In metallurgy, stainless steel is defined 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:
- Austenitic stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition is 18% chromium and 8% nickel, commonly known as 18/8 stainless.
- Ferritic stainless steels are highly corrosion resistant, but far less durable than austenitic grades and cannot be hardened by heat treatment. They contain between 10.5% and 27% chromium and very little nickel, if any. Most recipes include Molybdenum; some, aluminum or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.
- Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely strong and tough as well as highly machineable, and can be hardened by heat treatment. They contain 11.5 to 18% chromium and significant amounts of carbon. Some grades include additional alloying elements in small quantities.
- 200 Series - austeniitic iron-chromium-nickel-manganese alloys
- 300 Series - austeniitic iron-chromium-nickel alloys
- Type 301 - highly ductile, for formed products
- Type 303 - equivalent to ISO  A1.
- Type 304 - the most common; austenitic (contains nickel); equivalent to ISO A2.
- Type 316 - for food and medical uses; contains nickel and molybdenum; also called 18-8; equivalent to ISO A4.
- 400 Series - ferritic and martensitic alloys
- Type 408 - heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
- Type 409 - cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
- Type 410 - martensitic (high-strength iron/chromium); equivalent to ISO C1.
- Type 420 - "Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel".
- Type 430 - decorative, e.g. for automotive trim; ferritic.