ksi	25	30	25	30	25	25	20
Brinell hardness	131-183	149-212	118-174	171-248	118-159	149-212	99-124

Characteristics and applications of the different types are as under.

Types 1 and 2: are used interchangeably in many corrosion and wear resistant applications. Both are specially suited to heavy metal-to-metal wear service.

Type 1: has some advantages over the other types in handling mineral-acid corrosives and salt water.

Type 1b: has superior corrosion-erosion resistance in comparison with Type 1, and is harder and stronger.

Type 2: because of its wide use in corrosive environments, is the most commonly used. It is preferred for heat and oxidation resistance to 1300°F and in steam service. It is also used for handling caustics, alkalies, ammoniacal solutions, food products, rayon, plastics and similar environments where freedom from copper contamination is desired.

Type 2b: is especially recommended for heat applications up to 1500°F. Uses include gas turbine parts, exhaust manifolds and turbochargers. If machining is required, the chromium level should be kept between 3.0 and 4.0 percent. The greater hardness of Type 2b makes it suitable for resistance to abrasive wear with corrosion, but is not suited for metal-to-metal wear.

Type 3: is recommended for sever thermal shock service between room temperature and 450°F. At temperatures between 450 and 1500°F it can be used without severe thermal shock. It may be used for diesel exhaust manifolds and turbochargers. The type offers high resistance to erosion in wet steam and corrosive slurries.

Type 4: is recommended where strain-resistance is required. This type is superior to other types in resistance to erosion, corrosion and oxidation.

Type 5: offers minimum thermal expansivity which provides dimensional stability for machine tool parts, forming dies, steam turbines, scientific instruments and expansion joints.

Ductile Ni-Resist Irons (Austenitic Ductile Iron Castings)

The ductile Ni-Resist irons are similar to standard/conventional Ni-Resist compositions but have been treated with magnesium to convert the graphite from the flake form to spheroids, which results in higher strength and ductility. Austenitic ductile iron, also known as austenitic nodular iron or austenitic spheroidal iron, is characterized by having its graphite substantially in a spheroidal form and substantially free of flake graphite. It contains some carbides and sufficient alloy content to produce an austenitic structure.

The ductile family of Ni-Resist is available in every type except Type 1, which because of the higher copper content does not respond properly to the magnesium treatment. There are in addition several modifications of Type 2 to 5. Various types of ductile Ni-Resist irons are covered by ASTM A439 – Standard Specification for Austenitic Ductile Iron Castings. Their chemical composition in % is as under.

 

Specification	C	Mn	Si	Cr	Ni	P
ASTM A439 Type D-2	3.00	0.70-1.25	1.50-3.00	1.75-2.75	18.00-22.00	0.08
ASTM A439 Type D-2B	3.00	0.70-1.25	1.50-3.00	2.75-4.00	18.00-22.00	0.08
ASTM A439 Type D-2C	2.90	1.80-2.40	1.00-3.00	0.50	21.00-24.00	0.08
ASTM A439 Type D-3	2.60	1.00	1.00-2.80	2.50-3.50	28.00-32.00	0.08
ASTM A439 Type D-3A	2.60	1.00	1.00-2.80	1.00-1.50	28.00-32.00	0.08
ASTM A439 Type D-4	2.60	1.00	5.00-6.00	4.50-5.50	28.00-32.00	0.08
ASTM A439 Type D-5	2.40	1.00	1.00-2.80	0.10	34.00-36.00	0.08
ASTM A439 Type D-5B	2.40	1.00	1.00-2.80	2.00-3.00	34.00-36.00	0.08
ASTM A439 Type D-5S	2.30	1.00	4.90-5.50	1.75-2.25	34.00-37.00	0.08

Mechanical requirements for various types of ductile Ni-Resist irons covered by ASTM A439 are as under.

 

Property	Type
	D-2	D-2B	D-2C	D-3	D-3A	D-4	D-5	D-5B	D-5S
Tensile strength, min, ksi (MPa)	58 (400)	58 (400)	58 (400)	55 (379)	55 (379)	60 (414)	55 (379)	55 (379)	65 (449)
Yield strength (0.2 percent offset), min, ksi (MPa)	30 (207)	30 (207)	28 (193)	30 (207)	30 (207)	-	30 (207)	30 (201)	30 (207)
Elongation 2 in. or 50mm, min, %	8.0	7.0	20.0	6.0	10.0	-	20.0	6.0	10
Brinell hardness (300 kg)	139-202	148-211	121-171	139-202	131-193	202-273	131-185	139-193	131-193

Characteristics and applications of the different types are as under.

Type D-2: is recommended for service requiring resistance to corrosion, erosion, frictional wear and temperature up to 1400°F. This type also has high thermal expansivity.

Type D-2B: is recommended for superior resistance to neutral and reducing salts, and where higher resistance to erosion and oxidation than that offered by Type D-2 is desired.

Type D-2C: is recommended where resistance to heat and corrosion is less severe and where high ductility is desired.

Type D-3: is recommended for thermal shock service and where the thermal expansivity should match that of ferritic stainless steels. This type, in addition to having excellent elevated-temperature properties, also offers high resistance to erosion.

Type D-3A: is recommended for use where a high degree of wear and galling resistance is required along with intermediate thermal expansion.

Type D-4: is superior to Types D-2 or D-3 in resistance to corrosion, erosion and oxidation.

Type D-5: is utilized whenever minimum thermal expansion is desired. It may also be preferred over other types of Ni-Resist to reduce thermal stress.

Type D-5B: is recommended for applications requiring a very low order thermal stress. In addition, this type has a high level of heat and oxidation resistance as well as good mechanical properties at elevated temperatures.

Type D-5S: is recommended where oxidation resistance in air to 1800°F is desired. It also resists embrittlement and offers thermal stability and strength in cyclic heating to 1600°F.

Summary of the cooling rates to make various types of cast irons

A summary of the cooling rates to make various types of cast iron and their microstructure at room temperature is given below.

Tool steels are defined by U.S. steel producers as “carbon or alloy steels capable of being hardened and tempered”. Many alloy steels would fit this loose definition. However tool steels usually contain significantly more alloying elements than alloy steels.

Approximately 70 types of tool steels are available in the United States. One reason for so many types of tool steels is evolutionary development over a period of many years. The second reason is the wide range of needs that they serve. Tool steels have properties that permit their use as tools for cutting and shaping metals and other materials both hot and cold.

Characteristics of Tool Steels

One problem that exists in discussing the metallurgy of tool steels is that, since there are seven major headings, it is difficult to make statements that apply to all these steels. The following are some of the characteristics of tool steels.

• Composition and physical properties vary significantly (some tool steels have compositions that fit into the composition ranges of carbon and alloy steels, but most tool steels have alloy concentrations that are significantly higher than the carbon and alloy steels).
• One important factor that should be kept in mind is that the alloy additions do not improve corrosion resistance even though some grades have as much chromium as stainless steels. The reason for this is that alloy elements are usually combined with carbon to form carbides.
• The most significant metallurgical difference between tool steels and the other steels is their microstructure. A fully hardened carbon steel or alloy steel would have only martensite as the predominant phase. Most tool steels have a hardened structure of martensite and alloy carbides.
• Require special heat treatment processes.
• Better hardenability than most carbon and alloy steels.
• High heat resistance.
• Easier to heat treat.
• Most tool steels are sold as hot-finished shapes such as rounds and bars.

Classification of Tool Steels by American Iron and Steel Institute (AISI)

Designation system of one-letter in combination with a number is adopted for classification of tool steels by American Iron and Steel Institute (AISI). The commonly used tool steels have been grouped into seven major headings, and each group or subgroup has been assigned an alphabetical letter as under.

 

Group	Symbol and Type
Water-hardening	W
Shock-resisting	S
Cold-work	O Oil-hardening
	A Medium-alloy air-hardening
	D High-carbon High-chromium
Hot-work	H (H1-H19: chromium base, H20-H39: tungsten base and H40-H59: molybdenum base)
High-speed	T Tungsten-base
	M Molybdenum-base
Mold	P Mold steels (P1-P19: low-carbon and P20-P39: other types)
Special-purpose	L Low-alloy
	F Carbon-tungsten

The term toughness as applied to tool steels may be thought of as the ability to resist breaking rather than the ability to absorb energy during deformation.

Water Hardening Tool Steels (Group W)

They are used for low speeds and light cuts on relatively soft materials such as wood, brass, aluminium and unhardened low carbon steels.

Essentially these are plain carbon steels with 0.60 to 1.40 % carbon.

They have soft core (for toughness) with hard shallow layer (for wear resistance).

Their resistance to heat is poor.

Shock Resisting Tool Steels (Group S)

These steels are low in carbon content (0.45 to 0.55 %). The principal alloying elements are silicon, chromium, tungsten and sometimes molybdenum. Silicon strengthens the ferrite, while chromium increases hardenability and contributes slightly to wear resistance. Tungsten imparts red-hardness. The S group of tool steels were originally developed to withstand repeated shock (chisel-type applications), but the number of alloys in this category has evolved to include steels with a broad range of tool applications. This class of steels has a very good shock resistant quality with excellent toughness.

They are used in forming tools, chisels, punches, cutting blades and pneumatic tools.

Cold Work Tool Steels (Groups O, A and D)

Cold work tool steels are used for taps; blanking, forming and thread rolling dies; knurling tools; reamers; master gauges and drawing dies for wire, bars and tubes.

 

Oil Hardening Type (Group O)	Medium Alloy Air Hardening Types (Group A)	High Carbon High Chromium Types (Group D)
0.90 to 1.45 % Carbon with Mn, Si, W, Mo, Cr.   They contain graphite in the hardened structure along with martensite. (Graphite acts as a lubricator and also makes machining easier).	Contain 5 to 10 % alloying elements (Mn, Si, W, Mo, Cr, V, Ni) to improve the hardenability, wear resistance and toughness.	Contain 12% Cr and up to 2.25 % C.   Air or oil quench.   Low distortion, high abrasion resistance.

Hot Work Tool Steels (Group H)

In many applications the tool is subjected to excessive heat because the material being worked is in hot condition. Tool steels developed for these applications are known as hot work tool steels and have good red-hardness.

They are categorized by major alloying elements into three subgroups as under.

• Chromium types
• Tungsten types
• Molybdenum types

These steels are used in hot die work of all kinds, particularly extrusion dies, forging dies, die-casting dies, hot shear blades and mandrels.

High-Speed Tool Steels (Groups M and T)

The major application of high-speed steels is for cutting tools, but they are also used for making extrusion dies, blanking punches and dies.

These are the classes of steel that deep harden, retain that hardness at elevated temperatures and have high resistance to wear and abrasion. The carbon content of these steels varies from 0.70 % to 1.50 %.

These steels are subdivided into two groups: molybdenum base (group M) and tungsten base (group T). From the stand point of fabrication and tool performance, there is little difference between the molybdenum and tungsten grades. The important properties of red-hardness, wear resistance and toughness are about the same.

This class of tool material has a substantial amount of wear-resistant carbides in a hard heat resistant matrix. These steels are used in machine cutting tools such as tool bits, milling cutters, taps, reamers, drills, broaches.

The microstructure of a typical AISI M-2 high-speed tool steel is shown in the figure given below.

The microstructure consists of tempered martensite and carbides (white etching constituent).

Mold Tool Steels (Group P)

These steels have 0.10 to 0.35 % carbon. They show high toughness. The low carbon mold steels cannot be quench hardened. The carbon and alloy content is low to allow hubbing of mold details. The desired mold shape is pressed into the steel with a hub that is usually made from high speed steel. Thus mold cavities can be made without machining. Hubbed cavities are then carburized to make a production injection molding cavity.

They are used for low-temperature die casting dies and for molds for injection or compression molding of plastics.

Special Purpose Tool Steels (Groups L and F)

Many tool steels do not fall into the usual categories and are therefore designated as special-purpose tool steels.

The oil-hardening L group steels are low alloy steels with about 1 % Cr that makes them a good low cost substitute for cold work steels. The carbon-tungsten type (F group) steels are generally shallow-hardening, water-quenching steels with high carbon and tungsten content to promote high wear resistance. They are relatively brittle, so that in general they are used for high-wear, low-temperature and low-shock applications.

Typical uses for the L group steels are various machine tool applications where high wear resistance with good toughness is required as used in bearings, rollers, clutch plates, cams, etc. The high carbon types are used for gages, drills, taps, threading dies, knurls, etc.

The F group steels are used in wire-drawing dies, plug gages, forming tools, knives, etc.

Special Cutting Materials

Following materials are also used for tools.

Stellites

These are essentially cobalt-chromium-tungsten alloys. They contain from 25 to 35 percent chromium, 4 to 25 percent tungsten, 1 to 3 percent carbon and the remainder cobalt. The hardness of stellite varies from C 40 to 60, depending upon the tungsten and carbon content. Their outstanding properties are high hardness, high resistance to wear and corrosion and excellent red-hardness.

Stellite metal-cutting tool are widely used for machining steel, cast iron, cast steel, stainless steel and most machinable materials. They may be operated at higher speeds than those used with high-speed-steel tools. Stellite is also used as a hard-facing material.

Cemented Carbides

These materials are made of very finely divided carbide particles of the refractory metals, cemented together by a metal or alloy of the iron group, forming a body of very high hardness and high compressive strength. Cemented carbides are manufactured by powder-metallurgy techniques.

The exceptional tool performance of sintered carbide results from high hardness and high compressive strength combined with unusual red-hardness. The lowest hardness of sintered carbide is approximately the same as the highest hardness available in tool steel, Rockwell C 67.

Since cemented carbides have low toughness and tensile strength, the usual practice is to braze or mechanically fasten a small piece of carbide material (called an insert) to a steel shank, which provides rigid support under the cutting edge.

The high hardness and wear resistance make them well suited for earth drilling and mining applications. Cemented carbide dies are used for the hot drawing of tungsten and molybdenum and for the cold drawing of wire, bar and tubing made of steel, copper and other materials.

Ceramic Tools

Most ceramic or cemented oxide cutting tools are manufactured primarily from aluminium oxide. Ceramic cutting tools are most commonly made as a disposable insert which is fastened in a mechanical holder.

The principal elevated-temperature properties of alumina are high hardness, chemical inertness and resistance to wear. They are used in machining cast iron and hardened steel at high cutting speeds. They withstand the abrasion of sand and of inclusions found in castings. Heat treated materials as hard as Rockwell C 66 can be readily machined by them. However they are brittle and tend to chip easily.

Selection of Tool Steels

For most applications, hardness, toughness, wear resistance and red-hardness are the most important selection factors in choosing tools steels.

The selection of proper tool steel for a give application is a difficult task. Although many tool steels will perform on any given job, they will have to be judged on the basis of expected productivity, ease of fabrication and cost. In the final analysis, it is the cost per unit part made by the tool that determines the proper selection.

