Stainless Steel
There are different types of stainless steel used in piping. After carbon steel, Stainless Steel is the second most widely used material in Process industries because of its excellent corrosion resistance property. Stainless steel (SS) is alloy steel with a minimum chromium content of 10.5 % or more, and maximum carbon content of less than 1.20%.
SS has excellent resistance to corrosion and good ductility. Stainless steel becomes corrosion resistive due to the formation of non-reactive chromium oxide (Cr2O3) film that adheres tightly to the surface of the metal. This film acts as a barrier and protects metal against corrosion.
Types of Stainless Steel
There are different types of Stainless Steel used in industries. Based on microstructure SS can be classified as
- Austenitic stainless steel
- Ferritic stainless steel
- Martensitic stainless steel
- Precipitation Hardened stainless steel
- Duplex / Super Duplex stainless steel
Austenitic stainless steel
- Austenitic stainless steel is non-magnetic in nature and has very high corrosion resistance.
- High Corrosion resistance at temp up to 1500°F.
- Hardenable by cold working.
- Face centered Cubic –FCC (fine grain structure).
- Possess high impact strength at low temp.
- These types of SS are widely used in the process industry and a variety of industrial applications, cookwares are also manufactured from Austenitic stainless steel.
- Type 304, 304L, 316, 316L are some of the examples of Austenitic stainless steel.
Ferritic stainless steel
- Ferritic stainless steel is magnetic and contains high carbon, therefore it is brittle and relatively poor corrosion resistance than austenitic SS.
- Non-hardenable by heat treatment.
- Body-centered cubic -BCC (coarse grain structure).
- Ferritic stainless steel has a high resistance to chloride stress corrosion cracking so it is used in marine, petrochemical, heat exchangers, furnace application, where high chloride content is there.
- Type 409, 430, 439 are examples of Ferritic stainless steel.
Martensitic stainless steel
- Martensitic stainless steel has high hardness and poor corrosion resistance than austenitic SS.
- Heat treatable to high hardness level.
- Crystallographic form-distorted structure.
- It used in the manufacturing of sports knives and multi-purpose tools.
- Type 410,420 are examples of Martensitic stainless steel.
Precipitation Hardened stainless steel
- Precipitation Hardened stainless steel is magnetic and heat treatable to high strength, it has a very high strength-to-weight ratio with good corrosion resistance.
- It used in making aerospace components and springs.
- 17-7PH, 17-4PH are examples of Precipitation Hardened stainless steel.
Duplex / Super Duplex stainless steel
- Duplex and Super Duplex stainless steel contain both Austenite and Ferrite in the microstructure
- Offers benefits of both Austenite & Ferrite stainless steel
- Good resistance to pitting and crevice corrosion.
- High strength and high resistance to stress corrosion cracking.
- Used in the seawater system, heat exchanger, and structural application
- EX- UNS S32205, S31803, S32760 are examples of Duplex and Super Duplex stainless steel