Failure analysis of a steam valve stem

A failure analysis of a stop-and-control combined steam valve stem, which failed during service in a thermal power station, was performed by means of optical and electron microscopy techniques. The fractographic observations showed an intergranular decohesion indicative of a brittle fracture and that the origin of the fracture was located on the nitrided thread of the stem, which thus acted as a stress concentrator. Mechanical tests showed a low toughness of the material, due to a high density of precipitates on grain boundaries, and a heterogeneous austenitic (duplex structure) grain distribution, as revealed by light optical and electron metallographic analysis. Failure was attributed to the low toughness of the material and a nitrided layer on the roots of the threaded zone.