Corrosion in Stainless Steels

The addition of chromium to a steel results in the formation of a thin stable, non-porous, tightly adherent and ductile layer of primarily chromium oxide on the surface of the steel provided that it is exposed to air or another oxidizing environment. Since this layer conveys passivity to the steel, which means that it does not actively corrode, it is also called a passive layer. It is responsible for the ability of the steel to resist corrosion. The thickness of this very thin layer is of the order of 1 – 10nm (1 nanometre = 10^-9 m or 10^-6 mm).

It is also possible that the passive layer is damaged by tools during manufacturing (milling, grinding, polishing, drilling, tapping) or by accident. Under normal conditions (in the presence of air or an oxidizing environment) the passive layer forms itself anew, it is self-healing. This interesting capability of stainless steel is of great practical importance as no special measures are needed to renew or repair the corrosion resisting layer.

The concept of passive film formation is important because any conditions which prevent the formation of the film or cause it to break down will also lead to loss of corrosion resistance. Corrosion in stainless steel therefore occurs if the passive film is damaged and is not allowed to re-form. The behavior of the passive film depends on the composition of the steel, its surface treatment and the corrosive nature of its environment.

Stainless steels are generally very corrosion resistant and will perform satisfactorily in most environments. The limit of corrosion resistance of a given stainless steel depends on its constituent elements which mean that each grade has a slightly different response when exposed to a corrosive environment. Care is therefore needed to select the most appropriate grade of stainless steel for a given application.

Most specifiers and designers understand the importance of selecting a grade of stainless steel, for example 1.4301 (304) or 1.4401 (316). But surface finish is at least as important. In short, a bright polished surface gives maximum corrosion resistance.

The most common reasons for a metal to fail to live up to expectations regarding corrosion resistance are:

1. Incorrect assessment of the environment or exposure to unexpected conditions, e.g. unsuspected contamination by chloride ions.
2. The way in which the stainless steel has been worked or treated may introduce a state not envisaged in the initial assessment.

Experience indicates that any serious corrosion problem is most likely to show up in the first two or three years of service; problems can usually be explained by defects in bringing stainless steels into operation.

In certain aggressive environments some grades of stainless steel are susceptible to corrosion. Six common corrosion mechanisms are described below.

General (uniform) Corrosion

Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. If stainless steel is to come into contact with chemicals, design shall be based on published data to predict the life of the component. Published data list the removal of metal over a year. Tables of resistance to various chemicals are published by various organizations and a very large collection of charts, lists, recommendations and technical papers are available though stainless steel manufacturers and suppliers.

Pitting Corrosion

Pitting is a localized form of corrosion which can occur as a result of exposure to specific environments, most notably those containing chlorides (such as sodium chloride in sea water). Chloride ions facilitate a local breakdown of the passive layer, especially if there are imperfections in the metal surface. One reason why pitting corrosion is so serious is that once a pit is initiated there is a strong tendency for it to continue to grow, even although the majority of the surrounding steel is still untouched. This is not related to published corrosion data as it is an extremely localized and severe corrosion which can penetrate right through the cross section of the component. Grades high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.

The tendency for a particular steel to be attacked by pitting corrosion can be evaluated in the laboratory. A number of standard tests have been devised, the most common is that given in ASTM G48.

The Pitting Resistance Equivalent Number (PREN) has been found to give a good indication of the pitting resistance of stainless steels. Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels, based on their chemical compositions. The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.

These are 'linear' formulas, where the molybdenum and nitrogen levels are 'weighted' to take account of their strong influence on pitting corrosion resistance. The most commonly used version of the formula is,

PREN = %Cr + 3.3[%Mo] + 16[%N]

Tungsten is also included in the molybdenum-rating factor to acknowledge its affect on pitting resistance in the tungsten bearing super-duplex types (1.4501). A modified formula is then used as under.

PREN = %Cr + 3.3[%Mo +0.5(%W)] + 16[%N]

Grades with a PREN of 40 or more are known as 'super' austenitics or 'super' duplex types, depending to which basic family they belong.

PREN of some grades of stainless steels are given in the table given below.

NS – Not specified

In the above table, use of trade names (name of manufacturers) is as under.

Crevice Corrosion

The corrosion resistance of a stainless steel is dependent on the presence of a protective oxide layer on its surface. It is possible under certain conditions for this oxide layer to break down, for example in reducing acids. In such case, stainless steel requires supply of oxygen to make sure that the passive layer can form again.

Crevice corrosion is a localized form of attack which is initiated by the extremely low availability of oxygen in a crevice. It is only likely to be a problem in stagnant solutions where a build-up of chlorides can occur. The severity of crevice corrosion is very dependent on the geometry of the crevice; the narrower (around 25 micro-meters) and deeper the crevice, the more severe the corrosion. Crevices typically occur between nuts and washers or around the thread of a screw or the shank of a bolt. Crevices can also occur under deposits on the steel surface.

Crevice corrosion is a very similar mechanism to pitting corrosion. Alloys resistant to one are generally resistant to both. Crevice corrosion can be viewed as a more severe form of pitting corrosion as it will occur at significantly lower temperatures than does pitting. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.

Stress Corrosion Cracking (SCC)

Under the combined effects of stress, temperature and certain corrosive environments stainless steels can be subject to this form of corrosion. The stresses must be tensile, but do not need to be very high in relation to the proof stress of the material and can result from loads applied in service, or stresses set up by the type of assembly e.g. interference fits of pins in holes, or from residual stresses resulting from the method of fabrication such as cold working, welding or bending. The most damaging environment is a solution of chlorides in water such as sea water, particularly at elevated temperatures. As a consequence stainless steels are limited in their application for holding hot waters (above about 50°C) containing even trace amounts of chlorides (more than a few parts per million). This form of corrosion is only applicable to the austenitic group of steels and is related to the nickel content. Grade 316 is not significantly more resistant to SCC than is 304. The duplex stainless steels are much more resistant to SCC than are the austenitic grades, with grade 2205 being virtually immune at temperatures up to about 150°C, and the super duplex grades are more resistant again. The ferritic grades do not generally suffer from this problem at all.

Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production. A threshold stress can sometimes be identified for each material – environment combination. Some published data show a continuous fall of threshold stress with increasing H2S levels. To guard against SSCC, NACE specification MR0175 for sulphide environments limits the common austenitic grades to 22HRC maximum hardness.

Bimetallic (galvanic) corrosion

Bimetallic (galvanic) corrosion may occur when dissimilar metals are in contact in a common electrolyte (e.g. rain, condensation etc.). If current flows between the two, the less noble metal (the anode) corrodes at a faster rate than would have occurred if the metals were not in contact. The rate of corrosion also depends on the relative areas of the metals in contact, the temperature and the composition of the electrolyte.

If the area of the less noble material (the anodic material) is large compared to that of the more noble (cathodic material) the corrosive effect is greatly reduced, and may in fact become negligible. Conversely a large area of noble metal in contact with a small area of less noble will accelerate the galvanic corrosion rate.

Bimetallic corrosion may be prevented by excluding water from the detail (e.g. by painting or taping over the assembled joint) or isolating the metals from each other (e.g. by painting the contact surfaces of the dissimilar metals). Isolation around bolted connections can be achieved by non-conductive plastic or rubber gaskets and nylon or teflon washers and bushes.

Intergranular Corrosion

This is now quite a rare form of corrosion. When austenitic stainless steels are subject to prolonged heating between 450-850° C, the carbon in the steel diffuses to the grain boundaries and precipitates chromium carbide. This removes chromium from the solid solution and leaves a lower chromium content adjacent to the grain boundaries. Steels in this condition are termed 'sensitized'. The grain boundaries become prone to preferential attack on subsequent exposure to a corrosive environment. This phenomenon is known as weld decay when it occurs in the heat affected zone of a weldment. It should be noted that carbide precipitation depends upon carbon content, temperature and time at temperature. The most critical temperature range is around 700°C, at which 0.06% carbon steels will precipitate carbides in about 2 minutes, whereas 0.02% carbon steels are effectively immune from this problem. Thus the problem is avoided by choosing a low carbon grade the so-called ‘L’ grades which have low carbon content (~0.03%) or by using steel with Titanium or Niobium which preferentially combines with Carbon, leaving the chromium in solution and ensuring full corrosion resistance. These are the stabilized grades 321 (stabilized with titanium) and 347 (stabilized with niobium).

Importance of Segregating Carbon and Stainless Steel

Sometimes "rusting" of stainless steel is due to contamination. It is the rusting of carbon steel which has contaminated the surface of the stainless steel at some point in the production process. Possible sources of contamination from carbon steel are tools, lifting gear (ropes, chains), grinding dust, cutting sparks, wire brushes, etc.

Wherever possible, stainless steel and carbon steel should be fabricated in separate areas of the workshop or better still in separate workshops. Where this is not possible, it is important to clean down machines used for carbon steel before using them for stainless steel. Stainless steel surfaces should be protected with plastic coatings for as long as possible during storage.

In short, it is necessary to avoid contamination of the surface of stainless steel components by carbon steel at all stages of fabrication, handling, storage, transportation and erection.

Selection of Stainless Steels for Corrosion Resistance

Chromium (Cr) content sets stainless steels apart from other steels. Commercially available grades have around 11% chromium as a minimum. These can be either ferritic or martensitic, depending on carbon range control.

Increasing chromium enhances corrosion and oxidation resistance, so a 17% Cr 430 (1.4016) ferritic would be expected to be an improvement over the '410S' (1.4000) types. Similarly martensitic 431 (1.4057) at 15% Cr can be expected to have better corrosion resistance than the 12% Cr 420 (1.4021 / 1.4028) types.

Chromium levels over 20% provide improved 'aqueous' corrosion resistance for the duplex and higher alloyed austenitics and also forms the basis of the good elevated temperature oxidation resistance of ferritic and austenitic heat resisting grades, such as the quite rare ferritic 446 (25% Cr) or the more widely used 25 % Cr, 20% nickel (Ni) austenitic 310 (1.4845) grade.

In addition to this basic 'rule', nickel (Ni) widens the scope of environments that stainless steels can 'handle'. The 2% Ni addition to the 431 (1.4057) martensitic type improves corrosion resistance marginally. Additions of between about 4.5% and 6.5% Ni are made in forming the duplex types. The austenitics have ranges from about 7% to over 20%.

However, the corrosion resistance is not simply related to nickel level. It would be wrong to assume that a 304 (1.4301) with its 8% Ni therefore has better corrosion resistance than S32205 (1.4462) duplex with only 5% Ni.

More specific alloy additions are also made with the specific aim of enhancing corrosion resistance. These include molybdenum (Mo) and nitrogen (N) for pitting and crevice corrosion resistance. The 316 types are the main Mo bearing austenitics. Many of the currently available duplex grades contain additions of both Mo and N.

Copper is also used to enhance corrosion resistance in some 'common', but hazardous, environments such as 'intermediate' concentration ranges of sulphuric acid. Grades containing copper include the austenitic 904L (1.4539) type.

Detecting Susceptibility to Intergranular Attack

Susceptibility to intergranular attack in austenitic stainless steels can be detected by performing test as per ASTM A262. This specification covers the standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels. These practices include five intergranular corrosion tests, namely: (1) oxalic acid etch test for classification of etch structures of austenitic stainless steels, (2) ferric sulfate-sulfuric acid test, (3) nitric acid test and (4) copper-copper sulfate-sulfuric acid test for detecting susceptibility to intergranular attack in austenitic stainless steels; and (5) copper-copper sulfate-50% sulfuric acid test for detecting susceptibility to intergranular attack in molybdenum-bearing cast austenitic stainless steels. For more information please refer the specification.

Stainless steel castings are usually classified as either corrosion-resistant castings or heat-resistant. The usual distinction between corrosion-resistant and heat-resistant cast steels is based on carbon content. Cast stainless steels are most often specified by the designations established by the ACI (Alloy Casting Institute). Information about cast stainless steel grades is given in this article.

Cast Stainless Steels

There is a slight difference between wrought (rolled type) alloys and cast alloys in terms of composition, characteristics, behaviour, performance and service life. Because of the possible existence of large dendritic grains, intergranular phases, and alloy segregation, typical mechanical properties of cast stainless steels may vary more and generally are inferior to those of any wrought structure. Cast alloy chemical composition ranges are not the same as the wrought alloy composition ranges. For convenience, buyers often use wrought alloy designations for castings and frequently use them while specifying materials for castings. Most of the wrought alloy compositions have equivalent and corresponding cast alloy composition/designations used in alloy standards of different countries. Therefore, one must specify the desired alloy composition by casting type designation. Use of Alloy Casting Institute (ACI), now The Steel Founder’s Society of America or American Society for Testing Materials (ASTM) standards & specifications have been developed as a consensus of consumers, producers and disinterested experts and are often the most effective way to ensure understanding of the requirements.

In general, the cast and wrought stainless steels possess equivalent resistance to corrosive media and they are frequently used in conjunction with each other. One significant difference between the cast and wrought stainless steels is in the microstructure of cast austenitic stainless steels.

There is usually small amount of ferrite present in austenitic stainless steel castings, in contrast to the single-phase austenitic structure of the wrought alloys. The ferrite in cast stainless steel with duplex structures is magnetic, a point that is often confusing when cast stainless steels are compared to their wrought counterparts by checking their attraction to a magnet. Ferrite can be beneficial in terms of weldability because fully austenitic stainless steels are susceptible to a weldability problem known as hot cracking, or microfissuring. Ferrite also increases resistance to stress-corrosion cracking due to addition of silicon for fluidity.

However ferrite can be detrimental in some applications. For applications that require steels to be heated in the range from 425 to 650°C, carbide precipitation occurs at the edges of the ferrite pools in preference to the austenite grain boundaries.

The serviceability of cast corrosion-resistant steels depends greatly on the absence of carbon, and especially precipitated carbides, in the alloy microstructure. Therefore, cast corrosion resistant alloys are generally low in carbon (usually lower than 0.20% and sometimes lower than 0.03%).

ACI System of Designation

Service temperature provides the basis for a distinction between corrosion-resistant and heat-resistant cast grades. ACI standards include the C grades for aqueous (wet) corrosion service at temperatures up to 650°C and the H grades for high temperature service at temperatures generally over 600°C. Carbon and nickel contents of the H grade alloys are considerably higher than those of the C grades. H grade steels are not immune to corrosion, but they corrode slowly even when exposed to fuel-combustion products or atmospheres prepared for carburizing and nitriding. C grades are used in valve, pumps, and fittings. H grades are used for furnace parts and turbine components.

As per the ACI system of designation, the first letter of the designation indicates whether the alloy is intended primarily for liquid corrosion service (C) or high temperature service (H). The second letter indicates the approximate nickel and chromium contents of the alloy grade on the FeCrNi ternary diagram (ASTM A781). As nickel content increases, the second letter of the designation is changed from A to Z. The single or double digit number following the first two letters indicates the maximum carbon content of the grade (% x 100) for the corrosion resistant (C) grades. This number is the midpoint of the carbon content range in units of 0.01 % with a ±0.05% limit when used with heat resistant (H) grades. Finally, if further alloying elements are present, these are indicated by the addition of one or more letters as a suffix.

