1.4. Stainless Steels

In 1988 the United States produced 2.2 million tons of stainless steel, or 2.2% of total steel production (Ref 7, p.147). It was less than alloy steel (10.9%), but when corrosion resistance and mechanical properties of the parts are the most important factors, the use of stainless steels is a necessity.

Stainless steels are iron-base alloys containing at least 10.5% chromium. They achieve their stainless characteristics through the formation of adherent chromium oxide surface film. This oxide forms and heals itself in the presence of oxygen. Other elements added to improve particular characteristics, include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulfur, and selenium. Carbon is normally present in the amounts raging from less than 0.03% to over 1.0% in certain martensitic grades (Ref 7, p.841).

Original discoveries and developments in stainless steel technology began in England and Germany about 1910. The commercial production of stainless steels in the United States began in the 1920s by Allegheny, Carpenter, Firth-Sterling, U.S. Steel, and some other companies (Ref 7, p.841).

Stainless steels are divided into five groups: austenitic, martensitic, ferritic, duplex (ferriticaustenitic), and precipitation-hardening stainless steels.

Stainless steels are designated by AISI (the American Iron and Steel Institute) and by UNS (Unified Numbering System) differently.

The designation of AISI is a three-digit number and a numeric-letter code. The first numeral in a three-digit number identifies a group of stainless steels: the austenitic types are 2xx and 3xx ; the martensitic and ferritic types are 4xx ; and...