Is post-weld heat treatment necessarily beneficial

The 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, that is, the weldment is heated to a certain temperature and kept for a certain period of time in a heat treatment furnace, and the yield limit of the material is reduced at high temperature, so that plastic flow occurs in the place with high internal stress. The elastic deformation gradually decreases, and the plastic deformation gradually increases and the stress decreases.

01 Selection of heat treatment method

The effect of post-weld heat treatment on the tensile strength and creep limit of metals is related to the heat treatment temperature and holding time. The effect of post-weld heat treatment on the impact toughness of weld metal varies with different steel types.

Post-weld heat treatment generally selects single high temperature tempering or normalizing plus high temperature tempering treatment. For gas welding joints, normalizing and high temperature tempering heat treatment are used. This is because the grains of the weld seam and heat-affected zone of gas welding are coarse, and the grains need to be refined, so normalizing treatment is adopted.

However, single normalizing cannot eliminate residual stress, so high temperature tempering is required to eliminate stress. A single medium-temperature tempering is only suitable for the assembly welding of large-scale ordinary low-carbon steel containers assembled on the site, and its purpose is to achieve partial elimination of residual stress and hydrogen removal.

In most cases, a single high temperature tempering is used. The heating and cooling of the heat treatment should not be too fast, and strive to make the inner and outer walls uniform.

02 Heat treatment methods used in pressure vessels

There are two types of heat treatment methods used in pressure vessels: one type is heat treatment to improve mechanical properties; the other type is post-weld heat treatment (PWHT). Broadly speaking, post-weld heat treatment is the heat treatment of the welded area or welded component after the workpiece is welded.

The specific content includes stress relief annealing, complete annealing, solid solution, normalizing, normalizing and tempering, tempering, low temperature stress relief, precipitation heat treatment, etc.

In a narrow sense, post-weld heat treatment only refers to stress relief annealing, that is, in order to improve the performance of the weld zone and eliminate harmful effects such as welding residual stress, so as to uniformly and fully heat the weld zone and related parts below the metal phase transformation temperature 2 , And then the process of uniform cooling. In many cases, the post-weld heat treatment discussed is essentially a post-weld stress relief heat treatment.