What are the process methods for heat treatment of austenitic stainless steel

The stainless steel undergoes heat treatment, which can effectively improve processing performance, prepare a good microstructure for the final heat treatment, and eliminate internal stress, etc. 

The general processing methods for heat treatment of austenitic stainless steel include: stress relief annealing treatment, solution treatment, sensitization treatment, stabilization treatment, and heat treatment to eliminate σ phase. 

Strain-relief annealing treatment 

Strain relief treatment is a heat treatment process used to eliminate residual stress in steel after cold processing or welding. It typically involves heating to 300-350℃ and then tempering. For steels without stabilizing elements such as Ti and Nb, the heating temperature should not exceed 450℃ to prevent the formation of chromium carbides, which could cause intergranular corrosion. For cold-worked and welded parts of ultra-low carbon and stainless steels containing Ti and Nb, they need to be heated at 500-950℃, then slowly cooled to relieve stress (for eliminating welding stress, use the upper limit temperature), which can reduce the tendency of intergranular corrosion and enhance the stress corrosion resistance of the steel. 

2. Solid solution treatment 

The steel is heated to 1050 - 1150℃ and quenched in water. The main purpose is to allow the carbides to dissolve into the austenite and to retain this state at room temperature. This will significantly improve the corrosion resistance of the steel. As mentioned above, to prevent intergranular corrosion, a solution treatment is usually employed, where Cr23C6 is dissolved into the austenite and then rapidly cooled. For thin-walled parts, air cooling is used; in general, water cooling is adopted. 

3. Sensitization treatment 

Heating within the temperature range of 400 to 800 degrees Celsius is used to test the steel's resistance to intergranular corrosion, and is called sensitization treatment. This temperature range is referred to as the sensitization temperature. Except in special cases, it is advisable to avoid heating the steel within the sensitization temperature range. Solution treatment is to re-solubilize the precipitated chromium carbide into the austenite. The solution treatment process can be used to eliminate the influence of sensitization treatment. 

4. Stabilization treatment 

This is usually carried out after the solution treatment. It is commonly used for 18-8 steels containing Ti and Nb. After the solution treatment, the steel is heated to 850-880℃, held for a while, and then cooled slowly. At this point, the carbonitrides of Cr are completely dissolved, while those of Ti are not fully dissolved. During the cooling process, they are fully precipitated, preventing the formation of grain-boundary carbides and thus effectively eliminating intergranular corrosion. 

5. Heat treatment to eliminate σ phase 

The σ phase is a hard and brittle FeCr intermetallic compound. Its presence reduces the toughness, corrosion resistance and oxidation resistance of steel. The σ phase is most likely to occur in high-chromium ferrite. It can also appear in austenite-ferrite steel and austenitic steel. The σ phase can dissolve in austenite at high temperatures, and its existence in steel occurs at a temperature of 820℃. The heat treatment to eliminate the σ phase is to heat above its upper limit temperature. For 1Crl8Ni9Nb, the σ phase will disappear after heating at 850℃. The upper limit temperature at which the σ phase exists varies depending on the steel composition, so the specific heating temperature should be determined through experiments.