Reasons of Fracture of Safety Valve Spring of Auxiliary Boiler in a Nuclear Power Station

The fracture reasons of safety valve spring of auxiliary boiler in a nuclear power station were analyzed by morphology observation, chemical composition analysis, hardness test, microstructure and fracture analysis, and the slow strain rate test (SSRT) was used to compare the mechanical performance of the spring before and after dehydrogenation. The results showed that chemical composition and microstructure of the spring all met standard regulation, only its hydrogen content was slightly high (2.8 μg/g). The fracture was hydrogen embrittlement fracture. The hydrogen embrittlement fracture of spring under high shear force was the main reason for its failure. Compared with the original spring, the tensile strength of the spring after dehydrogenation was reduced after SSRT test, the cracking time increased by 40%, and the elongation increased from 8% to 15%, the reduction of area increased from 21% to 31%. The hydrogen embrittlement sensitivity of spring was significantly reduced.