Necessity of preheating and post-welding heat treatment for welded pipes with special materials

For welded pipes of special materials, heat treatment shall be carried out before and after welding, which is very important to the welding quality. Preheating before welding of welded pipes of special materials can not only slow down the cooling rate after welding, but also help the diffusion and escape of hydrogen from the weld metal to avoid hydrogen induced cracking. At the same time, the hardening degree of weld and heat affected zone is reduced and the crack resistance of welded joint is improved.

It can also reduce the welding stress, uniformly preheat locally or as a whole, and reduce the temperature difference between weldments in the welding area. On the one hand, it reduces the welding stress, on the other hand, it reduces the welding strain rate, which is conducive to avoiding welding cracks. It can reduce the restraint degree of welded structure, especially for the corner joint. With the increase of preheating temperature, the incidence of crack decreases.

The selection of preheating temperature and interlayer temperature of welded pipe with special materials is not only related to the chemical composition of steel and electrode, but also related to the stiffness of welding structure, welding method, ambient temperature, etc. In addition, the thickness direction of steel and the uniformity of preheating temperature in the weld zone have an important impact on reducing welding stress.

The post weld heat treatment of welded pipes with special materials has three purposes, namely hydrogen elimination, elimination of welding stress and improvement of weld microstructure and comprehensive properties. The main function of post weld hydrogen elimination treatment is to accelerate the escape of hydrogen in weld and heat effect, which is extremely effective in preventing welding cracks of low alloy steel. 

Stress relief heat treatment is to reduce the yield strength of welded workpiece at high temperature, so as to relax the welding stress. There are two common methods: one is overall high-temperature tempering, and the other is local high-temperature tempering. After heat treatment, the metallographic structure of the welded joint is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.