Stress corrosion resistance of welded steel tube after Heating Treatment

The stress corrosion of welded steel pipe can only occur under certain circumstances, and it also has the following characteristics:(1) Stress corrosion cracking can occur only when there is stress (especially tensile stress). This stress can be an applied stress, or a residual stress introduced during welding and T addition, or a dilation stress caused by the wedging of the corrosion product.(2) Stress corrosion fracture is a time-dependent hysteresis failure, which is similar to hydrogen hysteresis cracking.(3) Stress corrosion cracking is a low stress brittle fracture.

Stress corrosion cracking is a common cause of failure of petrochemical equipment, especially welded steel pipes for petroleum pipelines. Due to the unpredictability of stress corrosion cracking, it is a major hidden danger in the petrochemical industry. The condition of stress corrosion cracking is that the tensile stress and the corrosive medium coexist, and the tensile stress is an important condition for generating stress corrosion. In equipment such as petrochemicals and nuclear power plants, most of the stresses causing stress corrosion cracking are caused by residual tensile stress, which tends to be superimposed on the working stress. The residual tensile stress mainly comes from the residual tensile stress generated by the equipment during the welding process. At present, extensive annealing and cooling are used in engineering to eliminate residual stress, and post-weld cooling is an important process of residual stress, which wastes energy and is prone to large welding residual stress. Post-weld heat treatment is a new technique to eliminate residual stress. The material is preheated to the post-heat treatment temperature before welding and the weldment is continuously heated to maintain this temperature during the welding process. After the welding is completed, the insulation is used to keep it warm and slowly cooled. This method can effectively reduce the welding residual stress and improve the stress corrosion resistance of the metal.

The 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. Post-weld heat treatment can also effectively improve the stress corrosion resistance of welded steel pipes. The higher the temperature of post-heat treatment, the more significant the improvement of stress corrosion resistance.

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