Stainless steels are a class of chromium-containing steels widely used for their corrosion resistance in aqueous environments and for service at elevated temperatures. Stainless steels are distinguished from other steels by the enhanced corrosion and oxidation resistance created by chromium additions.
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.
