what is stainless steel ?

what is stainless steel ?

Stainless steel is a low carbon steel which contains chromium at least 11% or more depends on grades.It is the addition of chromium that gives the steel the unique stainless, corosion resisting properties.

The chromium content of the steel allows the formation of a rough, adherent, invisible, corrosion-resisting chromium oxide film on the steel surface.It damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present.

The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content the addition of other elements such as molybdenum, nickel and nitrogen. There are more than 60 grades of stainless steel. However, the entire group can be divided into five classes. Each is identified by the alloying elements which affect their microstructure and for which each in named.

There are five basic categories of stainless steel : austenitic, ferritic, duplex, martensitic and preciptation hardening.

History Stainless Steel

A few corrosion-resistant iron artifacts survive from antiquity. A famous example is the Iron Pillar of Delhi, erected by order of Kumara Gupta I around AD 400. Unlike stainless steel, however, these artifacts owe their durability not to chromium but to their high phosphorus content, which, together with favorable local weather conditions, promotes the formation of a solid protective passivation layer of iron oxides and phosphates, rather than the non-protective cracked rust layer that develops on most ironwork.

The corrosion resistance of iron-chromium alloys was first recognized in 1821 by French metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. Metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium 
found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be practical.

In the late 1890s Hans Goldschmidt of Germany developed an aluminothermic (thermite) process for producing carbon-free chromium. Between

1904 and 1911 several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel.
Friedrich Krupp Germaniawerft built the 366-ton sailing yacht Germania featuring a chrome-nickel steel hull in Germany in 1908.

In 1911, Philip Monnartz reported on the relationship between chromium content and corrosion resistance. On October 17,1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel as ThyssenKrupp Nirosta. Similar developments were taking place contemporaneously in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless steel. In 1912, Elwood Haynes applied for a US patent on a martensitic stainless steel alloy, which was not granted until 1919.

Also in 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, while seeking a corrosion-resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times.  
         
     The metal was later marketed under the 'Staybrite' brand by Firth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in London in 1929.
Brearley applied for a US patent during 1915 only to find that Haynes had already registered a patent. Brearley and Haynes pooled their funding and with a group of investors formed the American Stainless Steel Corporation, with headquarters in Pittsburgh, Pennsylvania.[10] In the beginning stainless steel was sold in the US under different brand names like 'Allegheny metal' and 'Nirosta steel'
 

In 1929 before the Great Depression hit, over 25,000 tons of stainless steel were manufactured and sold in the US.