The metallographic analysis of the fracture was carried out, and the bolt material was analyzed as tempered martensite from the 500-times enlarged photograph, so the strength and hardness of the bolt were high. The tempered martensite consists of a supersaturated alpha solid solution and its coherent η-carbide. The tempered martensite still retains the flaky or lath-like morphology of the original martensite, but the tempered martensite is quenched by the distribution of a large amount of highly dispersed fine η-carbide on the supersaturated α-solid solution. Martensite is easily corroded. Black under the light microscope. Under normal circumstances, what we ask for should be tempered sorbite. Since the tempering temperature is high and the ferrite has undergone recrystallization, the sheet or lath morphology of the original martensite has been lost. At the same time, the cementite particles are relatively large due to the growth of cementite.
Based on the above analysis, the fracture process of the bolt can be judged. Firstly, due to improper handling of the bolt during the quenching and tempering process, the structure forms tempered martensite. When the heat exchanger is assembled, the sealing surface leaks due to pressure. The bolts are tightened to make the bolts have a high preload force. After the material is introduced, the bolt is subjected to pre-tightening stress, temperature difference stress and material differential pressure stress simultaneously, and the three stresses simultaneously act to cause brittle cracks in the bolt which first contacts the medium. Under the action of tensile stress, the cracks continue to expand, and the pH of the medium fluctuates, which makes the medium corrosive. Stress corrosion occurs in the presence of tensile stress, which accelerates the breakage of the bolt and ultimately leads to cracking of the cracked bolt.
Because the bolt needs to have a large pre-tightening force, it also requires a certain degree of plastic deformation. After comprehensive consideration, the material of 25CrMoA is used to strictly implement the processing technology. 25CrMoA is also a medium-hardenable alloy quenched and tempered steel. 35CrMo slightly decreased, but the plasticity is better, and the overall performance is more suitable than 35CrMoA. From the post-installation operation, the bolt completely meets the requirements. For mechanical processing, strict quality control is essential, but it is also very important to strictly implement various process regulations in the process, especially to comply with the heat treatment process.
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