Welding heat treatment effect on stress corrosion resistance of welded steel pipe

Welded steel stress corrosion can occur only under certain circumstances, it has the following characteristics: only the presence of stress (tensile stress in particular) is to stress corrosion cracking. 

Welded steel stress corrosion can occur only under certain circumstances, it has the following characteristics: only the presence of stress (tensile stress in particular) is to stress corrosion cracking. This stress can be applied stress or residual stress introduced during the welding and processing of T, it can be expanded to stress corrosion products caused by the wedge effect. Stress corrosion cracking is a time lag of about destruction, which delayed cracking and hydrogen completely similar. Stress corrosion cracking is a low-stress brittle fracture. 
Stress corrosion is the damage under tensile stress, caused by metal corrosion medium. This corrosion generally across the grain, which is called transgranular corrosion. Strain the combined effects of stress corrosion and corrosion by the residual or applied stress generated by the destruction caused by the process. Cause stress corrosion fracture of materials known as stress corrosion cracking. Stress corrosion is generally considered the anodic dissolution and hydrogen induced cracking two kinds. Stress corrosion is a common mechanism: the parts or in the medium under the action of stress and corrosion, the surface of the oxide film is etched and damaged, and does not damage the surface of the surface damage formed in the anode and cathode, respectively, at the anode metal ion is Be dissolved, resulting in a current to the cathode. Since the anode is much smaller area than the cathode, the anode current density is large, the destroyed surface further corrosion. Plus tensile stress the role of the destruction of the gradual formation of cracks, crack over time gradually extended until it breaks. Such cracks can not only develop along the metal grain boundaries, but also across the grain development. Stress corrosion cracking is a petrochemical equipment, especially oil pipeline is a common cause of failure of welded steel pipe. Due to the unpredictability of stress corrosion cracking occurred, and therefore the largest petrochemical industry hazards. A necessary condition for the occurrence of stress corrosion cracking and corrosion medium tensile stress coexistence, the tensile stress is an important condition for stress corrosion. In the petrochemical and nuclear industries equipment, most of the stress caused by stress corrosion cracking is caused by the residual tensile stress, it is often superimposed on job stress. Mainly from tensile residual stress generated in the device residual stresses during welding. Currently, the project is widely used to eliminate residual stress annealed after welding cooling, and cooling after welding residual stress is an important process, this approach is not a waste of energy and easy to produce larger welding residual stresses. Post weld heat treatment is a new technique to eliminate residual stress. After the material is preheated before welding to heat treatment temperature and the welding process to maintain continuous heating of the weld member that temperature after soldering using its thermal insulation cotton allowed to cool slowly. This method can effectively reduce the welding residual stress, improve resistance to stress corrosion of metals. 
Post weld heat treatment can effectively reduce the welding residual stress, and heat treatment temperature is higher, the residual stress better. Heat treatment after welding can be effectively improved stress corrosion resistance welded steel pipe, the higher the heat treatment temperature, stress corrosion resistance increased more significant.

Source: http://www.articlesfactory.com/articles/business/welding-heat-treatment-effect-on-stress-corrosion-resistance-of-welded-steel-pipe.html