The importance of pre-weld preheating and post-weld heat treatment

Preheating before welding and heat treatment after welding have great influence on welding quality. Preheating before welding is required for important components. The main functions of preheating before welding are as follows:

It can slow down the cooling rate after welding, which is conducive to hydrogen diffusion and escape in weld metal, reduce the hardening degree of weld and heat affected zone, reduce the temperature difference between the weldments in the welding area, reduce the welding stress and welding strain rate, avoid welding cracks, and reduce the restraint degree of welding structure. With the increase of preheating temperature, the crack rate decreases.

Preheating temperature is related to welding method, rigidity, environment temperature, etc. the uniformity of preheating temperature in the weld area is conducive to reducing welding stress. The local preheating width shall be determined according to the binding degree of welding parts. If the preheating is not uniform, the welding stress will not decrease, but will increase.

Post weld heat treatment

Post weld hydrogen elimination treatment is a low-temperature heat treatment after welding without cooling the weld. Its main function is to accelerate the escape of hydrogen in the weld and heat affected zone, which has obvious effect on preventing welding cracks in low alloy steel welding.

In the welding process, due to the non-uniformity of heating and cooling, the welding stress will be generated in the component, which will reduce the bearing capacity of the welded joint area and produce plastic deformation, and even damage the component in serious cases.

Stress relief heat treatment is to reduce the yield strength of welded workpiece at high temperature, so as to relax the welding stress. The common method is overall high-temperature tempering. The whole weldment is put into the heating furnace, heated to a certain temperature, maintained for a period of time, cooled, which can eliminate 80% - 90% of the welding stress. Local high-temperature tempering only heats the weld and its vicinity, and slowly cools, The peak value of welding stress is reduced, the stress distribution is relatively smooth, and the welding stress is partially eliminated.

Some alloy steel after welding, the welded joint will appear hardening structure, make the material mechanical properties worse, in the welding stress and hydrogen under the action of joint damage, after heat treatment, the joint microstructure is improved, plasticity and toughness is improved, so as to improve the comprehensive mechanical properties of the welded joint.

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