For example, the designation CF8M indicates that the grade is corrosion resistant (C), contains between 17% and 21 % chromium and between 8% and 12% nickel (F), a maximum carbon content of 0.08% (8), and molybdenum (M); HD indicates that the grade is heat resistant (H), and contains between 26% and 30% chromium and between 4% and 7% nickel (D).

In North America, the common designations for cast stainless steel and nickel-base alloys are descriptive of their chemistry and purpose. This designation system was established by the Alloy Casting Institute (ACI) and has been adopted by ASTM.

ASTM standards A297, A351, A743, A744 and A890 specify castings for corrosion and heat-resisting service. The grade designations used in these standards are those originally developed by the ACI. The tables given below give chemical composition of C and H grade of stainless steels. C grade of stainless steels includes martensitic, ferritic, precipitation hardening, austenitic and austenitic-ferritic (duplex) steel grades and some nickel based alloys. Where appropriate, the nearest or related wrought grade is shown in the tables.

The data given is not intended to replace that shown in individual standards to which reference should always be made.

Aqueous (wet) Corrosion Grades – C Grades

 

Designation		Chemical composition % by mass (max unless otherwise stated)
ACI No	UNS No	C	Si	Mn	P	S	Cr	Mo	Ni	N	Cu	Nb	Others	Wrought Grade
CA6N	-	0.06	1.0	0.50	0.02	0.02	10.5/12.5	-	6.0/8.0	-	-	-	-	-
CA6NM	J91540	0.06	1.0	1.0	0.04	0.04	11.5/14.0	0.4-1.0	3.5/4.5	-	-	-	-	-
CA15	J91150	0.15	1.5	1.0	0.04	0.04	11.5/14.0	0.5	1.0	-	-	-	-	410
CA15M	J91151	0.05	0.65	1.00	0.040	0.040	11.5/14.0	0.15/1.0	1.0	-	-	-	-	-
CA28MWV	J91422	0.20/0.28	1.0	0.50/1.00	0.030	0.030	11.0/12.5	0.90/1.25	0.50/1.00	-	-	-	W 0.90/1.25; V 0.20/0.30	-
CA40	J91153	0.20/0.40	1.50	1.00	0.04	0.04	11.5/14.0	0.5	1.0	-	-	-	-	420
CA40F	J91154	0.20/0.40	1.50	1.00	0.04	0.20/0.40	11.5/14.0	0.5	1.0	-	-	-	-	-
CB6	J91804	0.06	1.00	1.00	0.04	0.03	15.5/17.5	0.5	3.5/5.5	-	-	-	-	-
CB30	J91803	0.06	1.50	1.00	0.04	0.04	18.0/21.0	-	2.00	0.90/1.20 optional	-	-	-	-
CB7Cu-1	J92180	0.07	1.00	0.70	0.035	0.03	15.50/17.70	-	3.60/4.60	0.05	2.50/3.20	0.15/0.35	-	17/4PH
CB7Cu-2	J92110	0.07	1.00	0.70	0.035	0.03	14.0/15.50	-	4.50/5.50	0.05	2.50/3.20	0.15/0.35	-	15/5PH
CC50	J92615	0.50	1.50	1.00	0.04	0.04	26.0/30.0	-	4.00	-	-	-	-	-
CD3MCuN	J93373	0.030	1.10	1.20	0.030	0.030	24.0/26.7	2.9/3.8	5.6/6.7	0.22/0.33	1.40/1.90	-	-	1C
CD3MN	J92205	0.03	1.00	1.50	0.04	0.020	21.0/23.5	2.5/3.5	4.5/6.5	0.10/0.30	1.00	-	-	2205 (4A)
CD3MWCuN	J93380	0.03	1.00	1.00	0.030	0.025	24.0/26.0	3.0/4.0	6.5/8.5	0.20/0.30	0.5/1.0	-	W 0.5/1.0	Zeron 100 (6A)
CD4MCu	J93370	0.04	1.00	1.00	0.04	0.04	24.5/26.5	1.75/2.25	4.75/6.00	-	2.75/3.25	-	-	1A
CD4MCuN	J93372	0.04	1.0	1.0	0.04	0.04	24.5/26.5	1.7/2.3	4.7/6.0	0.10/0.25	2.7/3.3	-	-	1B
CD6MN	J93371	0.06	1.00	1.00	0.040	0.040	24.0/27.0	1.75/2.5	4.0/6.0	0.15/0.25	-	-	-	3A
CE20N	J92802	0.20	1.50	1.50	0.040	0.040	23.0/26.0	0.50	8.0/11.0	0.08/0.20	-	-	-	-
CE3MN	J93404	0.03	1.00	1.50	0.04	0.04	24.0/26.0	4.0/5.0	6.0/8.0	0.10/0.30	-	-	-	Alloy 958 (5A)
CE30	J93423	0.30	2.00	1.50	0.04	0.04	26.0/30.0	-	8.0/11.0	-	-	-	-	-
CE8MN	J93345	0.08	1.50	1.00	0.040	0.040	22.5/25.5	3.0/4.5	8.0/11.0	0.10/0.30	-	-	-	Escoloy (2A)
CF3	J92500	0.03	2.00	1.50	0.04	0.04	17.0/21.0	-	8.0/12.0	-	-	-	-	304L
CF8	J92600	0.08	2.00	1.50	0.04	0.04	18.0/21.0	-	8.0/11.0	-	-	-	-	304
CF8C	J92710	0.08	2.00	1.50	0.04	0.04	18.0/21.0	-	9.0/12.0	-	-	8x C/1.0	-	347
CF20	J92602	0.20	2.00	1.50	0.04	0.04	18.0/21.0	-	8.00/11.0	-	-	-	-	302
CF3M	J92800	0.03	1.50	1.50	0.04	0.04	17.0/21.0	2.0/3.0	9.0/13.0	-	-	-	-	316L
CF3MN	J92804	0.03	1.50	1.50	0.040	0.040	17.0/22.0	2.0/3.0	9.0/13.0	0.10/0.20	-	-	-	316LN
CF8M	J92900	0.08	2.00	1.50	0.04	0.04	18.0/21.0	2.0/3.0	9.0/12.0	-	-	-	-	316
CF10	J92950	0.04/0.10	2.00	1.50	0.040	0.040	18.0/21.0	0.50	8.0/11.0	-	-	-	-	-
CF10M	J92901	0.04/0.10	1.50	1.50	0.040	0.040	18.0/21.0	2.0/3.0	9.0/12.0	-	-	-	-	-
CF10MC	-	0.10	1.50	1.50	0.040	0.040	15.0/18.0	1.75/2.25	13.0/16.0	-	-	10xC/1.20	-	-
CF10SMnN	J92972	0.10	3.50/4.50	7.00/9.00	0.060	0.030	16.0/18.0	-	8.0/9.0	0.08/0.18	-	-	-	-
CF16F	J92701	0.16	2.00	1.50	0.17	0.04	18.0/21.0	1.50	9.0/12.0	-	-	-	Se 0.20/0.35	303Se
CF16FA	-	0.16	2.00	1.50	0.04	0.20/0.40	18.0/21.0	0.40/0.80	9.0/12.0	-	-	-	-	-
CG6MMN	J93790	0.06	1.00	4.00/6.00	0.040	0.030	20.50/23.50	1.50/3.00	11.5/13.5	0.20/0.40	-	0.10/0.30	V 0.10/0.30	-
CG3M	J92999	0.03	1.50	1.50	0.04	0.04	18.0/21.0	3.0/4.0	9.0/13.0	-	-	-	-	317L
CG8M	J93000	0.08	1.50	1.50	0.04	0.04	18.0/21.0	3.0/4.0	9.0/13.0	-	-	-	-	317
CG12	J93001	0.12	2.00	1.50	0.04	0.04	20.0/23.0	-	10.0/13.0	-	-	-	-	-
CH8	J93400	0.08	1.50	1.50	0.040	0.040	22.0/26.0	0.50	12.0/15.0	-	-	-	-	-
CH10	J93401	0.10	2.00	1.50	0.04	0.04	22.0/26.0	-	12.0/15.0	-	-	-	-	-
CH20	J93402	0.20	2.00	1.50	0.04	2.00	22.0/26.0	-	12.0/15.0	-	-	-	-	-
CK20	J94202	0.20	2.00	2.00	0.04	0.04	23.0/27.0	-	15.0/19.0	-	-	-	-	-
CK3MCuN	J93254	0.025	1.00	1.20	0.045	0.010	19.5/20.5	6.0/7.0	17.5/19.5	0.18/0.24	0.50/1.00	-	-	254SMO
CK35MN	-	0.035	1.00	2.00	0.035	0.020	22.0/24.0	6.0/6.8	20.0/22.0	0.21/0.32	0.40	-	-	-
CN3M	J94652	0.03	1.0	2.0	0.03	0.03	20.0/22.0	4.5/5.5	23.0/27.0	-	-	-	-	-
CN3MN	J94651	0.03	1.0	2.0	0.040	0.010	20.0/22.0	6.0/7.0	23.5/25.5	0.18/0.26	0.75	-	-	AL-6XN
CN7M	N08007	0.07	1.50	1.50	0.04	0.04	19.0/22.0	2.0/3.0	27.5/30.5	-	3.0/4.0	-	-	-
CN7MS	J94650	0.07	1.50	1.50	0.040	0.040	19.0/22.0	2.0/3.0	27.5/30.5	-	1.5/2.0	-	-	-
CT15C	N08151	0.05/0.15	0.50/1.50	0.15/1.50	0.03	0.03	19.0/21.0	-	31.0/34.0	-	-	0.50/1.50	-	-

High Temperature Grades – H Grades

 

Designation		Chemical composition % by mass (max unless otherwise stated)
ACI No	UNS No	C	Si	Mn	P	S	Cr	Mo	Ni	N	Cu	Nb	Others	Wrought Grade
HC	-	0.50	2.00	1.00	0.04	0.04	26.0/30.0	0.50	4.0/7.0	-	-	-	-	-
HD	-	0.50	2.00	1.50	0.04	0.04	26.0/30.0	0.50	4.00	-	-	-	-	-
HE	-	0.20/0.50	2.00	2.00	0.04	0.04	26.0/30.0	0.50	8.0/11.0	-	-	-	-	-
HF	-	0.20/0.40	2.00	2.00	0.04	0.04`	18.0/23.0	0.50	8.0/12.0	-	0.50	-	-	309
HH	-	0.20/0.50	2.00	2.00	0.04	0.04	24.0/28.0	0.50	11.0/14.0	-	-	-	-	-
HI	-	0.20/0.50	2.00	2.00	0.04	0.04	26.0/30.0	0.50	14.0/18.0	-	-	-	-	-
HK	-	0.20/0.60	2.00	2.00	0.04	0.04	24.0/28.0	0.50	18.0/22.0	-	-	-	-	310
HK30	J94203	0.25/0.35	1.75	1.50	0.040	0.040	23.0/27.0	0.50	19.0/22.0	-	-	-	-	-
HK40	J94204	0.35/0.45	1.75	1.50	0.040	0.040	23.0/27.0	0.50	19.0/22.0	-	-	-	-	-
HL	-	0.20/0.60	2.00	2.00	0.04	0.04	28.0/32.0	0.50	18.0/22.0	-	-	-	-	-
HN	-	0.20/0.50	2.00	2.00	0.04	0.04	19.0/23.0	0.50	23.0/27.0	-	-	-	-	-
HP	-	0.35/0.75	2.50	2.00	0.04	0.04	24/28	0.50	33/37	-	-	-	-	-
HT	-	0.35/0.75	2.50	2.00	0.4	0.4	15.0/19.0	0.50	33.0/37.0	-	-	-	-	-
HT30	N08030	0.25/0.35	2.50	2.00	0.040	0.040	13.0/17.0	0.50	33.0/37.0	-	-	-	-	-
HU	-	0.35/0.75	2.50	2.00	0.4	0.4	17.0/21.0	0.50	37.0/41.0	-	-	-	-	-
HW	-	0.35/0.75	2.50	2.00	0.4	0.4	10.0/14.0	0.50	58.0/62.0	-	-	-	-	-
HX	-	0.35/0.75	2.50	2.00	0.4	0.4	15.0/19.0	0.50	64.0/68.0	-	-	-	-	-

Information on Common Corrosion Grades – C Grades

Technical information and application of common corrosion grades are as under. Where appropriate, the nearest or related wrought grade is shown in the bracket.

CB7Cu1 (17/4PH) and CB7Cu2 (15/5PH)

They are resistant to moderate atmospheric corrosion and mild organic media corrosion. Their corrosion resistance is lower than that of more highly alloyed grades, limiting their use in process environments. Their strength and tempering resistance are improved by molybdenum. These grades are ferromagnetic, hardenable by heat treatment, and have poor low-temperature impact strength. They combine hardness with improved corrosion resistance over non-stainless steels and are used for cutlery, turbine blades, and high temperature parts. Section thicknesses of about 0.2 inch (5 mm) and above can be cast satisfactorily.

CA15 (410) and CA40 (420)

CA15 is an iron-chromium alloy containing the minimum amount of chromium necessary for classification as a stainless steel. It is resistant to atmospheric corrosion and staining by many organic media in relatively mild service and provides fairly good machining and welding properties. CA40 is a higher carbon version of CA15. The higher carbon content permits the grade to be hardened to a maximum of 500 BHN and increases its strength.

CF3 (304L)

It is a lower carbon content version of CF8. Their applications are similar, but CF3 is preferred when there will be no post-weld heat treatment. Solution annealing is necessary for maximum corrosion resistance and to prevent intergranular attack. CF3 is used for applications below 650°F (345°C). It has been used in corrosive solutions including brackish water, phosphate solutions, and steam. It has been used in the food, nuclear power, petroleum, and soap and detergent manufacturing industries. Components include autoclaves, headers, spray nozzles, impellers, propellers, pump casings, retaining rings, suction manifolds, valve parts and valve bodies.

CF8 (304)

CF8 has good machining and welding characteristics. The as-cast structure is normally about 10% ferrite, which helps to reduce the potential for intergranular corrosion in castings exposed to temperatures in the sensitizing range. The ferrite promotes carbide precipitation in discontinuous pools rather than at the grain boundaries. At higher ferrite levels, strength and resistance to stress corrosion cracking are substantially improved. This grade has excellent low temperature properties and retains high impact strength levels at temperatures as low as -400°F (-240°C). When exposed to temperatures between 900 and 1200°F (480 to 650°C), it will become sensitized and suffer diminished corrosion resistance. CF8 cannot be hardened by heat treatment. It has good strength and ductility. It also has good cavitation resistance, which is important for hydro turbines, pump impellers, and related equipment.

It is primarily used for water handling but also provides resistance to strongly oxidizing environments such as boiling nitric acid and corrosive media applications. Products made from CF8 include valves and fittings, flanges, mixing agitators, rotary strainers, shaft sleeves, and spray nozzles.

