Stainless Steels
Stainless steel are particularly known for their resistance to corrosion. This resistance is obtained because of formation of protective oxide layer which spreads all over the surface. This layer does not allow the surrounding atmosphere to further react with the steel which retains its luster and appearance. The oxide layer on the stainless steel surface is formed by the oxide of Cr when it is present in large proportions. This oxide film is impervious to both metal ions and atmospheric oxygen. Improved corrosion resistance is obtained with increasing percentage of Cr, provided that Cr is in solid solution and not combined as carbide. The corrosion resistance is further enhanced by addition of certain amounts of nickel. According to structures obtainable at room temperatures the stainless steels are subdivided into three groups.
Ferrite stainless steels contain only chromium as alloying element in addition to small percentage of carbon. The carbon varies between 0.05% to 0.15%, while Cr varies between 13% to 30%. This alloy contains only α-phase at all temperatures.
Some Cr precipitates in form of carbides along with ferritic grains at room temperature. This alloy is very ductile and used where outstanding formability in complicated shapes is required. Many deep drawn objects are produced from ferritic stainless steel. This material possesses excellent resistance to corrosion.
When alloy steel contain at least 24% Cr and Ni together but not less than 8% of neither element, the γ-phase is retained on cooling at normal rates. At very low cooling rates the α-phase may separate fully. Austenitc phase is obtained when quenched from upper critical temperature. The commonest of these steels contain 18% Cr, 8% Ni and 0.1% C. It is called 18 : 8 steel. Austenitic steel is used for construction of chemical plants, decorative purposes and household utensils.
Neither of above two groups is heat-treatable. If steel contains Cr and Ni in such proportions that it has a γ-phase at high temperature and an α-phase on cooling at normal rates, it can be quenched to give a martensitic structure. Such heat-treatable steels are known as martensitic steels even when not in heat treated conditions. For developing martensitic these steels are oil-quenched from above upper-critical temperature. Three types of martensite steels are available commercially. These are:
a. 0.07% − 0.1%, C 13% Cr,
b. 0.2% − 0.4% C, 13% Cr, and
c. 0.1% C, 18% Cr, 2% Ni.
These steels are used for turbine blades, surgical instruments, springs, ball bearings, pump shafts, aircraft fittings, etc.While martensitic steel can be heat treated to obtain high strengths, the strength of ferritic and austenitic steel can be improved only by mechanical working. Various precipitation hardening stainless steels have also been developed.
At high temperatures somewhere between 500 and 700oC, the stainless steels lose their resistance to corrosion. This happens mainly because the chromium has a tendency to separate from solid solution and precipitate in form of carbides at grain boundaries. This makes welding of the stainless steels difficult and causes what is known as weld decay. If welded part is reheated to a temperature of about 900-1000oC the carbides are re-dissolved and can be converted into stable solid solution on quenching.