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WHAT ARE STAINLESS STEELS?
Stainless Steel may be considered as a family of alloy steels developed from the basic iron, manganese and carbon system. Chromium is the essential alloying element and it must comprise at least 11½% by weight of the composition before the alloy may be considered "stainless", i.e. corrosion resistant.
As the chromium content is increased to 20% and beyond, steels will demonstrate unusual resistance to surface deterioration at high temperatures.
Nickel is also used in some grades of stainless steels. In amounts in excess of 7% nickel enhances the corrosion resistance of the corresponding straight chromium grades and at the same time, greatly improves its ductility. A principle feature of the nickel-chromium grades is that they may be "work hardened", i.e. strengthened by cold working the steel.
SUMMARY
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300 Series |
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12 to 35% Chromium
7 to 20% Nickel |
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400 Series |
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12 to 25% Chromium |
WHY STAINLESS STEEL IS "STAINLESS"
Like carbon steel, stainless steel rusts, but instead of forming loose brown iron oxide, stainless steel forms an invisible, tenacious and highly protective surface film of chromium oxide. It is the presence of the element chromium in excess of 11½% that is responsible for the formation of the film, but oxygen must be available from the surroundings to complete the combination.
It follows that stainless steels will perform best in oxygen rich environments. The addition of nickel assists the formation and stabilization of this surface layer.
The surface layer of chromium oxide naturally resists attack by a wide variety of chemicals, and as long as continuous film is present, the metal will exhibit maximum corrosion resistance and is said to be a "passive" condition.
Passivation treatments involve exposing stainless steel to oxygen rich or oxidizing environments to produce a continuous, passive surface film. It is interesting to point out that stainless steels can resist the attack of corrosive solutions if the solution is aerated to maintain the chromium oxide surface film.
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5506-238th St.
Langley, BC
V2Z 2P1
(604) 533-4400
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Langley BC
V2Y 0E5
(604) 533-4400
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Langley Airport (CYNJ)
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Email: contacts@mcwelding.com
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