CF8C (347)

It is CF8 modified with an addition of niobium. The niobium prevents grain boundary precipitation of chromium carbides and subsequent intergranular corrosion. It provides corrosion resistance equivalent to CF8 and is used when field welding is required or in applications requiring long exposures to elevated temperatures. Although it can be used in the as-cast condition, it is normally heat treated. After heat treatment, the microstructure contains 5-20% ferrite uniformly distributed throughout the matrix in discontinuous pools. CF8C is used in the aircraft, nuclear, chemical processing, marine, oil refining, for handling hydrogen sulfide gas, petroleum products at high temperatures and pressures, and high-octane gasoline combustion products. Applications include aircraft shroud assemblies, autoclaves, engine exhaust fittings, jet engine parts, marine fittings, pump parts, rotors and valve bodies.

CF3M (316L)

Ferrite accounts for about 20% of the microstructure. It is a modification of CF3 with 2.0-3.0% molybdenum added to improve pitting and crevice corrosion resistance in chloride containing environments. It is in the same family as CF8M but with a lower carbon content. CF3M has good resistance to corrosive sulfurous media and acetic acids. For maximum corrosion resistance, CF3M should be heat treated. Post-weld heat treatment is not required because the alloy’s low carbon content limits formation of significant amounts of chromium carbide. CF3M castings have good machining and welding characteristics. CF3M is used for mixer parts, pump casings and impellers, tubes, and valve bodies and parts by the chemical, copper mining, food processing, paper mill, petroleum, pipeline, power plants, and water supply industries.

CF8M (316)

It is readily weldable and is not hardenable by heat treatment. Its microstructure is usually 5-20% delta. Temperatures of 800-1600°F (430- 870°C) cause formation of chromium carbides (sensitization) and a loss of corrosion resistance. The molybdenum improves resistance to corrosion in moderately or rapidly flowing seawater, however, CF8M should not be used for slow moving or stagnant seawater. It has been used by the aircraft, chemical, food processing, marine, mining, oil refining, pharmaceutical, power, and textile industries for applications like agitators, centrifuges, evaporator parts, mixing propellers, pump parts, spray nozzles, high pressure steam valve bodies and parts.

CG8M (317)

It has excellent resistance to corrosion in reducing environments. Its composition is similar to that of CF8M, but the molybdenum content (3 – 4%) is higher. The additional molybdenum increases resistance to hot sulfurous and other organic acids and dilute sulfuric acid solutions, halide bearing media, and reducing acids. Solution annealing provides maximum corrosion resistance. After heat treatment, the microstructure contains 15-35% ferrite. Extended exposure at temperatures between 1200 – 1700°F (650 – 925°C) may cause embrittlement and reduce corrosion resistance as ferrite is transformed into sigma phase. CG8M is not used for nitric acid service or other strongly oxidizing environments. It is especially useful for dyeing equipment, flow meter components, propellers, pump parts, valve bodies and parts, and sulfite liquor in the nuclear, petroleum, power, paper, printing, and textile industries.

Classification of Stainless Steels as per AISI

For stainless steels, the system established by the AISI is not based on composition, but on microstructure. Thus, the stainless steels are classified as austenitic, ferritic, martensitic, duplex, and precipitation-hardening types.

AISI has established a three numeral numbering system to classify stainless steels. The last two numerals have no particular significance, but the first numeral indicates the group as follows.

 

Series Designation	Groups
2xx	Chromium-nickel-manganese; nonhardenable, austenitic, nonmagnetic
3xx	Chromium-nickel; nonhardenable, austenitic, nonmagnetic
4xx	Chromium; hardenable, martensitic, magnetic
	Chromium; nonhardenable, ferritic, magnetic

Nickel (plus carbon, manganese, and nitrogen) promotes the formation of austenite, and chromium (plus silicon, molybdenum, and niobium) encourages the formation of ferrite so the structure of stainless steels can be largely predicted on the basis of their chemical composition.

The system is not as clearly organized as the AISI/SAE system for plain carbon steels, because the number designations overlap. For example, within the 4xx series, 405 and 409 designate ferritic stainless steels, while 403 and 410 designate martensitic stainless steels; within the 3xx series, 321 and 330 designate austenitic stainless steels, and 329 designates a duplex stainless steel. Therefore, one must be aware that the system for stainless steels is somewhat inconsistent.

Types of Stainless Steels as per AISI

As mentioned earlier, stainless steels are divided into 5 types as under.

Austenitic

These stainless steels have a microstructure of austenite (FCC – face centered cubic crystal structure) at room temperature. Austenitic stainless steel (such as the popular type 304) has been called 18/8 stainless steel, because it contains nominally 18% Cr and 8% Ni. There are 30 compositional variations in the standard austenitic stainless steels, and a summary of the family relationships is shown in the figure given below.

All the austenitic stainless steels are essentially chromium-nickel alloys. The chromium varies between 15 and 24% and the nickel between 3 and 22%. The total content of chromium and nickel in these steels is at least 23 percent. The family is derived from two basic, general-purpose alloys, types 302 and 202. The type 302 expands into 26 other types (chromium-nickel stainless steels, series 3xx) with specific compositional variations to impart particular properties, for example, better weldability, increased strength, increased heat resistance, better corrosion resistance, and improved machinability (For example, lowering the carbon content to 0.08 percent maximum led to type 304 with improved weldability and decreased tendency towards carbide precipitation). The type 202 is limited to only three types (chromium-nickel-manganese stainless steels, series 2xx) and was designed to replace nickel, a rather expensive alloying element, with nitrogen and manganese.

These types are essentially nonmagnetic in the annealed condition and do not harden by heat treatment. They can be hot-worked readily and can be hardened by cold-working whilst retaining a useful level of ductility and toughness. Cold-working develops a wide range of mechanical properties and the steel in this condition may become slightly magnetic. The austenitic type of steels offers the most resistance to corrosion in the stainless types, owing to its substantial nickel content and higher levels of chromium.

Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. Superior performance in very low-temperature services is additional feature of these type of steels. The two most common types are 304 (the most widely specified stainless steel, providing corrosion resistance in numerous standard services) and 316 (similar to 304, with molybdenum added, to increase opposition to various forms of deterioration). Austenitic steels are used for cooking utensils, food processing equipment, exterior architecture, equipment for the chemical industry, truck trailers, and kitchen sinks.

Through the range of temperatures 800-1500ºF, chromium carbides form along the austenite grains. This causes depletion of chromium from the grains resulting in decreasing the corrosion protective passive film. This effect is called sensitization. It is particularly important in welding of austenitic stainless steels.

The 0.08% max C allowed in types 304 and 316 leaves stainless steel vulnerable to intergranular corrosion when welded. The heat of welding is sufficient for chromium to combine with carbon and precipitate at grain boundaries in the zone alongside the weld, referred to as the heat affected zone (HAZ). The chromium that precipitates as chromium carbide leaves a zone adjacent to the weld depleted in chromium and susceptible to intergranular corrosion, or intergranular attack (IGA).

To avoid carbide precipitation during welding, low-carbon types 304L and 316L were developed which contains only 0.03 percent carbon (maximum). Formation of chromium carbides is also avoided in stabilized austenitic stainless steels containing carbide forming elements like titanium, niobium, tantalum and zirconium. In type 321, Ti (titanium) is added whereas in type 347, Cb (columbium, also known as niobium) or Ta (tantalum) is added. Stabilization heat treatment of such steels results in preferred formation of carbides of the stabilizing elements instead of chromium carbides. A stabilizing heat treatment consists of holding either annealed or welded material at 1600 to 1650°F for 2 to 4 hours, followed by rapid cooling in air or water. The purpose is to precipitate all carbon as a carbide of titanium or columbium in order to prevent subsequent precipitation of chromium carbide.

In the UNS numbering system, which is replacing the older American Iron and Steel Institute (AISI) designations, the “03” in S30403 and S31603 designates the 0.03% max C or low carbon “L” grade. In the UNS designation system “00” in S30400 and S31600 indicates the 0.08% max C high carbon grade not suitable for welded fabrication. It is important when purchasing stainless steels that the low carbon grade be clearly specified; otherwise there is a risk that the higher carbon grade will be received.

Although all stainless steels can be hardened somewhat by cold work, the response becomes pronounced in austenitic alloys reaching a maximum in types 201 and 301. The table given below shows work hardening behavior of the type 301 (17 percent chromium and 7 percent nickel) stainless steel in tension.

 

Cold Reduction %	Condition of Metal	Yield Strength psi	Tensile Strength psi	Elongation in 2 in. %	Rockwell Hardness No.
0	Annealed	33000	117800	68	B 85
10	Cold-rolled	67000	147600	47	C 32
25	Cold-rolled	127000	165200	24	C 38
35	Cold-rolled	164000	196000	15	C 43
45	Cold-rolled	200000	225000	7	C 46

Figure given below shows the effect of cold work on the 0.2% proof stress, the ultimate tensile strength and elongation at failure for a specific cast of 304.

Similar relationships apply to other austenitic materials. To maintain a useful ductility of 15%, it is recommended that the amount of cold work should be restricted to 30% for the austenitic grades.

Development work covering the substitution of manganese for nickel in stainless steels during the shortage of nickel (World War II and Korean emergency) led to the production of types 201 and 202, the chromium-nickel-manganese stainless steels. Type 201 with a nominal composition of 17 percent chromium, 4.5 percent nickel and 6.5 percent manganese is a satisfactory substitute for type 301 (17 percent Cr, 7 percent Ni) where machinability and severe forming properties are not essential. Where those properties are essential, type 202 with a nominal composition of 18 percent Cr, 5 percent Ni and 8 percent Mn is more desirable because the higher manganese reduces the rate of work hardening. Although types 201 and 202 have somewhat less resistance to chemical corrosion than 301 and 302, their resistance to atmospheric corrosion is entirely comparable.

Ferritic

These types of stainless steels (series 4xx) are straight-chromium stainless steels containing approximately 14 to 27 percent chromium with small amounts of carbon (usually less than 0.10%) The crystallographic structure of the steels is ferritic (BCC – body centered cubic crystal structure) at room temperature. These alloys deliberately lack high nickel contents, because nickel renders the steels austenitic.

The number of types of ferritic stainless steels is much smaller than the austenitic types. All the types are variations on the basic, general-purpose type 430. Figure given below shows the family relationships for the ferritic stainless steel types.

Two of the most common types are 430 (general-purpose grade for many applications, including decorative ones) and 409 (low-cost grade well suited to withstanding high temperatures). The lack of nickel results in lower corrosion resistance than the austenitic stainless steels (chromium-nickel stainless steels). Low in carbon content, but generally higher in chromium than the martensitic grades, they cannot be hardened by heat treatment (because of low carbon content) and are only moderately hardened by cold working. They are magnetic and can be cold-worked or hot-worked, but they develop their maximum softness, ductility and corrosion resistance in the annealed condition. In the annealed condition, the strength of these steels is approximately 50 percent higher than that of carbon steels and they are superior to martensitic steels in corrosion resistance and machinability. Annealing is carried out primarily to relieve welding or cold-working stresses.

Ferritic steels are best suited for general and high-temperature corrosion applications rather than services requiring high strength. Ferritic steels are chosen for their resistance to stress corrosion cracking. High chromium steels with additions of molybdenum can be used in quite aggressive conditions such as sea water.

These types of steels are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. Since the ferritic steels may be cold formed easily (they are not as formable as austenitic stainless steels), they are used extensively for deep-drawn parts such as vessels for chemical and food industries, hot water tanks and automotive trims and exhausting systems. The ferritic stainless steels are the lower-cost stainless steels, because they contain less alloys, and do not contain nickel (nickel is more expensive than chromium).

Martensitic

The family relationship of the martensitic stainless steels is shown in the figure given below. All the types are variations on the general-purpose type 410.

These steels (series 4xx) like ferritic steels are primarily straight chromium steels containing between 11.5 and 18 percent chromium but have higher carbon levels as compared to ferritic steels (as high as 1%). This allows them to be hardened and tempered much like carbon and low-alloy steels. The steels have austenitic structure (FCC) at high temperature, which transforms to martensite structure (BCC) as a result of quenching. Types 410 and 416 are the most popular and are used for turbine blades and corrosion-resistant castings. They are used where high strength and moderate corrosion resistance is required. They attain the best corrosion resistance when hardened from the recommended temperature but are not as good as the austenitic or ferritic stainless steels.

This type of stainless steel is magnetic, can be cold-worked without difficulty, especially with low carbon content, can be machined satisfactorily, have good toughness and is easily hot-worked. However, they have generally low weldability and formability. This type of steel is used for turbine blades, knife blades, surgical instruments, shafts, pins, springs, etc.

Stainless steels as a group are much more difficult to machine than plain carbon steels. The use of a small amount of sulfur in type 416 and selenium in type 416Se improves the machinability considerably. These alloys are used for cutlery, valve parts and bearings.

The addition of about 2 percent nickel to the 16 to 18 percent chromium, low-carbon alloy type 431 steel is used for aircraft fittings, paper machinery parts, pumps and bolts.

The nominal chemical compositions (important elements) of some common representative stainless steel types (discussed above) are as per the tables given below.

 

Type	C %	Mn %	Si %	Cr %	Ni %	Mo %	Others %
202	0.15 max	7.5-10.0	1.0 max	17.0-19.0	4.0-6.0	-	N2 0.25 max
302	0.15 max	2.0 max	1.0 max	17.0-19.0	8.0-10.0	-	-
304	0.08	2.0 max	1.0 max	18.0-20.0	8.0-11.0	-	-
316	0.08	2.0 max	1.0 max	16.0-18.0	10.0-14.0	2.0-3.0	-
410	0.15 max	1.0 max	1.0 max	11.5-13.5	0.50 max	-	-
430	0.12 max	1.0 max	1.0 max	14.0-18.0	0.50 max	-	-

Duplex

Duplex stainless steels contain high amount of chromium (18% -28%) and moderate (as compared to austenitic steels) amount of nickel (4.5% – 8%) as major alloying elements. Molybdenum is used in some of duplex steels as additional alloying element. Since the quantity of nickel is insufficient for formation of fully austenitic structure, the structure of duplex steels is mixed. The duplex class is so named because it is a mixture of austenitic (chromium-nickel stainless steel) and ferritic (plain chromium stainless steel) structures. These steels have a microstructure which is approximately 50% ferritic and 50% austenitic. This gives them a higher strength than either ferritic or austenitic steels, but poorer toughness than austenitic stainless steels.

As per AISI there is only one standard type of duplex stainless steel, type 329 (UNS S32900), which contains 23 to 28% Cr, 2.5 to 5.0% Ni, and 1.0 to 2.0% Mo.

Duplex stainless steels (UNS S32205) provide high resistance to stress corrosion cracking (formation of cracks caused by a combination of corrosion and stress) and to chloride ions attack. The so called “lean duplex” steels (UNS S32304) are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. “Super Duplex” steels (UNS S32705) have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels.

