Should gasketing or welding be done first on the heat exchanger tubes?

Under normal circumstances, tube end expansion and welding in heat exchangers are carried out simultaneously; expansion and welding are complementary processes that must be performed at the same time. This is because performing expansion joining first and then welding, or welding first and then expansion joining, will affect the material, resulting in weak expansion joining or welding, which in turn affects the performance of the heat exchanger. For the expansion fitting and welding of pipes made from different materials, there is generally no specific order required. However, it is necessary to select the appropriate welding and expansion methods based on different materials and thicknesses, in order to ensure the strength of the welds and expansions. For example, when welding 15CrMo pipes to tube sheets, methods such as manual arc welding or submerged arc welding are commonly used ； When welding ordinary carbon steel 20 pipes, welding wire is generally used for gas welding or submerged arc welding ； When welding 304 tubes to tube sheets, welding methods such as TIG welding or MIG welding are commonly used. For expansion joining, common methods include mechanical expansion joining, hydraulic expansion joining, and flame expansion joining. Select the appropriate expansion method and expansion joint size based on different pipe diameters and wall thicknesses. .

This post was last edited by The wise are enlightened on 2023-10-3 at 11:37: Weld first, then expand.

Expansion before welding and welding before expansion are two different welding procedures. A comparison between the two: In the procedure of welding before expansion, it is easier to clean the groove on the tube sheet before welding, and the air in the gap between the tube and the tube sheet can be removed from both the front and back sides. This is very 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 remaining after swelling from relaxing, thus avoiding relaxation caused by the high temperatures of welding. However, for tube-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 cracked as a result. 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. By using the expansion-first then welding process, the leakage rate after welding the tubes to the tube sheet is about 10 times higher compared to the welding-first then expansion process. Moreover, the welds have a uniform appearance with a metallic luster and an attractive shape; there are very few pores and lack of fusion detected during color inspection. Therefore, the process of welding first and then expanding is also commonly used abroad.

Expansion followed by welding process: Since expansion joints leave a large amount of oil, rust, and other contaminants at the pipe ends and at the groove areas, cleaning is carried out before welding. However, due to the narrow space between the pipes, as well as the pipes extending 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 pipes 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 the gases can easily 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, thus forming pores inside the weld. Furthermore, the high temperatures generated during welding can cause the already expanded areas to deform, thereby reducing or even eliminating the residual stresses and elastic deformations resulting from the expansion process, which may lead to a decrease in the clamping force. Long-term mass production practice has shown that the process of expanding first and then welding indeed has many drawbacks, especially when the welding properties are poor; this is the case, for example, with the combination of 20MnMo, 15CrMo, and austenitic stainless steel pipes.

In summary, although the process of expanding first and then welding can be used, various manufacturing scenarios show that the process of welding first and then expanding is more advantageous. Therefore, during design and manufacturing, the process of welding first followed by expansion should be given priority; in cases where the weldability of the tube material is poor, a region of 10–15 mm at the tube end can be left unexpanded.