Stainless steel' is defined as an iron-based alloy containing over 12–13mass%Cr and other elements, provided that the content of iron (Fe) is not over 50mass%. Stainless steel is not corroded under an oxygen-containing atmosphere, but is locally corroded and sometimes forms pits in chloride solutions such as body fluids. Nickel (Ni), Mo, copper (Cu), Ti, niobium (Nb), nitrogen (N), etc. are added to stainless steels to improve their corrosion resistance, heat resistance, strength, and workability. The metallurgical structure, strength, and corrosion resistance of stainless steels depend on the concentrations of Ni and Cr they contain and stainless steels are categorised as ferritic (Fe–Cr system), martensitic (Fe–Cr system), and austenitic (Fe–Cr–Ni system), according to their crystal phase. Austenitic-type stainless steels have outstanding corrosion resistance, but do not have great strength. Therefore, austenitic stainless steels are strengthened by working and heat treatment, and hardened with the addition of nitrogen (N). The addition of Mo improves their corrosion resistance, because their passive film becomes more stable. The categories of popular stainless steel series are summarised in Fig. 1.4, according to their development history. In AISI numbering, the 200 s represent Fe–Cr–Ni–manganese (Mn) alloy systems, the 300 s represent Fe–Cr–Ni alloy systems, and the 400 s represents Fe–Cr systems. Recently, the stainless steels used for the stems of artificial hip joints and bone fixators have been replaced by Ti alloys.
Stainless steel has been in use for more than one hundred years. It comprises a wide range of iron-based alloys, but unlike conventional steel they are resistant to corrosion and do not rust when exposed to water alone. The alloying element that makes steel 'stainless' is chromium; however it is the addition of nickel that enables stainless steel to become such a versatile alloy.
