In the production of tube heat exchangers, high-strength steel pipes often exhibit poor cold bending performance. When the second pipe is expanded into the tube sheet, if the hardness of the steel pipe exceeds that of the tube sheet, it can lead to expansion failure. Additionally, high-strength steel pipes may develop even higher strength after the expansion process, increasing the risk of stress corrosion cracking at low-temperature hydrogen sulfide and chloride ion exposure sites.
To address this issue, we focused on controlling the upper limit of the material's mechanical properties and extended the tempering time to reduce its overall strength. This approach ensures a more balanced performance between the steel pipe and the tube sheet during the expansion process. Regarding the selection of tube sheet materials, we initially tried surfacing welding on forgings during early trial production. However, this method resulted in significant deformation, with the surface becoming concave and prone to cracks due to high residual stresses.
As an alternative, we opted for explosive recombination using medium plates that matched the pipe material and the forging of the tube sheet. While this method effectively addressed the corrosion issues on the tube side, it still left the shell side of the tube sheet vulnerable to corrosion. Moreover, the composite plates had inherent structural weaknesses, which led us to abandon this approach altogether. Instead, we decided to use forged or billet materials directly for the tube sheet. This not only resolved the welding challenges associated with different materials but also completely eliminated corrosion problems on both the tube and shell sides of the heat exchanger.
The mechanical properties of the tube sheet forgings—such as yield strength, tensile strength, hardness, and elongation—are consistent with the specifications outlined for plate materials. The chemical composition of the multi-injection tube sheet forgings also meets the required standards. By using knotted electrodes, we were able to eliminate the need for preheating or post-weld heat treatment, making the welding process similar to that of conventional materials. This simplification helped reduce costs and improve overall efficiency.
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