According to the energy spectrum analysis, it is known that the corrosion product contains S (1.72%) and Cl (0.46%). Due to the presence of chloride ions, the crack is a stress corrosion crack along the crystal form. In addition, there are more S in the corrosion products. When S ions and Cl ions act simultaneously, the generation and expansion of stress corrosion cracking of stainless steel will be accelerated.
Comprehensive analysis According to chemical composition analysis, metallographic analysis and macro and micro analysis of fractures, it can be determined that the bolt failure is intergranular stress corrosion leading to fracture failure. The specific analysis is as follows: (1) Cr-Ni austenitic stainless steel Intergranular stress corrosion, the main main findings are: selective dissolution of chromium-depleted zone in stainless steel grain boundary under stress; under the action of stress, preferential dissolution of impurities in stainless steel along grain boundary segregation; Next, the dissolution of the precipitate phase itself in the middle grain boundary.
For the bolts that have broken this time, the carbon content is high. If the bolt is heated during the manufacturing process at the sensitization temperature (450-850e), the supersaturated carbon in the steel will diffuse to the grain boundary, precipitate and be nearby. The chromium forms a chromium-containing carbon compound, causing the corrosion resistance of the grain boundary to decrease, so that the crack propagates along the crystal.
Conclusion This joint bolt failure is caused by intergranular stress corrosion and fracture failure. The material of the failed bolt has a high carbon content, and if the heat treatment is improper, it is easy to cause such intergranular stress corrosion. It is possible to determine whether the stress corrosion failure of the bolt is a case by sampling the bolts in the same batch and under similar working conditions.
In order to avoid the occurrence of stress corrosion of similar connecting bolts, measures can be taken to avoid the formation of corrosive environments containing chlorine and sulfur media. For example, external protection is applied to the site, such as cold insulation or epoxy paint on the bolt rod to avoid corrosive media entering and so on. Use other high-grade materials such as ultra-low carbon stainless steel and high nickel stainless steel.
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