Reasons Analysis on Fracture of High-pressure Main Steam Valve Stem in a Subcritical 300 MW Unit

With the fractured main steam valve stem in a subcritical 300 MW unit as the study example, the possible fracture causes of the main steam valve stem were analyzed from several aspects, including the structural design, macro morphology, microstructure and mechanical properties, combined with the service environment. The results show that the fracture was due to a combined effect of many factors. The mechanical properties of the valve stem material had declined below the standard requirement after long-term service, and the microstructure had a degradation, including remaining banded segregation and precipitating network carbide at the grain boundary. In addition, the valve stem material C422 steel displayed serious notch sensitivity, so cracks first initiated at the machining defect positions. At the same time, there existed structure design defect at the diameter changing position and the transition arc radius was rather small, which resulted in large stress concentration. So, during the valve stem working, the initiated cracks propagated along the weakest 45° slope, and the lower strength of the valve stem accelerated the crack propagation, which finally led to the fracture of the valve stem.