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Stainless steel

In metallurgy, stainless steel is defined[1] as a ferrous alloy with a minimum of 10.5% chromium content. Such steels have higher resistance to oxidation (rust) and corrosion in several environments. It was invented in 1912, at the research laboratory of Brown-Firth, Sheffield, England by Harry Brearley. He had been investigating ways to reduce corrosion in gun-barrels, when it was noticed that a discarded sample was not rusting.

High oxidation resistance in air at ambient temperature is normally achieved with additions of more than 12%(weight) chromium. The chromium forms a layer of chromium (III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, meaning the metal stays shiny. It is, however, impervious to water and air, protecting the metal beneath. Also, when the surface is scratched this layer quickly reforms. When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off causing the parts to weld together. This effect is known as galling.

There are different types of stainless steels: when nickel, for instance is added the austenite structure of iron is stabilized and these steels become non-magnetic. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels are used as razor blades, cutlery, tools etc.

In recent decades, significant quantities of manganese have come to be used in many stainless steel recipes. Manganese imparts similar qualities to the steel as does nickel, but at a lower cost.

Stainless steels are also classified by their crystalline structure:

The AISI defines the following grades:


[1] AISI (American Iron and Steel Institute)
[2] ISO 3506 standard