Stainless steel is usually divided into 5 types:
- Precipitation hardening (PH) Stainless Steels - These stainless steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the stainless steel. With a suitable “aging” heat treatment, very fine particles form in the matrix of the stainless steel which imparts strength. These stainless steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. This is in contrast to conventional hardening and tempering in martensitic stainless steels where distortion is more of a problem. Corrosion resistance is comparable to standard austenitic stainless steels like 1.4301 (304).
- Austenitic Stainless Steels - These stainless steels are the most common. Their microstructure is derived from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary stainless steels at much higher temperatures. This structure gives these stainless steels their characteristic combination of weldability and formability. Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard austenitic stainless steels are vulnerable to stress corrosion cracking. Higher nickel austenitic stainless steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the stainless steel.
- Martensitic Stainless Steel - These stainless steels are similar to ferritic stainless steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy stainless steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic.
- Duplex Stainless Steels - These stainless steels have a microstructure which is approximately 50% ferritic and 50% austenitic. This gives them a higher strength than either ferritic or austenitic stainless steels. They are resistant to stress corrosion cracking. So called “lean duplex” stainless steels are formulated to have comparable corrosion resistance to standard austenitic stainless steels but with enhanced strength and resistance to stress corrosion cracking. “Superduplex” stainless steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic stainless steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.
- Ferritic Stainless Steels – These stainless steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These stainless steels have a similar microstructure to carbon and low alloy stainless steels. They are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium stainless steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic.
Source: Yaang Pipe Industry (www.yaang.com)
