Reasons for fracture of 0Cr17Ni4Cu4Nb steel blade in low－pressure rotors of supercritical steam turbines

The last stage 0Cr17Ni4Cu4Nb steel blade of the low－pressure rotor of a certain supercritical steam turbine broke. The causes of blade fracture were analyzed using methods such as macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination and hardness testing. The results show that under flexible operating conditions, frequent changed in the load borne by the unit result in frequent and significant fluctuations in the inlet steam volume and steam flow of the low－pressure cylinder, led to flutter of the last stage blades with long blade shapes during operation. The root size of the last stage blade did not match the root groove, causing the blade root loose and blade shake, exacerbating blade flutter. Under the combined action of two factors, cracks appeared in the stress concentration area near the blade root on the outlet side of the unit. Under the alternating stress generated by blade flutter, the cracks continued to propagate in the form of fatigue, ultimately led to the overall fracture of the blade.