Influence of high temperature low cycle fatigue deformation on the microstructure of a CrMoV rotor steel subjected to long-term service exposure at 425°C and retempering at 677°C

Abstract

A study has been made of the influence on the microstructure of a CrMoV rotor steel of low cycle fatigue (LCF) deformation at 538°C with or without a 1-hour hold time applied at the peak tensile load of the fatigue cycle, following service exposure at 425°C for 23 years and retempering at 677°C for 24 hours. Samples for transmission electron microscopy were taken from the shoulder and gage sections of the broken fatigue specimens. The shoulder section of this steel revealed massive M23C6 along the grain boundaries, M3C in globular or ellipsoidal form, fine spheroidal MC type carbide, and long needles of M2C. LCF deformation at 538°C led to a tendency toward spheroidization of M3C, promoted the nucleation of M2C in ferrite, and slightly coarsened the MC carbide. Superimposing a 1-hour hold period at the peak tensile load resulted in nucleating M2C preferentially at cementite-ferrite interfaces. © 1992.