Plain-carbon steels are satisfactory when strength and other requirements are not too severe. They can be used successfully at ordinary temperatures and in atmospheres that are not highly corrosive. Almost all hardened steels are tempered to reduce internal stresses. Plain-carbon steels show a marked softening with increasing tempering temperature. Due to this, they are not suitable for parts that require hardness above room temperature. Most of the limitations of plain-carbon steels may be overcome by the use of alloying elements. Information about effect of alloying elements on steel is given in this article.
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.
