1. Ferritic Stainless Steel
This type of stainless steel has a matrix composed primarily of ferrite (α phase) with a body-centered cubic (BCC) crystal structure. It is magnetic and generally cannot be hardened by heat treatment, although it can be slightly strengthened by cold working. It contains 11–30% Cr and may include small amounts of Mo, Nb, and Ti, but is essentially Ni-free. Ferritic stainless steel offers high strength and excellent resistance to chloride stress corrosion, pitting, and crevice corrosion. However, it is sensitive to intergranular corrosion and exhibits poor low-temperature toughness.
2. Martensitic Stainless Steel
This stainless steel has a martensitic matrix and is magnetic. Its mechanical properties can be adjusted through heat treatment. At high temperatures, martensitic stainless steel exists as austenite; after quenching, the austenite transforms into martensite, which gives the steel its name. It exhibits high strength, hardness, and wear resistance, and the desired properties can be achieved through heat treatment. Martensitic stainless steel has good machinability but poor weldability.
3. Austenitic Stainless Steel
Austenitic stainless steel has a matrix composed primarily of austenite (γ phase) with a face-centered cubic (FCC) crystal structure. It is non-magnetic and is strengthened mainly through cold working. Austenitic stainless steel generally contains 16–25% Cr and 7–20% Ni. After appropriate heat treatment, it forms a single-phase austenitic structure at room temperature.
4. Duplex (Austenitic-Ferritic) Stainless Steel
This stainless steel has a matrix composed of both austenite and ferrite phases, with ferrite content ranging from approximately 40% to 60%. It is magnetic and can be strengthened through cold working.
5. Precipitation-Hardened Stainless Steel
Precipitation-hardened stainless steel has a matrix of either austenite or martensite and can be strengthened through precipitation (age) hardening. After solution treatment, it forms a martensitic or semi-austenitic structure, and subsequent precipitation enhances its strength. It has high strength, better uniform corrosion resistance than martensitic stainless steel, and favorable forming and welding properties.