Post-weld heat treatment generally uses single high-temperature tempering or normalizing plus high-temperature tempering. For gas welding joints, normalizing and high-temperature tempering heat treatment are used. This is because the grains in gas welding seams and heat-affected zones are coarse and need to be refined, so normalizing is used. However, normalizing alone cannot eliminate residual stress, so high-temperature tempering is required to eliminate stress. Single medium-temperature tempering is only suitable for assembly and welding of large ordinary low-carbon steel vessels assembled on site. Its purpose is to partially eliminate residual stress and remove hydrogen. In most cases, a single high-temperature tempering is used. The heating and cooling of the heat treatment should not be too fast, and the inner and outer walls should be uniform.