Cause of 20Cr13 steel moving blade fracture in the feed pump turbine of ultra supercritical thermal power units

During the operation of a certain ultra supercritical unit, a fracture accident occurred on the fourth stage 20Cr13 steel moving blade of the feedwater pump turbine. The causes of blade fracture were analyzed using methods such as macroscopic observation, chemical composition analysis, metallographic examination, mechanical property testing, scanning electron microscopy and energy spectrum analysis. The results show that some moving blades of the feed pump turbine were improperly assembled, and the gap between the blade root and the blade root groove was too large, resulting in obvious loosening of the blades. During the long term high speed rotation of the rotor, the blades bore a high level of alternating cyclic load. Some moving blade roots with large assembly gaps and the contact area of the blade root groove experienced micro fatigue damage. The circumferential edge of the blade root underwent bi－directional multi－source fatigue cracking, and the cracks gradually expanded, ultimately led to blade fracture.