Stainless steel is a steel alloy that is composed of a minimum of 10.5% chromium content by mass and a maximum of 1.2% carbon by mass. Stainless steel is defined by its crystalline structure. There are four families austenitic, ferritic, martensitic, and duplex.
Stainless steel alloys in North America are designated by one of two different numbering systems: the 3-Digit System and the U.N.S.Number System.
The 3-Digit System
In the traditional three-digit numbering system, the first digit indicates the series or group, and the last two digits indicate the specific type. Modifications of the basic alloys are indicated by an alphabetic suffix to the series designation.
2XX Chromium-Nickel-Manganese Steels — Non-hardenable by heat treatment, austenitic, and non-magnetic hardenable by coldworking.
3XX Chromium-Nickel Steels — Non-hardenable by heat treatment, austenitic, and non-magnetic hardenable by cold working.
4XX Chromium Steels — Hardenable by heat treatment, martensitic, and magnetic.
4XX Chromium Steels — Non-hardenable, ferritic, and magnetic.
5XX Chromium Steels — Low chromium, heat-resisting. (Note: Although these alloys are sometimes associated with the stainless family, they are not true stainless steels as they only contain from 4% to 6% chromium.)
Precipitation-hardening type alloys are not covered by the three-digit numbering system.
The Unified Numbering System
The unified numbering system (UNS) is an alloy designation system widely accepted in North America. It consists of a prefix letter and five digits designating a material composition. A prefix of S indicates the alloy is stainless steel.
The first 3 digits often match older 3-digit numbering systems, while the last 2 digits indicate modern variations. For example, Stainless Steel Type 310 in the original 3-digit system became S31000 in the UNS System. The more modern low-carbon variation, Type 310S, became S31008 in the UNS System. Often, the suffix digit is chosen to represent a material property specification. For example, “08” was assigned to UNS S31008 because the maximum allowed carbon content is 0.08%. The Heat and Corrosion category of stainless steels falls in the number range of S000001 to S999999.
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