The main alloying element in Stainless steels is chromium (typically between 10-20%). Stainless Steel is valued due to its high corrosion resistance. Stainless steel is about 200 times more resistant to corrosion than mild steel.
Stainless steel can be divided into five groups:
- Austenitic: Austenitic steels form the largest portion of the global stainless steel market and few of its versatile usages would be food processing equipment, kitchen utensils and medical equipment. Austenitic stainless steels are the most weld-able of the stainless grades and can be divided rather loosely into three groups: common chromium-nickel (300 series), manganese-chromium-nickel-nitrogen (200 series) and specialty alloys. These metals are non-magnetic and not heat-treatable.
- Ferritic: Ferritic steels contain trace amounts of nickel, 12-17% chromium, less than 0.1% carbon, along with other alloying elements, such as molybdenum, aluminium or titanium. They can have good ductility and formability, but high-temperature strengths are relatively poor when compared to austenitic grades. Some ferritic stainless grades (such as types 409 and 405) cost less than other stainless steels. These metals are magnetic but are not heat treatable, they can be strengthened by cold working.
- Martensitic: Martensitic steels contain 11-17% chromium, less than 0.4% nickel and up to 1.2% carbon.The carbon content of this harden-able steel affects forming and welding. To obtain useful properties and prevent cracking, preheating and post-weld heat treatment is required. Martensitic stainless steels, such as types 403, 410, 410NiMo and 420 are magnetic and heat-treatable. These Stainless steels are used in knives, cutting tools, as well as dental and surgical equipment.
- Duplex: Primarily used in chemical plants and piping applications. Duplex stainless steels typically contain approximately 22-25% chromium and 5% nickel with molybdenum and nitrogen. Duplexes have higher yield strength and greater stress corrosion cracking resistance to chloride than austenitic stainless steels.
- Precipitation Hardening: This is a chromium-nickel stainless that also contains alloying additions such as aluminium, copper or titanium. These alloys allow the stainless to be hardened by a solution and aging heat treatment. They can be either austenitic or martensitic in the aged condition.
Choosing a particular type of stainless steel can depend on the application is and the requirements. The things that are considered may be:
- Corrosion and Corrosion resistance
- Tarnish and oxidation resistance
- Crevice corrosion
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