Preheating before welding and heat treatment after welding

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

It can also reduce welding stress, uniformly preheat locally or overall, and reduce the temperature difference between welding parts in the welding area. This reduces the welding stress on the one hand, and also reduces the welding strain rate on the other hand, which is helpful to avoid welding cracks. . It can reduce the degree of restraint of welded structures, especially the degree of restraint of corner joints. As the preheating temperature increases, the incidence of cracks decreases.

The selection of preheating temperature and interlayer temperature for welded pipes made of special materials is not only related to the chemical composition of the steel and welding rods, but also to the stiffness of the welded structure, welding method, ambient temperature, etc. It should be determined after comprehensive consideration of these factors. In addition, the thickness direction of the steel and the uniformity of the preheating temperature in the weld zone also have an important impact on reducing welding stress.

The post-weld heat treatment of special material welded pipes has three purposes, namely eliminating hydrogen, eliminating welding stress, and improving the weld structure and overall performance. The main function of post-weld hydrogen elimination treatment is to accelerate the escape of hydrogen in the weld and heat, which is extremely effective in preventing welding cracks in low alloy steel.

Stress-relieving heat treatment is to reduce the yield strength of the welded workpiece at high temperatures, thereby relaxing 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.

Welded pipes：https://www.eastern-steels.com/newsdetail/difference-between-seamless-pipe-and-seam-pipe.html