Stainless steel is not a single specific material - it is the name given to a group of corrosion-resistant steels containing a minimum of 11% chromium - varying additions of nickel, molybdenum, titanium, niobium and other elements may also be present. The mechanical properties and behaviour in service of each type of steel depends upon its composition and consequently a carefully considered choice of steel is vital.
There are five classes of stainless steel, namely Martensitic, Ferritic, Austenitic, Duplex and Precipitation Hardening:
The Martensitic types may be strengthened and hardened by heat treatment, in the same way as plain carbon steels, but the Ferritic steels, which in general have lower carbon and higher Chromium contents, do not respond in this way. Martensitic and Ferritic steels are magnetic.
Ferritic stainless steels contain chromium usually in the range 11 to 18%. They are magnetic, have moderate corrosion resistance and are not susceptible to stress corrosion.
Austenitic stainless steels contain nickel as well as chromium, and are sometimes referred to by the generic title 18/8, i.e. 18% chromium, 8% nickel, although the actual composition may vary widely from these figures. They are non-magnetic and cannot be hardened by heat treatment, although they strain harden rapidly when cold worked. All stainless steels except hardened Martensitic steels are ductile and therefore can be formed, but the Austenitic types are outstanding in this respect. They are also amongst the most highly corrosion-resistant materials available to the designer and engineer.
Duplex stainless steels combine the optimum properties of austenitic and ferritic types. They contain 18 - 26% chromium plus 4.5 to 6.5% nickel and have good resistance to stress corrosion cracking.
Precipitation hardening stainless steels may be martensitic, semi-austenitic or austenitic. They combine the heat treat ability of normal martensitic grades with the corrosion resistance of austenitics. They are available in bar form for the production of heavy duty engineering components.
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