One of the key challenges in the production of tube heat exchangers is the poor cold bending performance of high-strength steel tubes. When the second tube is expanded into the tube sheet, if the hardness of the steel tube exceeds that of the tube sheet, it can lead to expansion failure. Moreover, high-strength steel tubes may develop even higher strength after the expansion process, increasing the risk of stress corrosion cracking at low-temperature sites where hydrogen sulfide and chloride ions are present.
To address this issue, we focused on controlling the upper limit of the material's properties and extended the tempering time to reduce the mechanical strength of the steel tube. This ensures a better combination with the tube sheet during expansion. In terms of selecting the tube sheet material, we initially tried surfacing welding on forgings during the trial production phase. However, the surfaced tube sheets tended to deform significantly, becoming concave and prone to cracking due to high internal stresses. As an alternative, we opted for explosive bonding of medium plates matched with the pipe material and forging components. Although this method effectively addressed the corrosion issues on the tube side, the shell side of the tube sheet remained unprotected. Additionally, the composite plates still carried hidden risks of insufficient strength. Therefore, we ultimately decided to use forged or billet materials directly. This not only eliminated the welding challenges associated with different materials but also fully resolved the corrosion problems on both the tube and shell sides of the heat exchanger.
The mechanical properties of the tube sheet forgings, including yield strength, tensile strength, hardness, and elongation, as well as their chemical composition, were consistent with the requirements specified for plate materials in the table. By using knotted electrodes, we were able to solve welding and processing issues. The welding process did not require preheating or post-weld heat treatment, and the procedure was similar to that used for conventional materials. This approach ensured reliable and efficient fabrication while maintaining high-quality standards.
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