What is stainless steel?
Stainless steel covers a group of metals that have become very important due to their anti-corrosion or rust resistant qualities. Invented about a century ago, the application of stainless steel is vast, but includes domestic, construction, transport, medical, food and drink, water and sewage and oil and gas.
These iron alloys have a minimum of 10.5% chromium. It is the chromium that gives stainless steels their anti-corrosive properties it creates a thin layer of oxide (the passive layer) on the surface of the steel which prevents corrosion.
Additional alloying elements such as nickel, molybdenum, titanium and copper, are added to enhance certain properties: e.g. strength and cryogenic toughness.
Classes of Stainless Steel:
1. Ferritic
A less expensive class of stainless steel due to its lack of nickel content. Ferritic grades of stainless steel are used for high-temperature applications (such as engine exhausts). As ferritic stainless steels do not weld to a high standard, cant be hardened by heat treatment, and are do not fare well in aggressively corrosive conditions, their use is a little limited.
2. Austenitic
Austenitic stainless steel accounts for about 70% of all stainless steel production. Its makeup is classically 18%Cr8%Ni. This versatile stainless steel is ductile and can be formed and welded with successful results. Austenitic stainless steels sometimes contain molybdenum.
3. Duplex stainless steels
A combination of ferritic and austenitic stainless steels, with intermediate nickel content. This stainless steel is very high strength.
4. Martensitic
Martensitic stainless steels are similar to ferritic steels, but have higher carbon levels (about 0.1-1.2%). They are high strength as they can be hardened by heat treatment, but with poor weldability and moderate corrosion resistance.
5. Precipitation Hardening
These stainless steels are very high strength because other elements, such as copper, aluminium and niobium, are added. They are easily welded.