Difference between overall heat treatment of pressure vessels and heat treatment of welds
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Hello everyone: For example, when pressure vessels reach a certain thickness, heat treatment is required; generally, a requirement for overall heat treatment is specified. According to GB150, the purpose of heat treatment is to eliminate residual welding stresses. Is there a big difference between overall heat treatment and heat treating all welds and heat-affected zones?
Heat treatment of welded joints is one of the types of heat treatment for pressure vessels; it involves overall post-weld heat treatment, which is a method of heating
Cylinder rolling sometimes also requires heat treatment
That’s the heat treatment for eliminating residual stresses, you know
Is heat treatment applied only to the weld considered local heat treatment? Class A welds are not permitted. According to GB150, the requirements for the visual inspection differ clearly between overall heat treatment after welding and heat treatment of the welds. In general, when considering both the base material and the weld, it seems there isn’t much difference compared to focusing only on the weld itself, right?
Overall heat treatment includes weld heat treatment as well as heat treatment for the recovery of the forming properties of stressed components.
Overall heat treatment also includes stress-relief heat treatment resulting from cold working deformation
There have been cases where, after overall heat treatment of a fixed-tube-sheet type heat exchanger, welding was performed on the tube sheet and heat exchange tubes at one of its ends, followed by local heat treatment of the tube sheet on that side. The reason is that there is a significant difference in the coefficients of linear expansion between the materials of the heat exchange tubes and the shell of the equipment. Even with the installation of corrugated expansion joints on the shell, it is not suitable to perform overall heat treatment after both sets of tube sheets and the heat exchange tubes have been welded. It is more reliable and reasonable to leave a gap between one of the tube sheets and the heat exchange tubes without welding first.
Overall heat treatment eliminates residual stresses throughout the entire structure, including the base material and structural components, while local treatment applies only to the welds, aiming solely to remove the residual stresses resulting from welding. If the equipment is too large, treatment must be carried out in sections, and this too is part of the overall heat treatment process. However, local heat treatment applies only to a portion of the welds, and the purposes of the two are fundamentally different
Just as cold-forming results in a large degree of deformation, heat treatment is required to restore the material’s mechanical properties; this involves overall heat treatment, which is aimed at the material itself. As mentioned, the stress-relief heat treatment after welding is actually aimed at the weld area itself; however, the standards require overall heat treatment inside a furnace or segmented heat treatment.