Stainless Steel for Pressure Vessels Materials
Stainless steels are alloys of iron and chromium, typically with at least 12% chromium. Additionally, the 300 series stainless steels contain nickel. A term commonly used for Type 304 stainless steel is 18-8, for 18% chromium-8% nickel. Other alloying elements such as molybdenum, titanium, and niobium are added for specific purposes.
Stainless steels are classified as either austenitic, ferritic, martensitic, or duplex depending on their microstructure.
Austenitic stainless steels have an austenite structure similar to the high temperature structure of carbon steel. Austenitic stainless steels will not harden with heat treatment. They are nonmagnetic. Examples are Type 304, 316, 321 and 347. Austenitic stainless steels are readily weldable and are used both for cladding and in solid wall construction.
Ferritic stainless steels have a ferrite structure similar to the low temperature structure of carbon steel. Typical examples are Types 405 and 430. Ferritic stainless steels will not harden with heat treatment. They are magnetic and usually do not contain nickel. Their use in pressure vessels is primarily as cladding, such as the Type 405. Solid ferritic stainless construction is limited due to poor weldability.
Martensitic stainless steels can be hardened with heat treatment. They are magnetic. Type 410 stainless is the most common example. Their use in pressure vessels is primarily as cladding. Solid martensitic construction is limited due to poor weldability.
Duplex stainless steels have structures of roughly 50% austenite and 50% ferrite. They are nonhardenable by heat treatment. The duplex stainless steels are not currently widely used for pressure vessels but could be considered for both cladding and solid wall construction. They have corrosion properties similar to the austenitics but are higher in strength. They share some of the limitations of both the ferritics and austenitics.
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