Failure analysis of outer casing bolts of a 325 MW steam turbine

Repetitive failures of high-temperature casing bolts of four 325 MW steam turbine units in a 1300 MW thermal power plant have been investigated. Ten of these Cr-Mo-V steel bolts were failed in less than two years, indicating the probability of repeatable phenomena of bolts. Working pressure and temperature of the turbine were 12.74 MPa and 535 degrees C, respectively. Failure analysis of the bolts including fractography, mechanical and microstructural analyses have been conducted. Results from mechanical tests indicated a dramatically reduction in impact toughness of the bolts. Results of fractography showed that the fracture mode is mostly intergranular brittle without any evidence of creep. Metallographic and chemical composition analyses of fracture surface showed the accumulation of carbides in grain boundaries. Two groups of specimens made of the failed bolts were undertaken different heat treatment including i) tempering and, ii) austenitizing and then tempering. It showed that the toughness properties of specimens were remarkably retrieved, specifically after the second heat treatment. Based on the results it was concluded that reversible temper embrittlement (RTE) were occurred in the bolts. Proposed criteria to prevent RTE were also applied and the results were discussed.