Welding residual stress is caused by the uneven temperature distribution of the weldment caused by welding, the thermal expansion and contraction of the weld metal, etc., so residual stress will inevitably occur with the welding construction. The most common method to eliminate residual stress is high-temperature tempering, which means placing the weldment in a heat treatment furnace to be heated to a certain temperature and held for a certain period, and the material’s yield limit is reduced at high temperature to cause plastic flow in areas with high internal stress. The elastic deformation gradually decreases, and the plastic deformation gradually increases, causing the stress to decrease.
1. Selection of heat treatment methods for steel pipes
The impact of post-weld heat treatment of steel pipes on metal tensile strength and creep limit is related to the heat treatment temperature and holding time. The impact of post-weld heat treatment of steel pipes on the impact toughness of the weld metal varies with different steel types. Post-weld heat treatment of steel pipes 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.
2. Heat treatment methods used in pressure vessels
There are two types of heat treatment methods used in pressure vessels: one is heat treatment to improve mechanical properties; the other is post-weld heat treatment (PWHT) of steel pipes. Broadly speaking, post-weld heat treatment of steel pipes is the heat treatment of the welding area or welded components after the workpiece is welded. Specific contents include stress relief annealing, complete annealing, solid solution, normalizing, normalizing plus tempering, tempering, low-temperature stress relief, precipitation heat treatment, etc. In a narrow sense, the post-weld heat treatment of steel pipes only refers to stress relief annealing, that is, to improve the performance of the welding zone and eliminate harmful effects such as welding residual stress, to uniformly and adequately treat the welding zone and related parts below the metal phase change 2 temperature point. The process of heating and then cooling evenly. The post-weld heat treatment of steel pipes discussed in many cases is essentially post-weld stress relief heat treatment of steel pipes.
3. Purpose of post-weld heat treatment of steel pipes
①Relax welding residual stress.
② Stabilize the shape and size of the structure and reduce distortion.
③Improve the performance of base metal and welded joints, including:
a. Improve the plasticity of the weld metal.
b. Reduce the hardness of the heat-affected zone.
c. Improve fracture toughness.
d. Improve fatigue strength.
e. Restore or improve the yield strength reduced during cold forming.
④ Improve the ability to resist stress corrosion.
⑤ Further release harmful gases in the weld metal, especially hydrogen, to prevent the occurrence of delayed cracks.
4. Consideration of the comprehensive effect of PWHT
Post-weld heat treatment of steel pipes is not always beneficial. In general, post-weld heat treatment of steel pipes helps alleviate residual stress and should only be performed when there are strict requirements for stress corrosion. However, the impact toughness test of the specimen shows that the post-weld heat treatment of the steel pipe is detrimental to the toughness improvement of the deposited metal and the welding heat-affected zone, and sometimes intergranular cracking may occur within the grain coarsening range of the welding heat-affected zone. Furthermore, PWHT relies on the reduction of material strength at high temperatures to achieve stress relief. Therefore, during PWHT, the structure may lose rigidity. For structures adopting overall or partial PWHT, the stress of the weldment at high temperatures must be considered before heat treatment. Supporting capacity. Therefore, when considering whether to perform post-weld heat treatment on steel pipes, the advantages and disadvantages of heat treatment should be comprehensively compared. From the perspective of structural performance, some aspects improve performance, and some aspects that reduce performance. Reasonable judgments should be made based on the comprehensive consideration of both aspects.
Post time: May-20-2024