They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not as much as the ferritic, martensitic and PH types due to the 50% austenitic structure.

They are suitable for heat exchangers, desalination plants, petrochemical plants and marine applications.

The nominal chemical compositions (important elements) of some representative duplex stainless steels (as per ASTM A240: Heat-Resisting Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels) are as per the tables given below.

 

UNS No.	C % max	Mn % max	Si % max	Cr %	Ni %	Mo %	N %
S32905	0.08	1.0	0.75	23.0-28.0	2.5-5.0	1.0-2.0	-
S32205	0.03	2.0	1.0	22.0-23.0	4.5-6.5	3.0-3.5	0.14-0.20
S32304	0.03	2.5	1.0	21.5-24.5	3.0-5.5	0.05-0.6	0.05-0.20
S32750	0.03	1.2	0.80	24.0-26.0	6.0-8.0	3.0-5.0	0.24-0.32

Precipitation Hardening (PH)

As a result of research during World War II a new group of stainless steels with precipitation-hardening characteristics was developed. The first of these nonstandard grades of stainless steels, 17-7PH was made available in 1948.

Precipitation hardening stainless steels contain chromium and nickel as major alloying elements. These steels can develop very high strength by adding elements such as copper, niobium and aluminium to the steel. These elements tend to form coherent alloy precipitates. The nominal chemical compositions (important elements) of some representative precipitation-hardening stainless steels are as per the table given below.

 

Type	C %	Mn %	Si %	Cr %	Ni %	Mb %	Others %
17-4 PH	0.04	0.40	0.50	16.50	4.25	-	0.25 Cb, 3.60 Cu
17-7 PH	0.07	0.70	0.40	17.00	7.00	-	1.15 Al
PH 15-7 Mo	0.07	0.70	0.40	15.00	7.00	2.25	1.15 Al
17-10 P	0.12	0.75	0.50	17.00	10.50	-	0.28 P

These steels are usually solution-annealed at the mill and supplied in that condition. These steels may be either austenitic or martensitic and they are hardened by heat treatment (aging) after forming. With a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts hardness and strength. The 17-4 PH alloy should not be put into service in any application in the solution-treated condition because its ductility can be relatively low and its resistance to stress-corrosion cracking is poor. Aside from the increase in strength and ductility, aging also improves both toughness and resistance to stress-corrosion.

These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. This is in contrast to conventional hardening and tempering in martensitic steels where distortion is more of a problem. They have good weldability and their corrosion resistance is comparable to standard austenitic steels grade like 304. They are magnetic.

They are used for pump shafts, valves, turbine blades, paper industry equipment, aerospace equipment, etc.

Figure given below provides a graphical overview of different types of stainless steel groups with respect to chromium and nickel content.

Maraging Steels

Like stainless steels, maraging steels are also alloy steels and hence though they are not stainless steel brief information about them is given here for the sake of completeness.

They are iron-base alloys capable of attaining yield strengths up to 300000 psi in combination with excellent fracture toughness. These steels are low-carbon containing 18 to 25 percent nickel together with other hardening elements and are called maraging (martensitic plus aging). They are considered to be martensitic as annealed and attain ultrahigh strength on being aged in the annealed or martensitic condition. The martensite formed is soft and tough rather than the hard and brittle martensite of conventional low-alloy steels. This ductile martensite has low work-hardening rate and can be cold-worked to a high degree.

Table given below shows compositions of nickel maraging steels.

For all steels given in the table, maximum % of C = 0.03, Mn = 0.10, Si = 0.10, S = 0.010 and P = 0.010.

 

Designation	Ni	Co	Mo	Ti	Al	Cb
25 Ni	25.0-26.0	-	-	1.3-1.6	0.15-0.30	0.30-0.50
20 Ni	19.0-20.0	-	-	1.3-1.6	0.15-0.30	0.30-0.50
18 Ni (300)	18.0-19.0	8.5-9.5	4.6-5.2	0.5-0.8	0.05-0.15	-
18 Ni (250)	17.0-19.0	7.0-8.5	4.6-5.2	0.3-0.5	0.05-0.15	-
18 Ni (200)	17.0-19.0	8.0-9.0	3.0-3.5	0.15-0.25	0.05-0.15	-

Mechanical properties of maraging steels in annealed condition and after maraging are as per the table given below.

 

Annealed Condition
Type of steel	18 % Ni			20 % Ni	25 % Ni
Yield Strength (0.2 % off-set), 1000 psi	110			115	40
Tensile Strength, 1000 psi	140			152	132
Elongation (in 1 in.), %	17			8	30
Reduction in area, %	75			-	72
Hardness, R/C	28-32			26-35	10-15
After Maraging
Type of steel	18 Ni (300)@	18 Ni (300)#	18 Ni (300)$	20 % Ni	25 % Ni
Yield Strength, 1000 psi	295-303	240-268	190-210	246	249-256
Tensile Strength, 1000 psi	297-306	250-275	200-220	256	265-270
Elongation, %	12	10-12	14-16	11	12
Reduction in area, %	60	45-58	65-70	45	53
Hardness, R/C	52	52	52	52	52

@ Maraged 900°F, 3 h
# Maraged 900°F, 1 h
$ Conditioned 1300°F, 4 h, refrigerated, maraged 800-850°F, 1 h

Applications for maraging steels are hulls for hydrospace vehicles, motor cases for missiles, low-temperature structural parts, rifle tubing and pressure vessels.

Classification of Stainless Steels as per Other Systems

Classification as per German System is also widely used. In many countries, Abbreviated System of Designation is adopted. Information about both the systems is as under.

German System

As per this system (W.N. 17007), each designation consists of 5 numbers and for details of composition and properties of the steels one has to refer to the standard. As an example, material number 1.4306 will be considered.

The first digit is 1 and indicates that it is a steel.
The two following digits “43″ signify “chemically resisting steels without molybdenum, columbium or titanium”.
The last two digits “06″ define the exact alloy.

A steel to material number 1.4306 has the following composition.
C = 0.03% maximum
Cr = 18 – 20%
Ni = 10 – 12.5%
The steel corresponds therefore to AISI type 304L stainless steel although the lower limit of nickel is higher by 2%.

In addition to the designation “43″ there are also the following ones:
“40″ without molybdenum, columbium or titanium, nickel less than 2,5%
“41″ with molybdenum, without columbium or titanium, nickel less than 2,5%
“44″ with molybdenum, without columbium or titanium, nickel more than 2,5%
“45″ with copper, columbium or titanium, nickel more than 2,5%

Abbreviated System of Designation

This system is widely used by a number of countries. It consists of a series of letters and numbers as in the following examples.

X 2 Cr Ni 18 11
X means that it is a highly alloyed steel
2 indicates the carbon content in 1/100th of a percent, e.g. C = 0.02%
Cr stands for chromium and 18 is the content in %
Ni stands for nickel and 11 provides an indication of the content in %.
Thus a steel to X 2 Cr Ni 18 11 corresponds to AISI type 304L.

India is using Abbreviated System of Designation

Comparison between USA, Germany, Japan and Indian standards for common grades of stainless steel is given in the table given below.

 

UNS	AISI (USA)	Germany W.N. 17007 / DIN 17006		India	Japan
S30100	301	1.431	X12CrNi177	10Cr17Ni7	SUS301
S30400	304	1.4301	X5CrNi1810	04Cr18Ni10	SUS304
S30403	304L	1.4306	X2CrNi1911	02Cr18Ni11	SUS304L
S31600	316	1.4401	X5CrNiMo17122	04Cr17Ni12Mo2	SUS316
S31603	316L	1.4404	X2CrNiMo17132	02Cr17Ni12Mo2	SUS316L
S32100	321	1.4541	X6CrNiTi1810	04Cr18Ni10Ti20	SUS321
S34700	347	1.455	X6CrNiNb1810	-	SUS347
S43000	430	1.4016	X6Cr17	05Cr17	SUS430
S41000	410	1.4006	X10Cr13	-	SUS410

Roll of Alloying Elements in Stainless steel

Chromium, nickel, and molybdenum are the primary alloying elements that determine the structure, mechanical properties, and corrosion resistance of stainless steel. Intentional additions of small amount of carbon, nitrogen, niobium, tantalum, titanium, sulfur, and slightly larger additions of copper, manganese, silicon, and aluminum are used to modify properties.

Chromium

A stainless steel contains a minimum of 10.5% chromium because this level of chromium causes the spontaneous formation of a stable, passive, protective film. Increasing the level of chromium enhances corrosion resistance. At elevated temperatures, chromium provides resistance to oxidation, corrosion and creep.

Nickel

Nickel promotes the stability of austenite. Less nickel is needed to retain an austenitic structure as the nitrogen or carbon levels increase. When sufficient nickel is added to a chromium stainless steel, the structure changes from ferritic to austenitic. Nickel increases resistance to oxidation, strong acids (particularly reducing acids) and thermal fatigue. Adding nickel improves toughness, ductility, and weldability.

Molybdenum

Molybdenum additions improve resistance to pitting and crevice corrosion in chloride containing environments and corrosion by sulfuric, phosphoric, and hydrochloric acids. The elevated temperature mechanical properties of austenitic stainless steels and the strength and tempering resistance of martensitic stainless steels are improved by molybdenum.

Minor Elements

Increasing the carbon content in high temperature alloys improves high temperature strength and creep resistance, but reduces ductility. Conversely, carbon can be detrimental to corrosion resistance when it combines with chromium to form chromium carbides along grain boundaries. This reduces the chromium adjacent to the grain boundary (sensitization) and can lead to corrosion of chromium-depleted areas (intergranular corrosion).

Titanium, columbium, and tantalum additions preferentially combine with carbon and nitrogen to prevent sensitization and eliminate susceptibility to intergranular corrosion.

Nitrogen additions to austenitic and duplex stainless steels improve pitting resistance.

Additions of sulfur, selenium, and lead in stainless steel improve machinability.

Columbium additions can improve high temperature creep strength.

Copper additions improve resistance to sulfuric acid.

A combination of manganese and nitrogen may be used as a partial substitute for nickel in some stainless steels.

Aluminum improves resistance to oxidation.

Silicon is added to cast stainless steel grades to increase casting fluidity and improve castability. Silicon is generally limited to 1.5% in castings intended for service above 1500°F (815°C) because it lowers the high temperature creep property.

Main Advantages of the Stainless Steel Types

When considering stainless the most important features are corrosion (or oxidation) resistance; mechanical & physical properties; available forming, fabrication & joining techniques and material costs (total life cycle cost).

The basic approach is to select a grade with as low a cost as possible, but with the required corrosion resistance. Main advantages and disadvantages of the various stainless steel types are summarized in the table given below.

 

Type	Examples	Advantages	Disadvantages
Ferritic	430, 446	Low cost, moderate corrosion resistance & good formability.	Limited corrosion resistance, formabilty & elevated temperature strength compared to austenitics.
Austenitic	304,316	Widely available. Good general corrosion resistance and good cryogenic toughness. Excellent formability & weldability.	Work hardening can limit formability & machinability. Limited resistance to stress corrosion cracking.
Duplex	S32205	Good stress corrosion cracking resistance. Good mechanical strength in annealed condition.	Application temperature range more restricted than austenitics.
Martensitics	420, 431	Hardenable by heat treatment.	Corrosion resistance compared to austenitics & formability compared to ferritics limited. Weldability limited.
Precipitation hardening	17-4 PH	Hardenable by heat treatment, but with better corrosion resistance than martensitics.	Limited availability. Corrosion resistance, formability & weldability restricted compared to austenitics.

“L” and “H” Grades

The standard/straight grades of austenitic stainless steel contain a maximum of 0.08% carbon. There is a misconception that straight grades contain a minimum of 0.03% carbon, but the specification does not require this. As long as the material meets the physical requirements of standard grade, there is no minimum carbon requirement.

Within the usual designations of the common austenitic grades of stainless steel, such as 304 and 316, there are “sub-grades” – “L” and “H” variants – with particular applications.

The “L” grades are used to provide extra corrosion resistance after welding. The letter “L” after a stainless steel type indicates low carbon (as in 304L). The carbon is kept to 0.03% or under to avoid carbide precipitation.

“L” grades can be used as standard grades as long as the mechanical properties (tensile and yield) conform to the standard grade requirements and high temperature strength is not a requirement. “L” grades virtually always do fully comply with standard grade requirements, but this would need to be checked on a case by case basis. Mill test certificates give this information. Standard grades can be used as “L” grades as long as their carbon content meets the “L” grade limits of 0.030 or 0.035% maximum.

It has become quite common for steel mills to supply “L” heats when standard grades have been ordered. Sometimes the product and test certificates are dual marked “304/304L”.

“H” grades are the higher carbon versions of each of the standard grades. The “H” grades contain a minimum of .04% carbon and a maximum of .10% carbon and are designated by the letter “H” after the alloy. The high carbon results in increased strength of the steel, particularly at elevated temperatures (generally above about 500°C). Creep strength is higher for these high carbon grades as compared to standard grades. In addition all austenitic “H” grades must have a grain size of ASTM No 7 or coarser. “H” grades are produced primarily in plate and pipe, but may be available in some other products. Applicable grades are most commonly 304H and 316H, but high carbon versions of 309, 310, 321, 347 and 348 are also specified in ASTM A240/A240M. As discussed earlier, these grades are susceptible to sensitization if held in the temperature range of about 450-850°C. If it occurs, it will result in impaired aqueous corrosion and some reduction in ambient temperature ductility and toughness. In general however, this is not an issue for a steel that is primarily intended for high temperature strength.

If an application requires an “H” grade – generally for high temperature applications – this must be specified at time of order.

Standard grades can often be used in place of “H” grades so long as their carbon contents meet the “H” limits, generally 0.04-0.10%. The grain size requirement may have to be satisfied by extra testing. “H” grades can be used as standard grades so long as their carbon contents are 0.08% maximum, and nitrogen 0.10% maximum. This is highly likely, but would need to be checked.

Annealed Stainless Steel and Solution Annealing

Stainless steels are normally produced and used in the “annealed” condition. The term anneal, when used for stainless steels, means heat treated at temperatures of 1900°F (1040°C) or higher and water quenched, not slow cooled as the term “annealed” means for carbon and low alloy steels.

The phrase “solution annealing” means only that the carbides which may have precipitated (or moved) to the grain boundaries are put back into solution (dispersed) into the matrix of the metal by the annealing process. “L” grades are used where annealing after welding is impractical, such as in the field where pipe and fittings are being welded.

Important ASTM Standards for Stainless Steels

Important ASTM standards for stainless steels are as under.

ASTM A240: Chromium and chromium-nickel stainless steel plate, sheet and strip for pressure vessels
ASTM A276: Standard Specification for Stainless Steel Bars and Shapes
ASTM A312: Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipes

IS Standards for Stainless Steels

Indian standards for various products are as under.

