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Stainless steel grades are the way in which we categorise the mechanical properties and production specifications as well as the composition of stainless steel. There is an old three digit number system which is still used for the classification of stainless steel grades but over time new classification systems have been developed.
The standards are explained in detail by the BSSA or the British Stainless Steel Association as they are known in their “Guide to Stainless Steel Specifications”. Each one of the grades is grouped into one of five stainless steel families.
Stainless steel families are named according to their metallurgical microstructure and are named the following:
Austenitic stainless steels have an excellent resistance to corrosion and when they are annealed they become non-magnetic. These are great for welding and are extremely hygienic due to their properties they cannot be hardened by heat treatment but have an excellent performance level at low temperatures. The stainless steel grades usually found in this category include 304 (1.4301), 303 (1.4305) and 316 / 316L (1.404)
Ferritic stainless steels have moderate corrosion resistance but not as great as the Austenitic stainless steels. They will not be susceptible to stress corrosion and are magnetic. Similarly though they cannot be hardened by heat treatment and are not as good for welding. The common stainless steel grades found within this family include 409 (1.4512) and 430 (1.4016).
Martensitic Stainless Steels also have a moderate resistance to corrosion but again this resistance isn’t as strong as the Austenitic stainless steels however the Martensitic stainless steels are heat treatable and magnetic but are very difficult to weld. The common stainless steel grades found within this family include 420 (1.4021)
Duplex Stainless Steels are very similar to the Austenitic stainless steels in that they have excellent resistance to corrosion but in addition they have increased resistance to attacks from chloride. But it gets better Duplex stainless steels have a good resistance to stress corrosion cracking. It has higher tensile strength than Austenitic or Ferritic grades and is good for welding and formability. Common grades within this group include UNS 31803 also known as 2205 (1.4462)
Precipitation Hardening steels can be both Martensitic, semi-Austenitic or Austenitic and have good to moderate corrosion resistance and are good for welding. These are particularly good because they are both high strength and magnetic.