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Stainless steels
are chromium containing steel alloys. The minimum chromium content
of the standardised stainless steels is 11%. Chromium makes the
steel 'stainless' 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 and still represents
80% of the world market. It has a nickel content of at least of
7%, which makes the steel structure fully austenitic and gives it
ductility, a large scale of service temperature, non-magnetic properties
and good weldability. 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. This steel has some
nickel content for a partially austenitic lattice structure. The
duplex structure delivers both strength and ductility. Duplex steels
are mostly used in petrochemical, paper, pulp and shipbuilding industries.
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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