IS 1570 – Schedules for Wrought Steels – Part 5: Stainless and Heat-resisting Steels
IS 4454 – Steel Wires for Mechanical Springs – Specification – Part 4 : Stainless Steel Wire
IS 5522 – Stainless steel sheets and strips for utensils
IS 6603 – Stainless Steel Bars and Flats – Specification
IS 6911 – Stainless steel plate, sheet and strip
IS 6913 – Stainless steel tubes for the food and beverage industry
IS 6529 – Stainless steel blooms, billets and slabs for forging

Selection of Stainless Steel

Most decisions about which stainless steel to use are based on a combination of the following factors:

What is the corrosive environment? – Atmospheric, water, concentration of particular chemicals, chloride content, presence of acid, etc. General corrosion resistance is comparatively easy to determine, but real environments are usually more complex. An evaluation of other pertinent variables such as fluid velocity, stagnation, turbulence, galvanic couples, welds, crevices, variation in temperature, and variation from planned operating chemistry are others issues that need to be factored in to selecting the proper stainless steel for a specific environment.

What is the temperature of operation? – High temperatures usually accelerate corrosion rates and therefore indicate a higher grade. Low temperatures will require a tough austenitic steel.

What strength is required? – Higher strength can be obtained from the austenitic, duplex, martensitic and PH steels. Other processes such as welding and forming often influence which of these is most suitable. For example, high strength austenitic steels produced by work hardening would not be suitable where welding was necessary as the process would soften the steel.

Welding requirement – Austenitic steels are generally more weldable than the other types. Ferritic steels are weldable in thin sections. Duplex steels require more care than austenitic steels but are now regarded as fully weldable. Martensitic and PH grades are less weldable.

Degree of forming required to make the component – Austenitic steels are the most formable of all the types being able to undergo a high degree of deep drawing or stretch forming. Generally, ferritic steels are not as formable but can still be capable of producing quite intricate shapes. Duplex, martensitic and PH grades are not particularly formable.

What product form is required? – Not all grades are available in all product forms and sizes, for example sheet, bar, tube. In general, the austenitic steels are available in all product forms over a wide range of dimensions. Ferritics are more likely to be in sheet form than bar. For martensitic steels, the reverse is true.

There may also be special requirements such as non-magnetic properties to take into account.

It must also be borne in mind that steel type alone is not the only factor in material selection. Surface finish is also important in many applications, particularly where there is a strong aesthetic component.

Availability – There may be a perfectly correct technical choice of material which cannot be implemented because it is not available in the time required.

Cost – Sometimes the correct technical option is not finally chosen on cost grounds alone. However, it is important to assess cost on the correct basis. Many stainless steel applications are shown to be advantageous on a life cycle cost basis rather than initial cost.

Acknowledgement:

Most of the information on selection of stainless steel is reproduced from website of BRITISH STAINLESS STEEL ASSOCIATION – http://www.bssa.org.uk. The site is an excellent source for information on stainless steels. For more information about stainless steels, please refer this site.

An alloy steel may be defined as one whose characteristic properties are due to some element other than carbon.

Although all plain-carbon steels contain moderate amounts of manganese (up to about 0.90 percent) and silicon (up to about 0.30 percent), they are not considered alloy steels because the principal function of the manganese and silicon is to act as deoxidizers. They combine with oxygen and sulfur to reduce the harmful effects of those elements.

An iron-based mixture is considered to be an alloy steel when manganese is greater than 1.65%, silicon over 0.5%, copper above 0.6%, or other minimum quantities of alloying elements such as chromium, nickel, molybdenum, or tungsten are present.

Purpose of Alloying and General Effect of Alloying Elements

Alloying elements are added to steels for many purposes. The most important are as under.

•  Increase hardenability
•  Improve strength at ordinary temperatures
•  Improve mechanical properties at either high or low temperatures
•  Improve toughness at any minimum hardness or strength
•  Increase wear resistance
•  Increase corrosion resistance
•  Improve magnetic properties

Alloying elements may be divided in to two groups according to the way they may be distributed in the two main constituents of annealed steel.

Group 1: Elements which dissolve in ferrite.
Group 2: Elements which combine with carbon to form simple or complex carbides.

Effect of Alloying Elements upon Ferrite

Nickel, aluminium, silicon, copper and cobalt are group 1 elements and are found largely dissolved in ferrite.

In the absence of carbon, considerable proportions of the group 2 elements will be found dissolved in ferrite. Manganese, chromium, tungsten, molybdenum, vanadium and titanium are group 2 elements. The carbide forming tendency is apparent only when there is a presence of significant amount of carbon.

Any element dissolved in ferrite increases its hardness and strength in accordance with the general principles of solid solution hardening. The figure given below shows probable hardening effect of the various elements dissolved in alpha iron.

The hardening effect of the dissolved elements is actually small as compared to overall strength of steel. This is shown in the figure given below for low-carbon chromium alloy.

In the above figure, the upper curves indicates the influence of chromium to change the tensile strength by changing the structure (by air cooling), while the lower curves indicates the minor influence of chromium in essentially constant structure (furnace cooled).

Effects of Alloying Elements upon Carbide

Since all carbides found in steel are hard and brittle, their effect on the room-temperature tensile properties is similar regardless of the specific composition. However, chromium and vanadium carbides are outstanding in hardness and wear resistance. The hardness and wear resistance of alloy steels rich in carbides are in a large measure determined by the amount, size and distribution of these hard particles. These factors, in turn, are controlled by chemical composition, method of manufacturing and heat treatment.

The presence of elements that form carbides influences the hardening temperature and soaking time.

The carbide-forming elements are very powerful deep-hardening elements when they are dissolved in austenite.

Influence of Alloying Elements on the Iron-Iron Carbide Diagram

When a third element is added to steel, the binary iron-iron carbide diagram no longer represents equilibrium conditions. The presence of alloying elements will change the critical range, the position of eutectoid point and the locations of the alpha and gamma fields indicated by the binary iron-iron carbide diagram.

Nickle and manganese tend to lower the critical temperature on heating while molybdenum, aluminum, silicon, tungsten and vanadium tend to raise it.

The eutectoid point is shifted from the position it normally has in the iron-iron carbide diagram. All the alloying elements tend to reduce the carbon content of eutectoid, but only nickel and manganese reduce the eutectoid temperature as shown in the figure given below.

Increasing amounts of nickel and manganese may lower the critical temperature sufficiently to prevent the transformation of austenite on slow cooling. They are known as austenite-stabilizing elements. Therefore, austenite will be retained at room temperature. This situation occurs in austenitic stainless steels.

Effect of Alloying Elements in Tempering

In tempering, hardened steels are softened by reheating. In case of plain-carbon steels, as the tempering temperature is increased, the hardness drops continuously. The general effect of alloying elements is to retard the softening rate, so that alloy steels will require a higher tempering temperature to obtain a given hardness.

Te elements that remain dissolved in ferrite, such as nickel, silicon and to some extent manganese have very little effect on the hardness of tampered steel.

The complex carbide-forming elements such as chromium, tungsten, molybdenum and vanadium however have a very noticeable effect on the retardation of softening. Not only they raise the tempering temperature, but when they are present in higher percentage, the softening curves for these steels will show a range in which the hardness may actually increase with increase in tempering temperature. This characteristic behavior of alloy steel containing carbide-forming elements is known as secondary hardness and is believed to be due to delayed precipitation of fine alloy carbides.

Specific Effect of Common Alloying Elements

Since a large number of alloy steels are manufactured, it is not feasible to discuss the individual alloy steels. In view of this, a brief consideration of the specific effect of the common alloy elements and their application is given below.

Nickel Steels (2xxx Series)

Nickle has unlimited solubility in gamma iron and is highly soluble in ferrite, and is contributing to the strength and toughness of this phase.

Nickel lowers the critical temperature of steel and also reduces carbon content of the eutectoid. Therefore, the structure of unhardened nickel steels contains a higher percentage of pearlite than similarly treated plain-carbon steels. Since the pearlite forms at a lower temperature, it is finer and tougher than the pearlite in unalloyed steels.

Above factors permit the attainment of given strength levels at lower carbon contents, thus increasing toughness, plasticity and fatigue resistance.

Nickel has only a mild effect on hardenability but is outstanding in its ability to improve toughness, particularly at low temperatures.

Nickel steels are used for high-strength structural steels which are used in the as-rolled condition or for large forgings which are not adapted to quenching. Usually, high-strength structural steels contain1.5 to 5.0 percent nickel with 0.1 to 0.4 percent carbon.

The 3.5 percent nickel steels (23xx series) with low carbon are used for carburizing of drive gears, connecting-rod bolts and studs.

The 5 percent nickel steels (25xx series) provide increased toughness and are used for heavy duty applications such as bus and truck gears, cams and crankshafts.

Nickel steels of the 2xxx series have been largely replaced in many applications by the lower cost triple-alloy steels of the 86xx series.

Chromium Steels (5xxx Series)

Chromium forms simple and complex carbides. These carbides have high hardness and good wear resistance. In low-carbon steels chromium tends to go into solution, thus increasing the strength and toughness of the ferrite. When chromium is present in amounts in excess of 5 percent, the high-temperature properties and corrosion resistance of the steel are greatly improved.

The plain-chromium steels of the 51xx series contain between 0.15 and 0.64 percent carbon and between 0.70 and 1.15 percent chromium. The low-carbon alloy steels in this series are usually carburized. The presence of chromium increases the wear resistance of the case but the toughness in the core is not so high as the nickel steels. With medium carbon, these steels are oil-hardening and are used for springs, engine bolts, axles, etc.

A high-carbon (1.0 percent) high-chromium (1.5 percent) alloy steel (52100) has high hardness and wear resistance. This steel is used for ball/roller bearings and for crushing machinery.

A special type of chromium steel containing 1 percent carbon and 2 to 4 percent chromium has excellent magnetic properties and is used for permanent magnets.

The high-chromium steels containing over 10 percent chromium have high resistance to corrosion and are known as stainless steels. Information about stainless steels is covered in a separate (next) article.

Nickel-chromium Steels (3xxx Series)

In these steels the ratio of nickel to chromium is approximately 2.5:1.0 (2.5 parts nickel for 1.0 part chromium). In these type of steels, nickel increases toughness and ductility while chromium improves hardenability and wear resistance.

The low-carbon nickel-chromium steels are carburized. The chromium gives the wear resistance to the case while both alloying elements improve the toughness of the core. Steels with 1.5 percent nickel and 0.6 percent chromium (31xx series) are used for worm gears, piston pins, etc. For heavy-duty applications, such as aircraft gears, shafts and cams, the nickel content is increased to 3.5 percent and the chromium content to 1.5 percent (33xx series).

The medium-carbon nickel-chromium steels are used in the manufacture of automotive connecting rods and drive shafts. In many cases, these steels have been replaced by the triple-alloy steels of the 87xx and 88xx series.

Manganese Steels (31xx Series)

Manganese is present in all steels as a deoxidizer.

Manganese also reduces the tendency towards hot-shortness (red-shortness) resulting from the presence of sulfur, thereby enabling the metal to be hot-worked. If manganese is absent or very low, iron forms iron sulfide (FeS). Iron sulfide forms films around the primary crystals during solidification of steel. These films are liquid at the rolling temperature of steel and produce a condition of hot-shortness which is a tendency to crack through the grain boundaries during working. Manganese is outstanding in its power to combine with sulfur. The manganese sulfide remains solid at the rolling temperature and thus has less adverse effect on the hot-working properties of steel.

A steel is called an alloy steel when the manganese content exceeds about 0.8 percent. Manganese gives strength and hardness to steel but to a lesser degree than carbon, and is most effective in the higher-carbon steels. The element is a weak carbide former and has a moderate effect on hardenability. Like nickel, manganese lowers the critical range and decreases the carbon content of the eutectoid. Fine grained manganese steels attain unusual toughness and strength. These steels are used for gears, spline shafts and axles.

With a moderate amount of vanadium added, manganese steels are used for large forgings that must be air-cooled. After normalizing, these steels will give properties equivalent to those obtained in a plain-carbon steel after a full hardening and tempering operation.

When the manganese content exceeds about 10 percent, the steel will be austenitic after slow cooling. A special steel, known as Hadfield manganese steel, usually contains 12 percent manganese. After a properly controlled heat treatment, this steel is characterized by high strength, high ductility and excellent resistance to wear. It is an outstanding material for resisting severe service that combines abrasion and wear as found in power-shovel buckets and teeth, grinding and crushing machinery and railway track work. If this alloy after allowing the carbides to dissolve is quenched from 1850°F, the structure will be fully austenitic with a tensile strength of about 120000 psi, elongation of 45 percent and a BHN of 180. The steel is usually reheated below 500°F to reduce quenching stresses. In the austenitic condition following rapid cooling, the steel is not very hard. However, when it is placed in service and subjected to repeated impact, the hardness increases to about 500 BHN. This increase in hardness is due to the ability of manganese steels to work-harden rapidly and to the conversion of some austenite to martensite.

Indian standard for austenite-manganese steel castings is IS 276. There are 7 grades. Generally Grade 3 material as per this standard is used for the uses given above. The material composition for this grade is: 1.05-1.35 % C, 1.00 % Si, 11.50-14.00 % Mn, 0.08 % P, 0.025 % S and 1.5-2.5 % Cr. The mechanical properties for this grade are as under.

Tensile strength, min. (MPa): 600
Yield stress, min. (MPa): 300
Elongation percent, min.: 24
Hardness HB max.: 229
Angle of bend degrees, min.: 150

The ASTM standard number for austenite-manganese steel castings is A128.

Molybdenum Steels (4xxx Series)

Molybdenum is a strong carbide former. It has a strong effect on hardenability and like chromium, increases the high-temperature hardness and strength of steels. This element is most often used in combination with nickel or chromium or both nickel and chromium. For carburizing applications it improves the wear resistance of the case and the toughness of the core.

The plain-molybdenum steel (40xx and 44xx series) with low carbon content are generally carburized and are used for spline shafts, transmission gears and similar applications where service conditions are not too severe. With higher carbon they are used for automotive coil and leaf springs.

The chromium-molybdenum steels (41xx series) are relatively cheap and possess good deep-hardening property, ductility and weldability. They are used for pressure vessels, aircraft structural parts and automobile axles.

The nickel-molybdenum steels (46xx and 48xx series) have the advantage of the high strength and ductility from nickel combined with deep-hardening and improved machinability imparted by molybdenum. They have good toughness combined with high fatigue strength and wear resistance. They are used for transmission gears, chain pins, shafts and bearings.

The triple-alloy nickel-chromium-molybdenum steels (43xx and 47xx series) have the advantages of the nickel-chromium steels along with the high hardenability imparted by molybdenum. They are used in the aircraft industry for structural parts of wing assembly and landing gear.

Tungsten Steel

Tungsten has a marked effect on hardenability, is a strong carbide former and retards the softening of martensite on tempering. In general, the effect of tungsten in steel is similar to that of molybdenum, although large quantities are required. Approximately 2 to 3 percent tungsten is equivalent to 1 percent molybdenum. Due to this, it is not used in general engineering steels. Tungsten is used primarily in tool steels.

