Effect of long-term service exposure on microstructure and mechanical properties of a crmov steam turbine rotor steel

Abstract

The influence of prolonged service exposure on the microstructure and mechanical properties of a 1Cr-1Mo-0.25V steam turbine rotor steel has been studied. The samples for this study were taken from four locations of a rotor which had operated for 23 years. The operating temperatures at these locations were 288 °C, 425 °C, 510 °C, and 527 °C. The impact of retempering at 677 °C of steel exposed at 425 °C was also investigated. Service exposure at 288 °C brought no noticeable changes in either tensile properties or microstructure; the steel contained coarse bainitic cementite, extremely fine spheroidal MC, and thin platelets of M₂C. Service exposure at 510 °C led to profuse precipitation of cementite along grain boundaries in addition to increasing M₂C precipitation. These changes resulted in a slight decrease in the yield and tensile strengths and a marginal increase in ductility. Service exposure at 527 °C produced grain boundary precipitation of M₂₃C6, coarsening of MC, and more profuse precipitation of M₂C and caused a considerable decrease in strength and an increase in ductility. Retempering at 677 °C for 24 hours resulted in more precipitation of M₂₃C₆ and considerable coarsening of MC, without affecting further the size or shape of M₂C. The strength of the steel decreased drastically and the reduction in area increased considerably due to retempering. These changes in microstructure and mechanical properties indicate that service exposure at 527 °C for 23 years did not produce a stable microstructure. The microstructure and mechanical properties of the rotor steel would continue to deteriorate in future operation.