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 M 23C 6 along the grain boundaries, M 3C in globular or ellipsoidal form, fine spheroidal MC type carbide, and long needles of M 2C. LCF deformation at 538°C led to a tendency toward spheroidization of M 3C, promoted the nucleation of M 2C in ferrite, and slightly coarsened the MC carbide. Superimposing a 1-hour hold period at the peak tensile load resulted in nucleating M 2C preferentially at cementite-ferrite interfaces.