Why is heat treatment not applied to the circumferential seams of multi-layer high-pressure vessels?
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The circumferential joints of multi-layer high-pressure vessels include those between the various tube sections, as well as those between the tube sections and the forged or thick-plate end caps. Although the total thickness of the welds in these tube sections exceeds the threshold specified in the standards for requiring heat treatment, practical considerations and structural factors often result in no post-weld heat treatment being carried out. The reasons why post-weld heat treatment is not required for this type of weld are as follows: 1. For the welds of high-pressure vessel sections composed of multiple layers of plates, it is usually the thickness of one of the layers (when all layers have the same thickness) or the thickness of the thickest layer (when the layer thicknesses vary) that determines whether post-weld heat treatment is necessary. In China, the thickness of these plates ranges from 4 to 12 mm, which is below the threshold specified in the standards for requiring heat treatment; therefore, no heat treatment is needed for the circumferential seams between the section rings. 2. The welds between forgings or thick plate end caps and multi-layer shell sections require that, in addition to using materials equivalent to ASME Class 1, a surfacing layer with a thickness of at least 3 mm be applied to the groove surfaces of these welds before they are joined to the multi-layer shell sections. This process effectively treats such welds as if they had undergone heat treatment; therefore, no further heat treatment is necessary after welding them to the multi-layer shell sections. In ring weld welding, multi-pass welding is commonly used, with the thickness of each weld layer not exceeding 10 mm. In China, preheating is employed during the ring weld welding of multi-layer cylinders; therefore, it can be considered that the subsequent weld provides a certain degree of heat treatment to the metal from the previous weld. For more information exchange, please add WeChat sjytangtang
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This post was last edited by wanlirn on 2019-9-8 15:21. The first one is correct； Article 2: The process of the previous step has no relation to whether heat treatment is required for this lap joint ； For example, if the cylinder is made of 15CrMo with a thickness of 100 mm, and the end caps are formed by surfacing, heat treatment is still required in that case. The third point is the same as the second: when the cylinder is made of 15CrMo with a thickness of 100 mm and welded in multiple layers, preheating before welding is necessary, and heat treatment is still required. The original poster doesn’t seem to understand the purpose of post-welding heat treatment; the purpose of heat treatment is to eliminate residual welding stresses (with the exception of processes like normalizing). Since it’s multi-layered material with each layer having a relatively thin thickness (as per the first point mentioned), any stresses generated are absorbed by the local deformation of each layer, so heat treatment isn’t really necessary. If heat treatment is carried out anyway, it’s because every material deforms to some extent after cooling, and the direction of deformation in each layer will definitely be different, which in turn increases the stresses between the layers