Connection of heat exchange tubes to tube sheets: expansion first then welding, or welding first then expansion

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In the manufacturing process of equipment such as heat exchangers, \"expand first then weld\" and \"weld first then expand\" are two different processing sequences, and they differ in the following ways: 1. Processing sequence and principle – With the approach of expand first then weld, expansion joining is carried out first. Expansion joining involves using an expansion tool to cause plastic deformation in the tube, while simultaneously inducing elastic deformation in the wall of the tube sheet, thereby tightly connecting the tube to the tube sheet. After expansion joining, there is a good fit between the tube and the tube sheet. Then welding is carried out to further strengthen the connection between the tube and the tube sheet, while also sealing any minor gaps that may exist after expansion jointing. Weld first, then expand: First, weld the tube and the tube sheet to create a secure connection. Welding achieves connection primarily by fusing metals, providing high strength. Expansion joining is then carried out; the purpose of this process is to eliminate any stress concentrations that may arise during welding, and at the same time to further enhance the sealing between the tube and the tube sheet, thereby making the connection between them tighter. II. Regarding connection quality and sealing performance, expansion followed by welding is employed: the expansion process allows for the pre-filling of the gaps between the pipe and the tube sheet, which reduces the likelihood of defects such as pores occurring during welding, thus helping to improve sealing performance. However, if impurities such as oil remain on the surface after crimping and are not cleaned properly, it may affect the welding quality and thus the sealing performance. Weld first, then expand: Expansion joining after welding allows for some degree of reshaping of the welded area, enabling a tighter fit between the tube and the tube sheet; this helps to eliminate defects such as minor cracks that may arise in the weld heat-affected zone, thereby improving the sealing performance. However, if the welding quality is poor and severe welding defects occur, expansion joining may not be able to fully compensate for the impact of these defects on sealing performance. For connection strength, expansion followed by welding: Expansion bonding can create a certain degree of pre-tension between the tube and the tube sheet, which helps to maintain the proper relative position of these components during welding. This leads to a more even distribution of welding stresses, thereby enhancing the connection strength. Moreover, the joint surface obtained after expansion bonding provides a good foundation for welding, allowing the weld pool to fill more thoroughly, improving the quality of the weld and thereby enhancing the overall strength of the connection. Weld first, then expand: Welding itself provides high strength of connection, firmly bonding the tube and the tube sheet together. Expansion jointing is carried out after welding, primarily to further strengthen the welded joint and enhance its strength even more. However, if the expansion joining process is not performed properly, it may cause damage to the welded area; for example, excessive expansion joining can lead to cracks in the weld, reducing the strength of the connection. III. Stress distribution: The expansion followed by welding process generates certain residual stresses. During welding, heat input induces new thermal stresses in the areas that have been expanded. These two types of stress act on top of each other, and if not properly controlled, they can lead to significant residual stress concentrations, which negatively affect the long-term performance of the equipment by causing issues such as fatigue cracks. The thermal stress generated by welding first and then expanding is mainly concentrated in the weld and its heat-affected zone. The expansion joining process can, to a certain extent, release and redistribute welding stresses, resulting in a more uniform stress distribution at the junction between the tube and the tube sheet. This helps to reduce stress concentration and enhance the fatigue resistance of the equipment. IV. Scope of application: Expansion followed by welding is suitable for situations where extremely high sealing performance is required, and the welding properties of the tube and tube sheet materials are good. For example, in some small, high-precision heat exchangers, welding is used to further ensure sealing and connection strength, on the basis that expansion joining already guarantees a good fit. Welding first and then expanding is suitable for equipment subjected to high pressure and vibration loads. By first ensuring basic connection strength through welding, and then using expansion jointing to optimize stress distribution and improve sealing performance, equipment such as heat exchange units in large-scale chemical plants can maintain reliable operation under complex operating conditions.