In order to find out the cause of fracture of two high-pressure main steam valve stems of GH901 material on the left and right sides of a 300 MW unit steam turbine after only 100 hours of operation

methods such as macro-observation of appearance

chemical analysis

microstructure observation and mechanical performance test were applied to study the fractured valve stem. The results show that

the valve stem was not processed strictly in accordance with the drawings during manufacturing

resulting in transitional cutting of the outer wall of the spring hole

which was then remedied by surfacing welding. The base material and fusion line of the surfacing zone had multiple cracks

unfused and other original welding defects

which became the origin of the fracture. The stress concentrated in the variable diameter transition zone between the valve stem and the spring hole

which promoted the rapid expansion of welding defects. At the same time

the poor plastic toughness of the base material of the valve stem also accelerated the crack propagation speed

resulting in rapid fracture of the valve stem.