Abstract
In this research, a 25Cr2MoVA steel bolt used to assemble a high-pressure steam turbine cylinder in a power generation plant fractured after approximately 23 years of operation. Macrographic and micrographic analysis, scanning electron microscopy techniques, chemical analysis, tensile, impact and hardness testing were used to fully characterize the component and material properties. Based on thorough investigations, it has been identified that mechanical properties of connecting bolt of the cylinder were weakened by bulk ferrite during heat treatment and coarse grain boundaries during long-term use and these induce the bolt fracture under an impact force during the equipment startup process. It is recommended that all bolts in the same situation should be changed, or that high-pressure steam turbines should be carefully operated in order to avoid great impact during maintenance operations, especially during the equipment startup and shut-down process. This research clearly provides guidance for standard-setting committees and contributes to the prevention of similar types of accidents.
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