Vanadium Steels

Vanadium is a powerful deoxidizer and a strong carbide former which inhibits grain growth. Vanadium additions of about 0.05 percent produce a sound, uniform and fine-grin casting. When dissolved, it has a marked effect on hardenability, yielding high mechanical properties on air cooling. Carbon-vanadium steel are used for heavy locomotive and machinery forgings that are normalized.

The low-carbon chromium-vanadium steels (61xx series) are used in the case hardened condition in the manufacture of pins and crank shafts. The medium-carbon chromium-vanadium steels have high toughness and strength and are used for axles and springs. The high-carbon grade with high hardness and wear resistance is used for bearings and tools.

Silicon Steels (92xx series)

Silicon, like manganese is present in all steels as a cheap deoxidizer. When a steel contains more than 0.60 percent silicon, it is called a silicon steel. Silicon dissolves in ferrite increasing strength and toughness.

A steel containing 1 to 2 percent silicon known as navy steel is used for structural applications requiring a high yield point.

Hadfield silicon steel with less than 0.01 percent carbon and about 3 percent silicon has excellent magnetic properties for use in the cores and poles of electrical machinery.

A properly balanced combination of manganese and silicon produces a steel with unusually high strength with good ductility and toughness. This silicon-manganese steel (9260) is widely used for coil and leaf springs and also for chisels and punches.

Classification of Steels

Steels are generally classified by method of manufacture, use and chemical composition.

If steel is classified by method of manufacture, it gives rise to crucible steel. bessemer steel, open-hearth steel, basic oxygen steel and electric-furnace steel.

When steel is classified by use, it is generally classified by the final use for the steel such as machine steel, spring steel, boiler steel, structural steel or tool steel.

The easiest and the most popular way to classify steels are by their chemical composition. Various alloying elements are added to iron for the purpose of attaining certain specific properties and characteristics. These elements include, but are not limited to, carbon, manganese, silicon, nickel, chromium, molybdenum, vanadium, columbium (niobium), copper, aluminum, titanium, tungsten, and cobalt. A numbering system is used in classification by chemical composition method, giving information on the approximate content of the important alloying elements in the steel.

Classification Systems in USA

Three classification systems are used in USA as under.

The American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) System

For many decades, plain carbon and low-alloy steels have been classified by chemical composition using a system devised by SAE and eventually AISI.

The American Society for Testing and Materials (ASTM) System

ASTM system is not based on composition but is based on the steel product and application, for example, railroad rails, boiler tubes, plate, and bolts.

ASTM has very elaborate specifications for steels and cast irons. The specification for steels include the type of product (sheet, plate, bar, wire, rail, etc.), the composition limits, and the mechanical properties. The specification code consists of the letter “A” followed by a number. The ASTM system reaches far beyond ferrous materials and includes other materials also.

The American Society of Mechanical Engineers (ASME) devised a similar system, but it is generally limited to boiler and heat exchanger steels and other materials that are covered by the boiler code specifications. The ASME adopts the ASTM code and places “S” before it. For example, ASME SA213 shows that it is adopted from ASTM A213.

The Unified Numbering System (UNS)

Because of the confusion of different systems, a number of technical societies and U.S. governmental agencies devised what is known as the Unified Numbering System. There is a UNS designation for each steel composition, and it consists of a letter followed by five digits. The system fully incorporates the AISI/SAE system. For example, the UNS designation for AISI/SAE 1040 is G10400. The letter “G” represents the AISI/SAE plain carbon and alloy steels. Other ferrous alloys have different letters, such as “F” for cast irons and cast steels (cast steels can also have the letter “J”), “D” for steels with specific mechanical properties, “S” for heat and corrosion resistant steels, “T” for tool steels, and “H” for steels with enhanced hardenability.

Generally information on the steels as classified simply by composition using the AISI/SAE system is given below. This system has been established for many years and is widely used in industry.

Classification of Steels by AISI/SAE Method

The alloy steels are generally divided into two classes, the low alloy steels and the high alloy steels. If alloying elements is less than 8 %, they are called low alloy steels and if alloying elements is more than 8 %, they are called high alloy steels. Classification of plain carbon steels, low alloy steels, high strength low alloy (HSLA) steels and a variety of other low alloy steels mainly as per AISI/SAE method (and ASTM system where appropriate) is described in the following sections.

American Iron and Steel Institute (AISI) together with Society of Automotive Engineers (SAE) have established four or five-numeral (with additional letter prefixes) designation system as under for carbon and low alloy steels.

The first digit indicates the type to which the steel belongs. Thus 1 indicates a carbon steel. 2 to 9 are used for alloy steels. For example, 2 for a nickel steel, 3 for a nickel-chromium steel and so on.

In case of carbon steels, the second digit indicates modification of the steel as under.

0 – Plain carbon, non-modified
1 – Resulfurized
2 – Resulfurized and rephosphorized
5 – Non-resulfurized, Mn over 1.0%

In the case of simple alloy steels, the second digit indicates the approximate percentage of the predominant alloying element (1 means 1%).

The last two or three digits usually indicate the mean carbon content divided by 100.

Thus the symbol AISI/SAE 1030 means non modified carbon steel, containing 0.30% of carbon and AISI/SAE 5130 means alloy chromium steel, containing 1% of chromium and 0.30% of carbon.

In addition to the numerals, AISI specification may include a letter prefix before the four-digit number to indicate the steel making process as under.

A – Alloy, basic open hearth
B – Carbon, acid Bessemer
C – Carbon, basic open hearth
D – Carbon, acid open hearth
E – Electric furnace

Thus AISI B1020 means non modified carbon steel, produced by acid bessemer process and containing 0.20% of carbon.

Within the AISI/SAE plain carbon steel designations there are five subclasses, namely 10xx, 11xx, 12xx, 13xx, and 15xx. These are broadly based on the following categories of steel composition:

* Actually, the 13xx series of steels is classified as low alloy steels because of the high manganese level. (Generally a steel with an alloying element content above 1.5% is considered a low alloy steel.) However, in the case of the 13xx series, one is basically dealing with a simple extension of the 10xx and 15xx plain carbon steels.

The AISI/SAE 15xx and 13xx series represent high manganese, plain carbon steels. The higher manganese levels impart higher hardness and strength to the steels.

The 11xx series of plain carbon, resulfurized steels contains intentionally added sulfur. The sulfur does not actually alloy with the iron but combines with manganese to form manganese sulfide (MnS) inclusions. The sulfur level is much higher in the 11xx series than the 10xx series of plain carbon steels where sulfur is generally considered as an impurity. The higher sulfur level in the resulfurized steels imparts improved machinability to the steel because of the chip-breaking effect of the manganese sulfides. An example of a resulfurized steel is AISI/SAE 1140 steel. For a given carbon content, the manganese levels are slightly higher in the 11xx series than in the 10xx series. The higher manganese levels compensate for the higher sulfur levels, because manganese is added to tie up all the sulfur to form manganese sulfides.

The AISI/SAE 12xx series represents resulfurized and rephosphorized, plain carbon steels that are also free-machining steels, with both sulfur and phosphorus additions. The phosphorus addition increases the strength of the steel and promotes chip breaking during machining operations. In order to limit the strength of the steel, the carbon content is restricted to a level under 0.15%.

The figure given below shows the microstructure of a typical resulfurized, rephosphorized steel containing manganese sulfides (the gray, oblong particles marked by arrows). The remaining microstructure is ferrite (white etching constituent) and pearlite (dark etching constituent).

The following table gives basic representation of the various grades of carbon and low alloy steels. Be sure to refer the most recent AISI and SAE publications for the latest revisions.

There are many low-alloy steels that are not classified under the above mentioned AISI/SAE system. Thus, the situation with low-alloy steels becomes much more complicated. For example, HY-80, a steel widely used for high-strength plate and forging applications, is a Ni-Cr-Mo steel but does not have an AISI/SAE designation. This particular steel is covered by a specification designation, ASTM A543. ASTM has dozens of specifications for low-alloy steels.

ASTM specifications also cover many of the low-alloy steels. However, as mentioned previously, the ASTM system is driven by the application for the particular steel. The system for low-alloy steels is quite large. For example, a fairly common low-alloy steel is 2¼ Cr-1Mo steel. In the ASTM system there are separate specifications covering this steel, depending on the product form that is manufactured, as shown below.

As an example, ASTM A 213 has the title “Seamless Ferritic and Austenitic Alloy Steel for Boiler, Superheater, and Heat Exchanger Tubes.” The standard actually covers 14 different grades of ferritic steels and 14 different grades of austenitic steels. The 2 ¼ Cr-1Mo steel is grade T22. Because the grade is used in tubing for boilers and heat exchangers, it is also part of the specification system of ASME. The ASME code is SA213 type T22. The ASTM and ASME grade (type) T22 has the following composition.

Carbon: 0.15% max
Manganese: 0.30–0.60%
Silicon: 0.50% max
Chromium: 1.90–2.60%
Molybdenum: 0.87–1.13%

The microstructure of a typical ASTM A213 grade T22 steel (ASME SA213 type T22) is shown in the figure given above. The microstructure consists of ferrite (light etching constituent) and a small amount of pearlite (dark etching constituent). Light tan areas are martensite.

It is interesting to note that if the same steel was used for a forging or plate, it may have a different microstructure because of the different specified heat treatment. Even for tubes (ASTM A213), it can be furnished in the full-annealed, isothermal annealed, or normalized and tempered condition. Each condition would have a different microstructure.

Some representative standard-steel specifications for plain-carbon steel, free-machining steels and low alloy steels as per AISI/SAE grades are given in the following tables.

Generally, carbon is the most important commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite.

As per The American Iron and Steel Institute (AISI) a steel is considered to be a carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Carbon steels are iron-carbon alloys containing up to 2.00% of carbon, up to 1.65% of manganese, up to 0.5% of silicon and sulfur and phosphorus as impurities.

Steels are sometimes classified by the broad range of carbon content as under.

Low-carbon steels: up to 0.30 percent carbon
Medium-carbon steels: 0.30 to 0.60 percent carbon
High-carbon steels: above 0.60 percent carbon

Low-carbon Steels

Often they are called mild steels. Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels (deep drawing parts), tin plate, and wire products. For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher manganese content up to 1.5%. These materials may be used for stampings, forgings, seamless tubes, and boiler plate.

The figure given below shows the microstructure of a typical low-carbon steel (AISI/SAE 1010) showing a matrix of ferrite grains (white etching constituent) and pearlite (dark etching constituent).

These steels are not hardenable by quenching and tempering as carbon content is less than 0.30 percent. In this type of steels, cold working is the principle hardening mechanism. They are relatively soft and weak due to pearlite and ferrite microstructures, but they are tough and ductile. They are easily formable, machinable, weldable and cheap.

AISI/SAE 1020, ASTM A36 (carbon steel shapes, plates and bars of structural quality for bridges, buildings and general structural purposes) and IS 2062 (Indian Standard on steel for general structural purposes) are examples of low carbon steels.

Chemical composition (maximum unless minimum is indicated) of mild steels made as per IS 2062 and ASTM A36 are as under.

* Minimum 0.2 % Cu when copper steel is specified

Medium-carbon Steels

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon-manganese steels include machine components, shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.

The figure given below shows the microstructure of a typical medium-carbon steel (AISI/SAE 1040) showing ferrite grains (white etching constituent) and pearlite (dark etching constituent).

Mechanical properties of medium-carbon steels can be increased by heat treatment (by austenitizing or quenching & tempering). The rule of thumb is that 0.30 percent is the lower limit for hardening steel by heat treatment. However, plain carbon steels can be hardened only in thin sections with rapid quenching. Often there is distortion and cracking on quenching. They have poor impact resistance at low temperatures. To improve heat treating capabilities alloying elements like Cr, Ni, Mo, etc. are added to carbon steels and they are called alloy steels.

With 0.45 to 0.75 percent carbon, steels can be challenging to weld. Preheating, post heating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.

High-carbon steels

High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. They are hardest and strongest but least ductile. They are used in hardened and tempered state for wear resistance and cutting edges. They are used for spring materials, rope wires, screw drivers, hammers, wrenches, band saws, etc.

The figure given below shows the microstructure of a typical high-carbon steel (AISI/SAE 1095) showing a matrix of pearlite and some grain-boundary cementite.

A partial list of the plain carbon steels according to the ASTM specification system is given below to give an idea about classification of carbon steel as per ASTM specification.

As examples, two of these ASTM specifications are described below in more detail.

ASTM A1 requires that railroad rails have certain composition limits and a minimum hardness. For example, for a common rail size of 60 kg/m (132 lb/yd) the requirements are as under.

Carbon: 0.72–0.82%
Manganese: 0.80–1.10%
Phosphorus: 0.035% max
Sulfur: 0.040% max
Silicon: 0.10–0.20%
Hardness: 269 HB min

The microstructure of a typical ASTM A1 rail steel is 100% pearlite.

ASTM A36 for structural steels is very different from ASTM A1 for rail steel in that it specifies only a maximum carbon content and certain tensile properties. ASTM A36 has the following requirements.

Carbon: 0.26% max
Yield point: 248 MPa (36 ksi) min
Tensile strength: 400–552 MPa (58–80 ksi)
Total elongation (in 50 mm, or 2 in.): 21% min

The microstructure of a typical ASTM A36 structural steel is a mixture of pearlite and ferrite, with some manganese sulfide stringers.

The ASTM specifications illustrated above are rather simple. As the product becomes more critical and the composition more complex, the requirements expand considerably.

High Strength Low Alloy (HSLA) Steels

Although many of the previously mentioned AISI/SAE low alloy steels also have high strength and, in some cases, ultrahigh strength (a yield strength above 1380 MPa, or 200 ksi), there is a rather loose class of steels called HSLA steels that do not fit the previously mentioned AISI/SAE classification.

These HSLA steels are a group of low and medium carbon steels that generally use small amounts of alloying elements to attain yield strengths usually above about 345 MPa (50 ksi) in the hot-rolled, cold-rolled, annealed, stress-relieved, accelerated-cooled, direct-quenched, or normalized condition. In some cases they are called microalloyed steels because of the small amounts of vanadium, columbium (niobium), and/or titanium that are added for grain refinement and precipitation strengthening.

High strength low alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or improved formability and greater resistance to atmospheric corrosion than conventional carbon steels.

They have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldability. In these steels small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

Thus they have higher strengths, but still ductile, formable, machinable and generally more corrosion resistant. They are used for towers, bridges, columns, pressure vessels, etc.

ASTM specifies most of the HSLA steels according to composition, mechanical property requirements, and application. A partial list of ASTM specifications for various HSLA steels is given below.

Within each ASTM specification, one can find the mechanical property requirements as well as the range of chemical composition allowed. There are numerous other ASTM specifications involving low-alloy steels, depending on the particular application.

