Effect of Heat Treatment after Welding on Stress Corrosion Resistance of Welded Steel Pipes

Stress corrosion of welded steel pipe can only occur under certain circumstances. It also has the following characteristics: (1) Stress corrosion cracking can only occur when stress (especially tensile stress) is present. This stress can be applied stress, or the residual stress introduced during the welding and adding T process, or it can be the expansion stress caused by the wedge effect of the corrosion product. (2) Stress corrosion cracking is a time-dependent lag failure, which is completely similar to hydrogen hysteresis cracking. (3) Stress corrosion cracking is a kind of low stress brittle fracture.

Stress corrosion cracking is a common cause of failure of petrochemical equipment, especially welded steel pipes used in petroleum pipelines. Due to the unpredictability of stress corrosion cracking, it is the biggest hidden danger in the petrochemical industry. The necessary condition for stress corrosion cracking is the coexistence of tensile stress and corrosive media, and tensile stress is an important condition for stress corrosion. In equipment such as petrochemicals and nuclear power plants, most stresses that cause stress corrosion cracking are caused by residual tensile stress, which is often superimposed on working stress. The residual tensile stress mainly comes from the residual tensile stress generated by the equipment during the welding process. At present, it is widely used in engineering to perform annealing treatment after welding cooling to eliminate residual stress, and post-weld cooling is an important process of residual stress generation. This practice wastes energy and easily produces large welding residual stress. Post-weld heat treatment is a new technology to eliminate residual stress. The material is preheated to the post-heat treatment temperature before welding and the weldment is continuously heated during the welding process to maintain this temperature. After the welding is completed, the heat-preserving cotton is used to insulate it and slowly cool it. This method can effectively reduce the welding residual stress and improve the stress corrosion resistance of the metal.

Post-weld heat treatment can effectively reduce the welding residual stress, and the higher the heat treatment temperature, the better the residual stress elimination effect. The post-weld heat treatment can also effectively improve the stress corrosion resistance of the welded steel pipe. The higher the post-heat treatment temperature, the more significant the improvement of the stress corrosion resistance.