Is heat treatment always beneficial? This article will tell you the answer

During welding construction, the generation of welding residual stress is unavoidable due to the uneven temperature distribution of weldments caused by welding, thermal expansion, and contraction of weld metal, etc. The most common way to eliminate residual stress is high-temperature tempering, that is, putting the weldment into a heat treatment furnace, heating it to a specific temperature, and keeping it warm for a certain period. With the characteristics of the material's reduced yield limit at high temperature, plastic flow occurs in the parts with high internal stress, reducing elastic deformation and increasing plastic deformation, thereby reducing stress.

 

 

1. There are various heat treatment methods, each with its applicable scenarios

 

The effect of post-weld heat treatment on the tensile strength and creep limit of metals is related to the temperature and insulation time of heat treatment; the effect on the impact toughness of weld metal varies depending on the type of steel. Post-weld heat treatment generally selects single high-temperature tempering or normalizing plus high-temperature tempering. For example, for gas welding joints, due to the coarse grains in the weld and heat-affected zone, the grains need to be refined, so normalizing plus high-temperature tempering heat treatment is used. After all, normalizing alone cannot eliminate residual stress, so high-temperature tempering is required. Single medium-temperature tempering is only suitable for assembly welding of large ordinary low-carbon steel containers assembled on site, to partially eliminate residual stress and dehydrogenation. In most cases, single high-temperature tempering will be selected, and the heating and cooling of the heat treatment should not be too fast, and the inner and outer walls should be as uniform as possible.

 

The heat treatment methods used for pressure vessels are divided into two categories: one is heat treatment to improve mechanical properties, and the other is post-weld heat treatment (PWHT). In a broad sense, post-weld heat treatment is the heat treatment of the welding area or welded components after the workpiece is welded. Specifically, there are 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, which is to improve the performance of the welding area and eliminate harmful effects such as welding residual stress. The welding area and related parts are uniformly and fully heated below the metal phase change temperature point, and then uniformly cooled. In many cases, the post-weld heat treatment discussed is the post-weld stress relief heat treatment.

 

2. The purpose of post-weld heat treatment is clear, and the role is extensive

 

The main purposes of post-weld heat treatment are: first, relax welding residual stress; second, stabilize the shape and size of the structure and reduce distortion; third, improve the performance of the parent material and welded joints, such as improving the plasticity of the weld metal, reducing the hardness of the heat-affected zone, improving the fracture toughness and fatigue strength, and restoring or improving the yield strength reduced in cold forming; fourth, improve the ability to resist stress corrosion; fifth, further release harmful gases (especially hydrogen) in the weld metal to prevent the occurrence of delayed cracks.

 

Whether pressure vessels need post-weld heat treatment should be specified in the design, and the current pressure vessel design specifications also have requirements. For welded pressure vessels, there is a large residual stress in the welding area, and its adverse effects will only appear under certain conditions. When the residual stress combines with the hydrogen in the weld, it will promote the hardening of the heat-affected zone, causing cold cracks and delayed cracks; the static stress remaining in the weld or the dynamic stress in the load operation, combined with the corrosion of the medium, may cause stress corrosion. Welding residual stress and parent material hardening are important factors in the generation of stress corrosion cracks, which will cause the metal to change from uniform corrosion to local corrosion.

 

To determine whether a pressure vessel needs post-weld heat treatment, it is necessary to comprehensively consider the purpose of the container, size (especially the wall thickness), performance of the materials used, and working conditions. When these situations exist, we should consider: thick-walled containers with harsh use conditions, containers that bear large loads and alternating loads; welded pressure vessels with a thickness exceeding a certain limit (including boilers, petrochemical pressure vessels, etc., which have special regulations and specifications); pressure vessels with high dimensional stability requirements; containers made of steel with a high tendency to harden; pressure vessels with stress corrosion cracking hazards; other pressure vessels with special regulations, specifications and drawings. Studies have shown that 650-degree tempering has a good effect on eliminating residual stress in steel welded pressure vessels. If proper heat treatment is not performed after welding, it is difficult to obtain corrosion-resistant welded joints. Generally, stress relief heat treatment involves heating the welded workpiece to 500-650 degrees and then slowly cooling it. The stress reduction is due to creep at high temperature. Carbon steel begins to appear at 450 degrees, and molybdenum-containing steel begins to appear at 550 degrees. The higher the temperature, the easier it is to eliminate stress, but if it exceeds the original tempering temperature of the steel, the strength of the steel will decrease, so the temperature and time must be controlled. Moreover, for the PWHT temperature of quenched and tempered steel, the principle is not to exceed the original tempering temperature of the steel, which is generally about 30 degrees lower than the original tempering temperature; otherwise, the quenching and tempering effect will be lost, and the strength and fracture toughness will be reduced.

 

3. Heat treatment has both advantages and disadvantages, and comprehensive consideration is required

 

Post-weld heat treatment is not beneficial. Generally, it is beneficial to relieve residual stress and will be carried out when there are strict requirements for stress corrosion, but the impact toughness test of the specimen shows that it is not conducive to improving the toughness of the deposited metal and the heat-affected zone of the weld, and sometimes it may cause intergranular cracking within the grain coarsening range of the heat-affected zone of the weld. In addition, PWHT relies on the reduction of material strength at high temperature to eliminate stress, and the structure may lose rigidity. Therefore, for the structure that adopts overall or partial PWHT, the support capacity of the weldment at high temperature should be considered before heat treatment.

 

Therefore, when considering whether to perform post-weld heat treatment, it is necessary to comprehensively compare the advantages and disadvantages. From the perspective of structural performance, there is both an improved performance and a reduced performance. It is necessary to make a reasonable judgment based on comprehensive considerations so that heat treatment can play a more scientific and effective role in ensuring welding quality and improving pressure vessel performance.