Failure analysis on fracture causes of stainless-steel studs in seawater system of nuclear power plants

Abstract

Nine of the twelve B8M stainless steel studs on the flange of a seawater pipeline isolation valve of a nuclear power plant were encountered with premature fracture during service. To find out the cause of the fracture failure, the fracture studs were comprehensively detected and analyzed by means of appearance morphology analysis, chemical composition analysis, mechanical properties testing, microstructure testing, and fracture micro-area analysis. The results show that the main reasons are as follows: (1) The solid solution temperature of the raw materials for processing bolts is low, so the carbides are not fully dissolved and distributed in a mesh along the grain boundary. Thus, the chromium-poor near the grain boundary forms, resulting in the mechanical properties of the bolts weakening. According to the experiment, it was found that the corrosion resistance of the bolts is weak due to the low nickel content. Carbon easily precipitates at the grain boundary and forms Cr23C6 with chrome, which weakens the bonding strength of the grain boundary and causes the bolt to break along the grain.