In this article we will discuss about:- 1. Meaning of Stainless Steels 2. Corrosion Resistance of Stainless Steels 3. Classification 4. Use of Rare Earths in the Manufacture.
Meaning of Stainless Steels:
Stainless steel is iron base alloy that has a great resistance to corrosion. 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. Within 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.
Mechanism of the Corrosion Resistance of Stainless Steels:
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 material is not affected.
Classification of Stainless Steels:
ADVERTISEMENTS:
Stainless steels can be classified into the following groups:
(i) Austenitic Stainless Steels:
These contain at least 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. These are non-hardenable and non-magnetic.
(ii) Martensitic Stainless Steels:
ADVERTISEMENTS:
These are alloys of iron, carbon and chromium. The difference between the percentage of chromium and seventeen times the percentage of carbon is less than 12.5%. These are also called chromium steels, and are hardenable and magnetic.
(iii) Ferritic Stainless Steels:
These are alloys of iron, carbon and steel. In these the difference between the percentage of chromium and seventeen times the percentage of carbon is greater than 12.5%. These are magnetic and non-hardenable.
(iv) Low Chromium Stainless Steels:
ADVERTISEMENTS:
These contain 4 to 6% of chromium. These have oxidation-resistant properties at high temperatures.
Use of Rare Earths in the Manufacture of Stainless Steels:
Rare earths are used to manufacture stainless steel. Such a steel has the plasticity of deformation at rolling and forming temperatures. Ordinary stainless steels are difficult, rather impossible to roll because they are hot short, that is, they are brittle at high temperature.
Table below shows the influence of addition of metals in iron for the formation of alloy steels on their properties and the use of resultant alloy steels:
