By using the macro-observation of appearance

the chemical composition analysis

the microstructure observation

and the mechanical test method

the causes leading to valve stem rupture of main steam valve used for one supercritical 600 MW unit have been analysed.Results show that some strengthened particles

such as Al-rich

Si-rich

Ti-rich

and Mo-rich particles have dispersed into the base-metal of superalloy stem

its micro-structure being abnormal

boasting poor tensile ductility and bad impact toughness

and finally leading to brittle rupture at the knife-withdrawing slot of screw threads.It is recommended the treatment measures should be taken for main steam valve stems of same batch and similar modes

so as to ensure safe operation of units.