Stainless steel is an alloy of Iron and Chromium with high corrosion resistance. More Chromium gives increased resistance to corrosion. Steel stays “stain-less” in slightly aggressive environments, when there is no less than 11% Chromium in the alloy, and no less than 17% Chromium make an alloy corrosion resistant in aggressive environments. Apart from other chemical elements stainless steel might include Nickel, Nitrogen, Titanium and Molybdenum. High corrosion resistance is explained by a thin layer of oxide on the surface of the steel known as the 'passive layer'. This layer protects steel from aggressive environments. It rebuilds immediately when damaged.
Stainless steel has the following properties:
- High corrosion resistance, durability and strength.
- Oxidation resistance at high temperatures.
- Hygienic due to smooth and minimally porous surface (resistant to contamination and bacteria)
- Hot strength.
- Attractive appearance.
- High ductility.
Stainless steel types
There four major stainless steel types:
- Austenitic stainless steels.
- Ferritic stainless steels.
- Duplex stainless steels.
- Martensitic stainless steels.
The most widespread are Austenitic and Ferritic.
Austenitic stainless steels are the most widespread. Their microstructure is derived from the addition of no less than 7% of Nickel, this structure gives these steels their characteristic combination of weldability and formability. These steels include:
- AISI 304 (08Х18Н10), AISI 321 (08Х18Н10Т) are the most widespread steels used for food processing industry equipment production;
- AISI 316 (10Х17Н13М2Т) include Molybdenum and Titanium. This provides production opportunities for more aggressive environments. Austenitic steels are nominally non-magnetic, so they don’t attract magnetic particles, which otherwise could result in surface contamination.
Ferritic stainless steels have properties close to mild steel, but with higher corrosion resistance. The most famous steel of this type is AISI 430 (12Х17), which is used in households, décor and household appliances production. It is rather cheap, but has a number of significant drawbacks:
- Lower corrosion resistance and ductility compared with austenitic grades.
- Special requirements for welding (They are hardened by cold working and are not heat treatable).
- Ferritic stainless steels are magnetic. They attract particles to the surface, which leads to contamination and results in corrosion.