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The importance of preheating
Welding of important components of welding, welding of alloy steel and thick parts, are required in the preheating before welding. The following main effect of preheating before welding:
(1) the preheating can slow cooling rate annealing after welding, is conducive to the escape of hydrogen diffusion in weld metal, avoid to produce the hydrogen induced crack. At the same time reduce the weld and heat affected zone, quenching degree, improve the crack resistance of welded joints.
(2) the preheating can reduce welding stress. Uniformly local preheating or integral preheating, welding area can reduce the temperature difference between the welders (also known as the temperature gradient). In this way, on one hand reduces the welding stress, on the other hand, reduces the welding strain rate, is conducive to avoid weld cracking.
(3) preheating can reduce the welding structure of the degree of restraint, to reduce the angle joint restraint is particularly evident, with the preheating temperature increase, decrease the incidence of crack.
Preheat and interpass temperature selection and chemical composition of steel and welding not only relevant, also and welded structure rigidity, welding method, the temperature of the environment, should be comprehensive consideration of these factors. In addition, the preheating temperature of steel plate in the thickness direction of the homogeneity and uniformity in the weld area, to reduce the welding has an important impact stress. Local preheating width, should according to the welders restraint conditions, the general should be the weld zone around each three times the thickness of the wall, and not less than 150-200 mm. If the preheating is not uniform, not only reduce the welding stress, but will increase the welding stress situation