Stainless Steel

Stainless steel also known as inox steel, is a metal alloy formed by alloying chromium with steel, It generally contains 10.5% chromium content by mass. Stainless steel does not readily corrode, rust or stain with water, because the chromium makes an oxide film on the surface of the steel.

stainless steel rods

There are other elements used to make stainless steel as well, including nickel, nitrogen and molybdenum. Bringing these elements together forms different crystal structures that enable a variety of properties in machining, welding and forming.

stainless steel pipes

High oxidation-resistance in air at ambient temperature is normally achieved with additions of a minimum of 13% by weight chromium, and up to 26% is used for harsh environments. The chromium forms a passivation layer of chromium(III) oxide (Cr₂O₃) when exposed to oxygen. The layer is too thin to be visible, and the metal remains lustrous. The layer is impervious to water and air, protecting the metal beneath. Also, this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium. Corrosion-resistance can be adversely affected if the component is used in a non-oxygenated environment.

Types of stainless steel 

There are four major types of stainless steel:

•  Austenitic stainless steel: Austenitic  stainless steels have an austenitic crystalline structure, which is a face-centered cubic crystal structure. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.

• Ferritic stainless steel: Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, because of the lower chromium and nickel content. Most compositions include molybdenum; some, aluminium or titanium.

• Martensitic stainless steel: Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machinable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12%–14%), molybdenum (0.2%–1%), nickel (less than 2%), and carbon (about 0.1%–1%) giving it more hardness but making the material a bit more brittle. It is quenched and magnetic.

• Duplex stainless steel: It have a mixed microstructure of austenite and ferrite. Duplex stainless steels have roughly twice the strength compared to austenitic stainless steels and also improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19%–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.

 Applications:

• cookware, 

• cutlery, 

• household hardware,

•  surgical instruments,

•  industrial equipment e.g, in sugar refineries,

•  as an automotive and aerospace structural alloy,

•  construction material in large buildings,

•  Storage tanks and tankers used to transport orange juice and other food are often made of stainless steel, because of its corrosion resistance.