Abstract: The split bolt in the intermediate pressure inner cylinder of a steam turbine in a certain thermal power plant broke during operation. The causes of bolt fracture were analyzed using macroscopic observation, non-destructive testing, chemical composition analysis, mechanical testing, scanning electron microscopy analysis, metallographic examination. The results show that improper temperature control during the installation, and hot tightening of the bolt resulted in the formation of quenched martensite in the material due to excessive temperature. Under the combined action of impact axial tensile force and bending stress, the bolt underwent brittle fracture.