Failure Reasons of Safety Valve Spring in a Delayed Coking Unit

The fracture failure causes of 50CrVA spring steel were analyzed by means of direct reading spectrometer, Rockwell hardness tester, metallographic microscopy and scanning electron microscopy. The results showed that the working medium of the failed spring contained H₂S, and the installation position of the equipment was close to sea, so it was easy to form wet H₂S environment. The coating on the spring surface failed after long-term service, exposing the spring steel substrate to the wet H₂S environment. Under the action of wet H₂S corrosion environment and high shear stress, coupled with the brittle martensite microstructure of the spring, the intergranular stress corrosion cracking occurred, which eventually leaded to fracture failure.