Construction sequence for expansion welding the heat exchanger tube sheet to the tubes

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Construction sequence for the expansion welding joint between the heat exchanger tube sheet and tubes. The connection methods between the tubes and the tube sheet in a heat exchanger include expansion welding, welding, strength expansion + sealing welding, and strength welding + expansion fitting. 1. Weld first, then expand. In the weld-first-then-expand process, the groove on the tube sheet can be cleaned thoroughly before welding, and the air present in the gap between the tube and the tube sheet can be removed from both the front and back sides. This is highly beneficial for preventing pores in the welds and ensuring the quality of the welded joint. At the same time, post-swelling prevents the residual stresses generated after swelling from relaxing, thus avoiding relaxation caused by the high temperatures associated with welding. However, for pipe-to-tube sheet joints with poor weldability, microcracks are likely to form in the weld seam during expansion welding, and in some cases the weld seam may even be torn apart. In such cases, deep expansion should be employed (that is, the pipe opening should not expand by more than 10–15 mm), so as to keep the expansion area away from the weld seam and thereby reduce the impact of expansion on the weld seam; this is also the biggest drawback of the process of welding first and then expanding. The results of the welding-then-expansion process inspection show that the weld appearance is uniform, with a metallic luster and an attractive shape; pore and lack of fusion defects were found only in very small quantities during the coloring inspection. Therefore, the process of welding first and then expanding is also commonly used abroad. 2. Expansion followed by welding: The expansion-then-welding process is used. Since a large amount of oil, rust, and other contaminants remain at the pipe ends and at the groove areas during expansion, cleaning is carried out before welding. However, due to the narrow width of the pipe bridge and the fact that the pipes extend beyond the tube sheet, it is difficult to ensure thorough cleaning of the grooves. During welding, these residual impurities undergo intense chemical changes; water and air expand locally due to the heat, creating pressure in the gaps between the tubes and their holes. As the back side of these gaps becomes blocked, the pressurized gases can only escape from the side of the weld seam. Metal that is in a molten state during welding has no strength at all, so it is easy for gases to pass through the weld seam, especially at the end of the weld. The gas rushing out of the weld bead causes the weld metal to boil, resulting in an uneven weld surface that may even appear honeycombed. At the same time, it also causes oxidation of the weld surface, leading to defects such as lack of fusion. During the cooling process of the weld, some gases fail to escape from the surface of the weld in time, thereby forming pores inside the weld. Furthermore, the high temperatures generated during welding can cause the already expanded areas to deform, thereby reducing the residual stresses and elastic deformations resulting from the expansion process, which may lead to a decrease or even disappearance of the clamping force. The results of the experimental studies show that the leakage rate of the expand-first then weld process is about 10 times that of the weld-first then expand process. Long-term mass production practice has also shown that the method of expanding first and then welding does have many drawbacks, especially when the welding process properties are poor; this is the case, for example, with the combination of 20MnMo, 15CrMo, and austenitic stainless steel pipes. Based on the above analysis, although the procedure of expanding first and then welding can be adopted, manufacturing and application experiences both at home and abroad indicate that the procedure of welding first and then expanding is more advantageous. Therefore, I believe that during design and manufacturing, the process of welding first and then expanding should be given priority; in cases where the weldability of the tube material is poor, a region of 10–15 mm without expansion can be left at the tube end. Additional experience: If post-weld heat treatment is required, and mechanical expansion is used for expansion joining, expand once before heat treatment, and then expand lightly once after heat treatment ； If flexible expansion joining (such as fluid bag expansion joining) is used, it can be expanded once after heat treatment.