Stainless steel is a large group of metals with a couple of properties in common: The main element is iron (> 50 % Fe = “steel”),and apart from that the steel contains at least 10-12 % chromium (Cr). In its simplest form, no other elements are required in order to make “stainless steel”, proved by the fact that the simplest possible stainless steel (EN 1.4003) contains only 10.5 % Cr, and the rest is iron.
Historically, the simple chromium alloyed steel types were invented back in 1912. In 1913, the German Krupp Company added nickel (Ni) in order to improve the mechanical properties (the birth of the austenitic stainless steel), and in 1920, the beneﬁ cial effect of adding even small amounts of molybdenum was observed. Thereby, the “acid resistant” stainless steel was invented. Apart from chromium and nickel, the steel may contain a large array of other elements, all of which affect the mechanical or chemical properties in one way or the other. The description of the effect and importance of the different elements is enclosed in Chapter 2.According to their microstructure, the stainless steel types may be classiﬁ ed into ﬁ ve main groups: