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 In metallurgy, Stainless Steel (inox) is defined as a ferrous alloy with a minimum of 10.5% chromium content. The name originates from the fact that Stainless Steel stains, or rusts, less easily than ordinary steel. Stainless Steel has higher resistance to oxidation (rust) and corrosion in several environments. High oxidation resistance in air at ambient temperature is normally achieved with additions of more than 12% (by 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. This phenomenon is called passivation by materials scientists, and is seen in other metals, such as aluminium.  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. When disassembled, the welded material may be torn and pitted, an effect that is known as galling. There are different types of Stainless Steels: when nickel, for instance is added the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels are used as razor blades, cutlery, tools etc.
Significant quantities of manganese have been used in many Stainless Steel recipes. Manganese preserves an austenitic structure in 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. "Superaustenitic" Stainless Steels, such as alloy AL-6XN, exhibit great resistance to chloride pitting, crevice corrosion and stress-corrosion cracking over the 300 series.
 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, aluminium 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.
The AISI defines the following grades among others:
200 Series—austenitic iron-chromium-nickel-manganese alloys.
300 Series—austenitic iron-chromium-nickel alloys
- Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working.
- Type 303—equivalent to ISO[2]2 A1. Free machining version of 304 via addition of sulfur.
- Type 304—the most common; the classic 18/8 Stainless Steel; equivalent to ISO A2.
- Type 316—for food and surgical Stainless Steel uses; Alloy addition of molybdenum to prevent specific forms of corrosion; 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". Also known as "surgical steel".
- Type 430—decorative, e.g. for automotive trim; ferritic.
- Type 440—a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly.
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