Evaluation of Creep Properties of Degraded Cr-Mo-V Turbine Cast Steels through Hardness Measurement and Its Application to Life Assessment

Abstract

Several 1.25Cr-1Mo-0.25V steam turbine cast components retired from fossil power plants after long time service at high temperatures were cut into specimens and degraded creep properties were investigated. Materials used were laboratory aged castings, used casings, valve chests, and a nozzle body.
The purpose of this paper is to characterize material degradation and to establish creep life estimation procedure for those old components.
It was observed that hardness was lower at high temperature portions of retired components than original production and this thermal softening was attributed to carbide coarsening and recovery of dislocation microstructure. Creep rate was increased and creep rupture time was reduced according to the thermal softening.
Quantitative relationships between hardness and creep rate, and between hardness and creep rupture time were developed. Using these relationships, creep life estimation procedure was established for cast components, which reasonably took into account material degradation during prolonged exposure to elevated temperatures through hardness measurement.