Stainless Steels
Such are particularly termed for their resistance to corrosion. This resistance is acquired due to formation of protective oxide layer that spreads all over the surface. That layer does not permit the close atmosphere to further react along with the steel that retains its luster and appearance. The oxide layer on the stainless steel surface is formed via the oxide of Cr whilst it is there in large proportions. Such oxide film is impervious to both metal ions and atmospheric oxygen. Enhanced corrosion resistance is acquired along with increasing percentage of Cr, given that Chromium is in solid solution and not combined like carbide. The corrosion resistance is further better with addition of specific amounts of nickel. As per to structures obtainable at room temperatures the stainless steels are subdivided into three sets.
Ferrite stainless steels comprise only chromium like alloying element in addition to minute percentage of carbon. The carbon varies in between 0.05 to 0.15 percent, whilst Cr varies in between 13 to 30 percent. Such alloy contains only a-phase at all temperatures. Several Cr precipitates in form of carbides through ferritic grains at room temperature. Such alloy is extremely ductile and utilized where outstanding formability in complicated shapes is required. Many deep drawn objects are generated from ferritic stainless steel. This material possesses superb resistance to corrosion.
When alloy steel contain at least 24% Cr and Ni together but not less than 8 percent of either element, the g-phase is retained on cooling on normal rates. At extremely low cooling rates the a-phase may separate entirely. Austenitc phase is acquired when quenched from upper critical temperature. The commonest of these steels comprise 18% Cr, 8% Nickel and 0.1% C. this is called 18: 8 steel. Austenitic steel is employed for construction of chemical plants, decorative reasons and household utensils.
Neither of on top two groups is heat-treatable. Whether steel contains Cr and Nickel in such proportions where it has a g-phase at high temperature and a a-phase on cooling at usual rates, this can be quenched to offer a martensitic structure. Such heat-treatable steels are termed as martensitic steels even while not in heat treated situations. For enhancing martensitic these steels are oil-quenched from as above upper-critical temperature. Three kinds of martensite steels are available commercially. These are as:
i. 0.07% - 0.1%, C 13% Cr,
ii. 0.2% - 0.4% C, 13% Cr, and
iii. 0.1% C, 18% Cr, 2% Ni.
These steels are employed for turbine blades, springs, surgical instruments, ball bearings, aircraft fittings, pump shafts.
Whilst martensitic steel can be heat treated to acquire high strengths, the strength of ferritic and austenitic steel can be enhanced only via mechanical working. Different precipitation hardening stainless steels have also been improved.
At high temperatures someplace in between 500 and 700oC, the stainless steels lose their resistance to corrosion. It happens mostly since the chromium has a tendency to separate from solid solution and precipitate in carbides form at grain boundaries. This makes welding of the stainless steels complex and reasons what is called as weld decay. Whether welded part is reheated to a temperature of about 900 - 1000oC the carbides are re-dissolved and can be transformed into steady solid solution on quenching.