Some HSLA steels have commercial trade names. Recent designations HSLA 80 and HSLA 100 are being used for steels of a very specific steel composition with a minimum yield strength level of 552 MPa (80 ksi) and 690 MPa (100 ksi). In reality, there can be many HSLA 80 and HSLA 100 steels, depending upon composition, thermomechanical treatment, and heat treatment.

Thus, there is not a standard classification system that encompasses all high-strength, low-alloy steels.

Chemical composition (maximum unless minimum is indicated) of HSLA steel made by SAIL Limited, Salem Steel Plant, India is as under.

For comparison of steels (low-carbon and HSLA), mechanical properties of low-carbon steel (as per ASTM A36 and IS 2026) and high strength low alloy (HSLA) steel made by SAIL Limited, India are given below.

* t = Nominal thickness of test piece
** HI – Denotes improved notch ductility
+ Gauge length = 2 in. for ASTM A36 and = 5.65 x (square root of S₀) where S₀ is the cross sectional area of test piece for IS 2062 and SAIL Limited products.

Other Low Alloy Steels

There are many low alloy steels that are not designed for just their room-temperature strength properties. These steels have additional properties that are important, such as corrosion or heat resistance and formability as under.

Low Alloy Steels for High-Temperature Properties

An example of a low alloy steel that is used for its high-temperature properties is ASTM A470 turbine rotor steel. These steels are used in steam turbines for electric power generation and usually contain combinations of nickel, chromium, molybdenum, and/or vanadium. The microstructure of ASTM A470 rotor steel consisting of tempered upper bainite is shown in the figure given below.

Low Alloy Steels for Improved Corrosion Resistance

There are a number of low alloy steels that have improved corrosion resistance. These steels usually have additions of copper, nickel, or chromium and are called weathering steels. The ASTM specifications cover several of these steels.

Low Alloy Steels with Formability

There are some steels that are designed for optimal formability in sheet-forming applications. One common steel is specified as drawing quality, special killed. This cold-rolled, low-carbon sheet steel has a specified aluminum content. The aluminum combines with nitrogen in the steel to form aluminum nitride precipitates during the annealing process. These aluminum nitride precipitates are instrumental in the development of a specific crystallographic texture in the sheet that favors deep drawing. Another type of steel used for applications requiring optimal formability is interstitial-free steel. In this very-low-carbon sheet steel, the interstitial elements, carbon and nitrogen, are combined with carbide- and nitride-forming elements, such as titanium and columbium (niobium). The steel is rendered “free” from these interstitial elements that degrade formability.

Bake Hardenable Low Alloy Steels

Specific sheet steels have been designed to increase strength during the paint-baking cycle of automobile production. These bake hardenable steels contain elements that develop compounds that precipitate at the paint-baking temperatures. These precipitates harden the steel.

Metals Classification

Metals can be classified in two main divisions as Ferrous metals (iron-based) and Non-ferrous metals (all the others) as shown below. It can be seen that classification of steels is a part of this classification.

Information on effect of alloying elements on alloy steels, high alloy steels (stainless steels and tool steels) and cast irons is given in separate articles.

There are five principal methods of case hardening as under.

1. Carburizing
2. Nitriding
3. Cyaniding and carbonitriding
4. Flame hardening
5. Induction hardening.

The first three methods change the chemical composition, carburizing by the addition of carbon, nitriding by the addition of nitrogen and cyaniding/carbonitriding by the addition of both carbon and nitrogen. The last two methods do not change the chemical composition of the steel and are essentially shallow-hardening methods. In flame and induction hardening the steel must be capable of being hardened and therefore, the carbon content must be about 0.30 percent or higher.

Carburizing

The purpose of carburizing is to provide a hard surface on normally unhardenable steels. In this method, low carbon steel, usually 0.20 percent carbon or lower is placed in an atmosphere that contains substantial amount of carbon monoxide.

The usual carburizing temperature is 1700°F. At this temperature, the maximum amount of carbon that can be dissolved in austenite can be found out from the A_cm line of the iron-iron carbide equilibrium diagram. Therefore, very quickly, a surface layer of high carbon (about 1.2 percent) is built up. Since the core is of low carbon content, the carbon atoms trying to reach equilibrium will begin to diffuse inward. The rate of diffusion of carbon in austenite, at a given temperature, is dependent upon the diffusion coefficient and the carbon concentration gradient. Under known and standard operating conditions, with the surface at fixed carbon concentration, the form of carbon gradient may be predicted, with reasonable accuracy, as a function of elapsed time. After diffusion has taken place for the required amount of time depending upon the case depth desired, the part is removed from the furnace and cooled.

There is no technical limit to the depth of hardening with carburizing techniques, but it is not common to carburize to depths in excess of 0.050 in. The carburizing time is between 4 to 10 hours. The relation of time and temperature to case depth is shown in the figure given below.

All of the carburizing processes (pack, gas, liquid) require quenching from the carburizing temperature or a lower temperature or reheating and quenching. Parts are then tempered to the desired hardness.

Commercial carburizing may be carried out by means of pack carburizing, gas carburizing and liquid carburizing.

Pack Carburizing

In pack carburizing, the work is surrounded by a carburizing compound in a closed container. The container is heated to the proper temperature for the required amount of time and then cooled slowly.

Commercial carburizing compounds usually consist of hardwood charcoal, coke and about 20 percent of barium carbonate. Barium carbonate promotes the formation of carbon dioxide (CO₂). This gas in turn reacts with the excess carbon in the charcoal to produce carbon monoxide, CO. Carbon monoxide reacts with the low-carbon steel surface to form atomic carbon which diffuses into the steel. The carburizing compound is in the form of coarse particles or lumps so that when the cover of the container is sealed, sufficient air is trapped inside to form carbon monoxide. The carburizing process does not harden the steel. It only increases the carbon content to some predetermined depth below the surface to a sufficient level to allow subsequent quench hardening.

It is difficult to quench the part immediately, as the sealed pack has to be opened and the part removed from the pack. Due to this, the entire pack is cooled slowly and the part is subsequently hardened and tempered.

This method is efficient and economical for individual processing of small lots of parts or of large, massive parts. However it is not well suited to the production of thin carburized cases that must be controlled to close tolerances. Because of inherent variation in case depth, the method is not used on work requiring a case depth of less than 0.030 in. and tolerances are at least 0.010 in.

Gas Carburizing

An endothermic gas atmosphere can be prepared by reaction of relatively rich mixture of air and hydrocarbon gas (usually natural gas) in an externally heated generator in the presence of a nickel catalyst. The gas produced consists of 40 percent nitrogen, 40 percent hydrogen and 20 percent carbon monoxide.

In gas carburizing, the steel is heated in contact with carbon monoxide and/or a hydrocarbon which is readily decomposed at carburizing temperature. The hydrocarbon may be methane, propane or natural gas. Commercial practice is to use a carrier gas, such as obtained from an endothermic generator and enrich it with one of the hydrocarbon gases.

This method allows the surface carbon to be reduced to any desired value by using a diffusion period, during which the gas is turned off but temperature maintained.

In this method, it is possible to directly quench the part after carburizing.

Most carburizing gases are flammable and controls are needed to keep carburizing gas at 1700oF from contacting air (oxygen).

Liquid Carburizing

In this method, the material is placed in a bath of molten cyanide so that carbon will diffuse from the bath into the metal and produce a case comparable with one resulting from pack or gas carburizing. Liquid carburizing can be differentiated from cyaniding by the case produced. In cyaniding, the case is higher in nitrogen and lower in carbon. The reverse is true in the case of carburizing.

Cases as deep as 0.025 in can be produces by this method. Compared to this, cyanide cases are seldom to a depth greater than 0.010 in.

Cycle times for liquid cyaniding is much shorter (1 to 4 hours) than gas and pack carburizing processes.

The other advantage of this method is freedom from oxidation and sooting.

In general, the method is suitable for small and medium size parts as it is difficult to process large parts in a salt bath.

However, as cyanide salts are poisonous, this method requires careful attention and the parts must be thoroughly washed after heat treatment to prevent rusting.

Nitriding

The steel is usually hardened and tampered between 1100 and 1300°F to produce a sorbitic structure of maximum core toughness and then nitrided.

In this process, nitrogen is diffused into the surface of the steel being treated. The effectiveness of this process depends on the formation of nitrides in the steel by reaction of nitrogen with certain alloying elements. Aluminium, chromium and molybdenum are the major nitride-forming alloying elements. The reaction of nitrogen with the steel causes the formation of very hard iron and alloy nitrogen compounds. The resulting nitride case is harder than carburized steels. The advantage of this process is that hardness is achieved without the oil, water or air quenching. As an added advantage, hardening is accomplished in a nitrogen atmosphere which prevents scaling.

The parts to be nitrided are placed in an airtight container through which the nitriding atmosphere is supplied continuously. Nitriding temperature is below the lower critical temperature of the steel and it is set between 925°F and 1050°F. The nitrogen source is usually ammonia (NH₃). At the nitriding temperature the ammonia dissociates into nitrogen and hydrogen.

A nitrided case consists of two distinct zones. In the outer zone, nitride forming elements, including iron, gets converted to nitrides. This region, which varies in thickness up to a maximum of about 0.002 inch, is commonly known as the “white layer”. In the zone below this white layer, alloy nitrides only have been precipitated.

The white layer is brittle and tends to chip from the surface if its thickness is more than 0.0005 inch. If the layer is thick, it is removed by lapping after nitriding.

Two stage gas-nitriding processes is used to reduce the thickness of this white layer.

Hardest cases, approximately RC 70 are obtained with aluminium alloy steels known as Nitralloys. These are medium carbon steels containing chromium and molybdenum also. For application where lower hardness is acceptable, medium carbon steels containing chromium and molybdenum (AISI 4100 and 4300 series) are used. Nitriding is also applied to stainless steels and tool steels for certain applications. However, corrosion resistance of stainless steel is reduced considerably by nitriding.

The nitriding cycle is quite long depending upon the desired case depth. Nitriding time for some steels is shown in the figure given above.

Nitriding is used extensively for aircraft engine parts like cams, cylinder liners, valve stems, shafts and piston rods.

Cyaniding and carbonitriding

Cases that contain both carbon and nitrogen are produced in liquid salt baths (cyaniding) or by use of gas atmosphere (carbonitriding). The temperatures used are generally lower than those used in carburizing, between 1400 and 1600°F. Exposure is for a shorter time and thinner cases are produced, up to 0.010 in. for cyaniding and up to 0.030 in. for carbonitriding.

Since nitrogen increases the hardenability, carbonitriding the less expensive carbon steels for many applications will provide properties equivalent to those obtained in gas-carburized alloy steels.

It has also been found that the resistance of a carbonitrided surface to softening during tempering is markedly superior to that of a carburized surface.

Cyaniding

In cyaniding, the proportion of nitrogen and carbon in the case produced by a cyanide bath depends on both composition and temperature of the bath, the temperature being the most important. Nitrogen content is higher in baths operating at lower end of temperature range.

Generally, carbon content of the case is lower than that produced by carburizing, ranging from about 0.5 to 0.8 percent. The case also contains up to about 0.5 percent nitrogen, therefore, file-hard cases can be obtained on quenching in spite of the relatively low carbon content.

This process is particularly used for parts requiring a very thin hard case, such as screws, small gears, bolts and nuts.

Carbonitriding

Carbonitriding is a case hardening process in which a steel is heated in a gaseous atmosphere of such composition that carbon and nitrogen are absorbed simultaneously. Actually carbonitriding is a modification of carburizing process. This process is also known as dry cyaniding, gas cyaniding and nicarbing. The atmospheres used in carbonitriding generally comprise a mixture of carrier gas, enriching gas and ammonia. At the furnace temperature, the added ammonia (NH₃) breaks up or dissociates to provide the nitrogen to the surface of the steel.

Flame Hardening

In this method, selected areas of the surface of a steel are heated into the austenite range and then quenched to form martensite. Therefore, it is necessary that the steel is capable of being hardened. Generally, steel for flame hardening have 0.3 to 0.6 percent carbon. After quenching, the part should be stress-relieved by heating in the range of 350 to 400°F and air cooled.

In flame hardening, heat may be applied by an oxyacetylene torch as shown in the figure given below or it may be a part of an elaborate setup which automatically carries out different tasks like heating, quenching and indexing.

Depth of the hardness zone may be controlled by an adjustment of the flame intensity, heating time, or speed of travel. Skill is required in adjusting and handling manually operated equipment to avoid overheating the work because of high flame temperature. Overheating can result in cracking after quenching and excessive grain growth in the region just below the hardened zone.

The hardened zone is generally much deeper than that obtained by carburizing, ranging from 1/8 to 1/4 inch in depth. Thinner cases of the order of 1/16 inch can be obtained by increasing the speed of heating and quenching.

The main advantages of flame hardening are adaptability and portability. The equipment can be taken to the job and adjusted to treat only the area which requires hardening. Parts too large to be placed in a furnace can be handled easily and quickly with the torch. Another advantage is the ability to treat components after machining since there is little scaling. The main disadvantages are the possibility of overheating and thus damaging the part and difficulty in producing hardened zones less than 1/16 inch in depth.

Laser Beam Hardening

Laser beam hardening is another variation of flame hardening. A phosphate coating is applied over the steel to facilitate absorption of the laser energy. The selected areas of the part are exposed to laser energy. This causes the selected areas to heat. By varying the power of the laser, the depth of heat absorption can be controlled. The parts are then quenched and tempered.

Induction Hardening

Induction hardening depends for its operation on localized heating produced by currents induced in a metal placed in a rapidly changing magnetic field.

The operation resembles a transformer in which the primary or work coil is composed of several turns of copper wire and the part to be hardened is made the secondary of a high-frequency induction apparatus. Work coil of different designs are used to suit different types of heating requirements like external heating, internal heating, etc.

As shown in the figure given above, when high-frequency alternating current passes through the work coil, a high-frequency magnetic field is set up. This magnetic field induces high-frequency eddy currents and hysteresis currents in the metal work piece. Heating results from the resistance of the metal to passage of these currents. The high-frequency induced currents tend to travel at the surface of the metal. This is known as skin effect. Therefore, it is possible to heat a shallow layer of the steel without heating the interior. However, heat applied to the surface tends to flow towards the center by conduction, and so time of heating is an important factor in controlling the depth of the hardened zone.

Depending on the frequency and amperage, the rate of heating as well as the depth of heating can be controlled. The range of frequencies commonly used is between 10,000 and 500,000 Hz. The case obtained by induction hardening is similar to that obtained by flame hardening, and thinner cases are possible.

Selecting the Right Surface Hardening Method

Carburizing is the best method for low carbon steels.

Nitriding is a lower distortion process than carburizing but it can be used for certain type of steels such as chromium-molybdenum alloy steels or Nitralloy-type steels.

Flame hardening is preferred for heavy cases or selective hardening of large machine components.

Induction hardening works best on parts small enough and suitable in shape to be compatible with the induction coil.