(2) Martensitic stainless steel:
These steels contain 12–18% chromium and 0.1 to 1.8%
carbon. These steels can be hardened by heat treatment but their corrosion resistance
is decreased. Steels with 12 to 14% chromium and 0.3% carbon are widely used for
table cutlery, tools and equipment. Steel with little less carbon percentage and higher
percentage of chromium are used as springs, ball bearings and instruments under
high temperature and corrective conditions.
(3) Austentic stainless steels:
These are the most costliest among all stainless steels. In
these steels besides chromium, nickel is also added. Nickel is a very strong austenitic
stabilizer and therefore the microstructure of these steels is austentic at room
temperature. These steels contain 12 to 21% chromium and 8 to 15% nickel and carbon
less than 0.2%. The most familiar alloy of this group is known as 18:8 stainless steel
i.e. 18% chromium and 8% nickel plus other. Other elements like carbon, manganese
and silicon in very small quantities.
2.8 TOOL STEELS
Tool steels are specially alloyed steels designed for high strength, impact toughness and
wear resistance at room and elevated temperatures. They are normally used in forming
and machining of metals. So the requirements in a tool steel are that it should be capable
of becoming very hard and further that it should be able to retain its hardness at high
temperatures normally developed during cutting of materials. This property is known
as “red hardness”. Further, tool steel should not be brittle for smooth working.
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