Austenitic Stainless Steels

Austenitic Stainless Steels. Today Piping & Fabrication will talk about and this is the rest of post. Austenitic stainless steels do not undergo phase changes like the ferritic steels. They remain austenitic at all temperatures and so heat treatments usually do not apply. When austenitic stainless steels are to be used in corrosive services, cold working and heating for bending may significantly lower their corrosion resistance. Cold working may result in residual stresses, and heating operations can result in sensitization. Both factors contribute to intergranular stress corrosion cracking (IGSCC). When austenitic stainless steels are heated in the range of about 800 to 1600°F (430 to 870°C), carbon in excess of about 0.02 percent will come out of solution and diffuse to the grain boundaries where it will combine with adjacent chromium to form chromium carbide (Cr23C6). This phenomenon is called sensitization. These grain boundaries are then preferentially attacked by corrosive media. The heat treatment often applied to cold-worked and sensitized stainless steels to restore corrosion resistance is a carbide solution heat treatment. In this procedure, the material is heated to a temperature above the sensitization range, usually about (1950 to 2100°F (1065 to 1150°C), and held there sufficiently long to permit the carbides to dissolve and the carbon to go back into solid solution. The material is then removed from the furnace and rapidly cooled through the sensitization range, preferably by quenching in water. The rapid cooling does not give the carbon sufficient time to come out of solution, and corrosion resistance is restored to the sensitized area.

Heat treatment cycles

Obviously carbide solution heat treatment is limited by the furnace size and quenching facilities. It is most freqently applied to bends but is also useful in reducing sensitization and residual stresses in welds.