Stainless Steels

Stainless steels are usually defined as alloy steels containing at least 10% chromium, either alone or with other alloying elements. In the United States, steels with as low as 8% chromium are included in this classification. Stainless steels are specified when special properties, such as corrosion resistance and resistance to scaling are required. Some grades, such as Type 422, have strength and impact toughness up to 650°C (1200°F).

Stainless steels are considered more difficult to forge than carbon or alloy steels in that they require higher forging pressures at normal forging temperatures. Three groups within the general classification of stainless steel are of interest in forging: ferritic, austenitic and martensitic. There are many special grades of stainless steels not classified in any of these groups. For example, a whole series of forging alloys is included in what might be termed special stainless or heat resistant grades.

Ferritic Grades

Ferritic stainless steels are low carbon grades containing 10% to 30% chromium. Some grades additionally contain molybdenum, silicon, aluminum or titanium to develop special properties. The microstructures are basically ferrite with some scattering of finely distributed carbides. They exhibit excellent corrosion resistance and considerable ductility, can be worked hot or cold, and are relatively inexpensive. They cannot be hardened by heat treating, but strength can be increased moderately by cold working.

Ferritic stainless steels containing 16% to 18% chromium are generally the most useful alloys in this group. Chromium contents in the range of 15% to 30% are used to resist oxidizing at progressively higher elevated temperatures. Typical ferritic forging grades include:

Material Grade	ASTM	Percent Composition
429	A-182	15Cr
430	A-182	17Cr
FXM 27	A-182	26Cr

While most other ferritic stainless grades are forgeable, they are not considered common forging grades.

Austenitic Grades

Austenitic stainless steels contain 8% to 18% of nickel in addition to typically 18% to 20% chromium. They cannot be hardened by heat treatment, but can be hardened by cold working, and excellent properties are developed in the grades with lower nickel content. This class is highly resistant to many acids, and alloys in this family exhibit excellent impact toughness at cryogenic temperatures, as low as liquid nitrogen (-200°C or -325°F). Alloys with higher (25%) chromium can be used at temperatures up to 1095°C (2000°F) without excessive scaling. Two types, 304L and 316L have carbon contents below 0.03%, and are specified where welding is required and postannealing is impractical.

Commonly used austenitic forging grades include:

Material Grade	ASTM	Percent Composition
		Cr	Ni
304	A182	19	10
310	A182	25	21
316	A182	17	12
317	A182	19	13
321	A182	18	11
347	A182	18	11

SAE, ASME and ASTM standards list many other Austenitic grades that have special purposes and, while not as commonly forged, are just as forgeable as these listed grades. 303 is not a common forging alloy because its sulphur causes problems.

Martensitic Grades

Martensitic stainless steels usually contain a maximum of 14% chromium, except types 440A and 440B, which contain 16% to 18% chromium. Carbon is added in sufficient amounts to promote hardening. Other elements, when present, are usually restricted to 2% or 3%. They exhibit excellent strength, and, unlike most other stainless grades, they are magnetic. Martensitic stainless steels are hardened and tempered using processes similar to those used for as alloy steels. Typical tempering temperatures range from 540 to 620°C (1000 to 1150°F).

Commonly used martensitic forging grades include:

Material Grade	ASTM	Percent Composition
		C	Cr
403	A473	0.2	12
410 (F6A)	A182	0.15	13

SAE, ASME and ASTM include many other Martensitic grades that have special purposes and, while not commonly forged, are just as forgeable as these listed grades.

Special Stainless Grades

There are many, many special grades of stainless steels that are not classified exclusive to any one of the above three families. For example, there is a series of forging alloys that include grades such as 17-4PH, 15-5PH and PH13-8Mo, which are precipitation hardenable (PH) grades. These grades include Cr-Ni additions similar to the austenitic grades but with additions of copper, titanium, columbium and/or aluminum. These additions permit a solution treatment, followed by a suitable aging cycle, to promote a compound strengthening effect.

The alloys are usually "soft martensitic" in the annealed condition and are then transformed to a hard martensitic condition either by an aging cycle or by a sub-zero cooling cycle. Subsequent intermediate temperature treatments or aging cycles then produce the final properties. These alloys are typically capable of achieving tensile and yield strengths higher than the martensitic grades of stainless, yet exhibit resistance to corrosion similar to the 18-8 family of austenitic steels.

There are also a few special purpose stainless grades that are approximately twice as strong as the 304-type austenitic grades and are as corrosion resistant. These are called "duplex" stainless grades. The most common applications are nuclear components. They have higher chromium contents than the PH grades, similar nickel and lower carbon. Other elements are used to enhance strength. The resulting microstructure is a mixture of austenite and ferrite with very little carbide; hence the term "duplex". UNS designatons for a few of the more common grades are S31803, S32550, S32750 and S32760.

Additional information and application data about any of the stainless steel types described in this category or more detailed information on an individual stainless steel grade can be obtained from:

The Specialty Steel Industry of North America (SSINA)
The Stainless Steel Information Center
3050 K. Street, N.W.
Washington, DC 20007
1-202-342-8630
https://www.ssina.com/