Friday, 12 February 2016

Stainless steels

(i) Stainless steels:

What is stainless steel ? Types of stainless steel

          Stainless steel is iron base alloy that has a great resistance to corrosion because of this these steels are called stainless. 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 to 980, 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 electrical 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 color and beauty of the basic materials is not affected.

Types of stainless steels

Stainless steels are further divided into the following three categories:

(a) Ferrite stainless steel:
          These steels contain a maximum of 0.15% carbon, 6–12% chromium, 0.5% nickel besides iron and usual amounts of manganese and silicon. These steels are stainless and relatively cheap. They are also magnetic. These steels have a similar micro-structure to carbon and low alloy steels. These days, one and two rupee coins are made from such steels. These steels are essentially Iron-chromium alloys and 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. Main usage for such steel is in manufacture of dairy equipment, food processing plants, chemical industry etc., 

(b) Martensitic stainless steel:
          These stainless steels have 12–18% chromium but contain higher carbon percentage (0.15-1.2%). These are also called chromium steel, and are hardenable and magnetic. These steels can be hardened by heat treatment, but their corrosion resistance is reduced. They are used where high strength and moderate corrosion resistance is required. They have generally low weldability and formability. They are magnetic. These steels are used for making surgical knives, hypodermic needles, bolt, nut screws and blades etc.,

(c) Austenitic stainless steels:
          These are the most important and costliest among all stainless steels. Austenitic steels have austenite as their primary phase. In these steels, besides chromium, nickel is also added. Nickel is a very strong austenite stabilizers and therefore the micro-structure of these steels is austenitic at room temperature. The most common amongst stainless steel is 18/8 steel. Its composition is 24% chromium, 8% nickel, 0.08- 0.2% carbon, manganese 1.25% maximum and silicon 0.75% maximum. Such steels have extremely good corrosion resistance but they cannot be hardened by heat-treatment. However, they are very susceptible to "strain hardening". In fact, due to strain hardening, their machining becomes very difficult. It is generally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel. It is used extensively for household utensils and in chemical plants and other places where high corrosion resistance is required.

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