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.