Stainless steel is a type of iron alloy that has a minimum of 10.5% chromium in it. The chromium layer forms a ‘passive’ layer that protects the steel from corrosion. This corrosion property can be enhanced by increasing the amount of chromium in the steel. Nickel and Molybdenum are other elements that can also be introduced into the composition of the steel to get desirable properties like enhanced formability and further corrosion resistance. Since a wide range of alloys can be called stainless steel depending on its constituents, stainless steel can be divided into different types.
- Austenitic Stainless Steel: This is the most common type of stainless steel. The microstructure of this steel is obtained from the addition of Nickel, Manganese and Nitrogen. This combination of elements gives rise to the same structure as of that ordinary steel at higher temperatures. Its characteristics are defined by its weld-ability and formability. Normal austenitic stainless steel is prone to stress corrosion cracking but this can be overcome by increasing nickel in the steel. They are considered non-magnetic but may show certain kinds of magnetic properties based on its composition and the work hardening of the steel.
- Ferritic Stainless Steel: These are alloys based on chromium and with a carbon content of less than 0.1%. They are magnetic and are not as formable as Austenitic steels. The microstructure of these alloys is similar to carbon and low alloy steels. The main downside with this steel is its relatively poor toughness in welds, so they have to be used in relatively thin sections. But wherever welding is not required these steels are widely used. Steels with high chromium and addition of Molybdenum can be used in very aggressive conditions like sea water.
- Martensitic Stainless Steel: These steels are similar to ferritic stainless stain as they are high chromium and low carbon. But here carbon content can go up to 1%. The high carbon content allows them to be hardened and thus is used in situations where high strength and moderate corrosion resistance is required. They offer general weld-ability and formability and are magnetic.
- Duplex Stainless Steel: These steels are a mix of approximately 50% Ferritic steel and 50% Austenitic steel. This composition gives them strength superior to both Ferritic and Austenitic steel. It is resistant to stress corrosion cracking. There are two types of Duplex steels namely ‘Lean Duplex’ and ‘Super Duplex’. Lean Duplex is made to have comparable corrosion resistance properties as that of Austenitic steels but with enhanced strength and resistance to stress corrosion cracking. Super Duplex is designed to have enhanced strength and resistance to almost all forms of corrosion. They are weld-able but needs care with weld inputs and heat and have moderate formability. They are magnetic but not as magnetic as Ferritic steels because of the 50% of Austenitic steel present.
- Precipitation Hardening: They are extremely strong steels made with elements like Copper, Niobium and Aluminum. The manufacturing process they undergo, adds tiny particles in the matrix structure of the steel imparting the enhanced strength. They have corrosion resistance comparable to Austenitic steels.
