Published: Jan 1984
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A metallurgical evaluation of a failed steam turbine disk was performed. The disk in question had failed in service and was one of eleven shrunk-on disks comprising the LP turbine rotor of an electric generating unit. The investigation included detailed fractographic examinations of the initiation area of the primary fracture and metallographic and fractographic examination of subcritical defects. A critical magnetic particle inspection of all blade grooves was performed, and mechanical properties tests and chemical analyses were conducted to characterize the disk material. Slow-strain-rate tests were also performed to investigate the cracking susceptibility of the disk material in selected environments. The observations made and the data obtained indicate that subcritical crack growth occurred in certain blade attachment grooves by some form of stress corrosion cracking. On the basis of the results of this investigation, the failure of the disk is attributed to the combined effects of high operating stresses at the blade grooves, low fracture toughness of the disk material, and environment.
steam turbines, turbine disks, stress corrosion, NiCrMoV steels, fractography
Staff Engineer, Southwest Research Institute, San Antonio, Tex.
Manager, Materials Engineering, Allis-Chalmers Corporation, Milwaukee, Wisc.