Heat treatment process of pressure vessels during manufacturing

When a pressure vessel is welded, a sharp temperature gradient with a temperature difference greater than 100 degrees is generated in the adjacent area of the base material, which causes uneven plastic strain in ferrite steel or other equivalent materials. During the subsequent cooling process, a residual stress field with a peak stress reaching the yield point will be generated.
For the hydrogen absorbed in the weld, the more effective method is to carry out post weld heat treatment, which can not only relax and ease the welding residual stress, improve the welding Heat-affected zone that is hardened and embrittled due to welding, improve the ductility and fracture toughness of the weld metal, but also allow the diffusion and escape of harmful gases such as hydrogen in the welding area and nearby. There are two types of heat treatment methods used for tubular furnace pressure vessels: one is heat treatment to improve mechanical properties, and the other is post weld heat treatment (PWHT). Post weld heat treatment is the heat treatment performed on the welding area or component after the workpiece is welded.
Post weld heat treatment only refers to stress relief annealing, and many discussions on post weld heat treatment essentially refer to post weld stress relief heat treatment. In order to improve the performance of the welding zone and eliminate harmful effects such as welding residual stress, the welding zone and related parts are uniformly and fully heated below the metal phase transition temperature point 2, and then uniformly cooled.
The uneven plastic strain in the manufacturing of pressure vessels leads to residual strain in elastic-plastic materials, which can be caused by mechanical factors such as cold rolling, cold straightening, and other cold processing heat (mainly generated during welding), or a combination of both, that is, thermal mechanical factors. Stress relief annealing, complete annealing, box furnace, solid melting, normalizing, normalizing and tempering, tempering, low-temperature stress relief, precipitation heat treatment, etc. The strength and plasticity are significantly reduced, and the hydrogen dissolved in the metal lattice causes brittle failure of the steel during slow deformation. The hydrogen in metal materials can be absorbed during the production process of metal materials, such as the hydrogen absorbed by the liquid metal during welding being retained in the weld seam, or the hydrogen absorbed by the material during service in a hydrogen environment will leave a residual elastic strain field in the final product of pressure vessel processing and bear corresponding elastic residual stress.
During the manufacturing process of pressure vessels, the following problems arise - excessive cold rolling and cold work hardening caused by cold work such as cold straightening. The changes in weld area and performance caused by welding are reflected. The generation and development of residual stress and stress corrosion cracks caused by box furnace welding. In order to eliminate peak strain in the welding area and achieve uniform distribution of internal strain, various methods can be adopted, such as mechanical vibration method, post weld heating method, etc.
Many potential problems in pressure vessels mainly come from metallurgical damage in the weld zone, so using mechanical methods to reduce internal strain is no longer sufficient to prevent many problems that may occur during future operation. The hydrogen embrittlement phenomenon of metals, which affects the performance of pressure vessels, has been a concern. After hydrogen enters steel, the mechanical properties will undergo significant deterioration.

The above content is organized by Superb heater Company, super heater. Mainly professional production: industrial heating elements, temperature measuring elements, production equipment, accessories, and raw materials such as tubular heaters, tubular heaters, belt heaters, ceramic heaters, silicone rubber heaters, thermocouples, etc.

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