Immediately after welding, all (or part of) the welding parts are heated or insulated to make them cool slowly. The purpose of post-heating is to reduce the content of diffused hydrogen in welded joints, especially in the heat-affected zone, so it is also called dehydrogenation treatment, which is an important technological measure to prevent delayed cracks in welding some low alloy structural steels.

20MnNiMoNb steel preheat 130 ℃ before welding, after the relationship between heat temperature and time to avoid cracks. After the heat effect is very significant, after welding at 280 ℃ heat preservation 45min, you can keep the weldment is not cracked, if heated to 300~350 ℃, you can keep a shorter time.

post-weld heat treatment

After welding, heat treatment to improve the organization and performance of welded joints or eliminate residual stress is called post-weld heat treatment.

Due to the large heat input, slow heating and cooling speed, and long residence time at high temperature during electroslag welding, the primary crystal grains of the weld are very coarse and the impact toughness is very low. Usually, heat treatment should be carried out after welding.

Some weldments, such as thick-walled pressure vessels, are subjected to heat treatment to eliminate welding residual stress after welding. The commonly used method is high temperature tempering. After tempering at 600~620 ℃, the plasticity and toughness of vanadium-containing low alloy steel decrease (temper brittleness), and the tempering temperature should be 550~560 ℃.

The relationship between post-heat and post-weld heat treatment.

Post-weld heat is not equal to post-weld heat treatment. The purpose of post-heat is to dehydrogen and prevent delayed cracking. The purpose of post-weld heat treatment is to improve the microstructure and properties of the joint and eliminate the welding residual stress. In addition, the heating temperature of post-weld heat treatment is much higher than that of post-heat. Of course, if the post-weld weldment is subjected to both treatments at the same time, if the post-weld heat treatment can be carried out in time, the post-heat treatment can be eliminated.