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News

Guide to stainless steel

Outokumpu Oyj : 25 October, 2010  (Technical Article)
The excellent qualities of stainless steel make it an ideal choice for various demanding applications from cutlery to food and chemical processing plants and oil platforms.
Many types of stainless steels have been developed to resist different corrosion environments and working conditions ensuring that factories are safe, structures last longer and our food is hygienic. Stainless steel is even used for in systems to clean up the exhaust gases from cars and power stations.

Stainless steel is also recyclable: when scrapped, it can be re-melted to make something new.

Stainless steels are chromium containing steel alloys. The minimum chromium content of the standardised stainless steels is 10.5%. The Chromium makes the steel “stainless” and this means improved corrosion resistance

The better corrosion resistance is due to a chromium oxide film that is formed on the steel surface. This extremely thin layer, under the right conditions, is also self-repairing.

Besides chromium, typical alloying elements are molybdenum, nickel and nitrogen. Nickel is mostly alloyed to improve the formability and ductility of stainless steel. Alloying these elements brings out different crystal structures to enable different properties in machining, forming, welding etc.

The four major types of stainless steel are:

•Austenitic
•Ferritic
•Austenitic-Ferritic (Duplex)
•Martensitic

Austenitic is the most widely used type of stainless steel. Nickel and manganese are two alloying elements that will make the structure fully austenitic if added in appropriate amounts. An austenitic structure promotes ductility, non-magnetic properties, good weldability and possibilities for a wide range of service temperatures. The range of applications of austenitic stainless steel includes housewares, containers, industrial piping and vessels, architectural facades and constructional structures.

Ferritic stainless steel has properties similar to mild steel but with the better corrosion resistance. The most common of these steels are 12% and 17% chromium containing steels, with 12% used mostly in structural applications and 17% in housewares, boilers, washing machines and indoor architecture.

Austenitic-Ferritic (Duplex) stainless steel has a ferritic and austenitic lattice structure - hence common name: duplex stainless steel. Duplex stainless steels have a smaller nickel-content than the austenitic grades. The structure is 50 % ferritic and 50 % austenitic and this gives a layered structure with high strength. The Duplex grades are widely used in tank applications, pulp & paper industry, oil& gas industry and in desalination.

Martensitic stainless steel contains mostly 11 to 13% chromium and is both strong and hard with moderate corrosion resistance. This steel is mostly used in turbine blades and in knives.
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