Stainless steel is iron base alloy that has a great resistance to corrosion. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust (or) stainless than other types of steel It is observed that a thin, transparent and very tough film forms on the surface of stainless steel which is inert or passive and does not react with many corrosive materials. In a temperature range of 235°c to 980°c, it exhibits strength, toughness and corrosion resistance superior to other metals. It is thus ideally suited for handling and storage of liquid helium, hydrogen, nitrogen and oxygen that exist at cryogenic temperature. The property of corrosion resistance is obtained by adding chromium only (or) by adding chromium and nickel together. Stainless steel is manufactured in electric furnaces.
Why stainless steel is corrosive resistance
Stainless steels are very slightly oxidisable. Slight oxidation forms very thin film of oxide and this oxide film acts as a protective coating and in this way further corrosion is stopped. This protective film of oxide is so thin that the colour and beauty of the basic materials is not affected.
Types of Stainless steel
Stainless steels can be classified into the following groups:
Austenitic Stainless steel
Austenitic steels have austenite as their primary phase. These are alloys containing atleast 24% chromium and nickel combined and the percentage of each of them alone is not less than 8%. These steels may contain other elements for particular purposes. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. These are non-hardenable by heat treatment and non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.
Martensitic Stainless steel
These are alloys of iron, carbon and chromium. The difference between the percentage of chromium and 17 times the percentage of carbon is less than 12.5%. These are also called chromium steel, and are harden able and magnetic. They are used where high strength and moderate corrosion resistance is required.They have generally low weld ability and form ability. They are magnetic.
Ferritic Stainless steel
These are alloys of iron, carbon, steel. These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They cannot be hardened by heat treatment.Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic.
Low Chromium stainless steel
These contain 4 to 6% of chromium. These have oxidation-resistant properties at high temperature.