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Purpose of preheating pipe prior to welding

Important components of the welding, alloy steel and the welding of thick parts must preheat before welding.

The purpose of preheating before welding as follows:

(1) preheating can slow down the cooling rate after welding, diffusion of hydrogen in the weld metal to escape, to avoid hydrogen induced cracking. Also reduce the the hardened extent of the weld and heat affected zone to improve the crack resistance of welded joints.

(2) preheating can reduce welding stress. Uniform local warm-up or overall warm-up can reduce the welding zone temperature between the welders (also known as the temperature gradient). In this way, on the one hand, reduces the welding stress, on the other hand, reducing the welding strain rate, to avoid welding cracks.

(3) preheating can reduce the degree of binding of the welded structure, especially to reduce the degree of the binding of the corner joints, cracks decrease the incidence of the preheat temperature increased.

Welding preheating temperature and interpass temperature selection is not only related to the chemical composition of steel and welding rod, and rigidity of welded structures, welding method, ambient temperature, etc., these factors should be considered and determined.

In addition, the preheating temperature uniformity in the thickness direction of steel in the weld region uniformity, reduce welding stress has an important influence. The width of the local warm-up, should be welders bound of the degree of may be, should generally be around the weld zone three times the wall thickness, and shall not be less than 150-200 mm. If the warm-up and uneven, not only does not reduce the stress of welding, but will increase the welding stress.

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