Precipitation in 1CrMoV rotor steels is reviewed with respect to crystal structure, composition, nucleation behaviour and morphology. In addition, the effects of aging on precipitation are described, and the major effects of tensile and cyclic deformation discussed. Where data relating to 1CrMoV is not available, reference has been made to work in which similar steels have been examined.

A critical review of precipitation behaviour in 1CrMoV rotor steels

The influence of operating temperature on in-service degradation of mechanical properties of high temperature stream turbine components has been investigated. Material samples for this study were taken from a Cr-Mo-V rotor and several 2.25Cr-1Mo cast steel components which had operated over 200,000 hours. The test results revealed that the degree of in-service degradation of strength, toughness, and the fracture appearance transition temperature of both steels were very sensitive to the service temperature. Both steels softened only when they were exposed at a temperature greater than 454°C (850°F) and the degree of softening increased with further increase in service temperature. In Cr-Mo-V steel, the loss in strength was accompanied by an improvement in ductility and toughness. Despite softening of 2.25Cr-1Mo steel in service, elevated temperature exposure resulted in a marked decrease in ductility and toughness. The loss of toughness in this steel was in part irreversible. In contrast, a severe increase in fracture appearance transition temperature, due to reversible temper embrittlement, occurred in both steels at a service temperature of around 427°C (800°F), but not at the highest service temperature. In fact, the Cr-Mo-V steel did not temper embrittle as a result of service exposure at the highest operating temperature investigated. These results are rationalized in terms of changes in microstructure and grain boundary chemistry that occur in service as a function of operating temperature.

Degradation of mechanical properties of Cr-Mo-V and 2.25Cr-1Mo steel components after long-term service at elevated temperatures

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.

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

The paper presents the results of research on the microstructure and mechanical properties of 7CrMoVTiB10 – 10 (T24) steel. The examined steel was subject to ageing at the temperature of 550 and 600 °C for 70,000 h. Performed tests of mechanical properties (YS, TS, YS 550 , HV10) of T24 steel have shown that, after ageing at the temperature of 550 °C, the observed decrease was not larger than 20% in comparison with the as-received state – the properties were higher than the required minimum. Ageing of the T24 steel at the higher temperature resulted in a greater reduction of its mechanical properties – they were lower than the required minimum. Performed microstructural tests have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure, the growth of the size of M 23 C 6 carbides, and precipitation of the secondary M 2 C and M 6 C precipitates. The magnitude of these changes depended on the temperature of ageing. The changes observed in the microstructure of the steel aged at the higher temperature, i.e. 600 °C were more advanced than those observed at the temperature of 550 °C.

Influence of long-term ageing on the microstructure and mechanical properties of T24 steel

A comparison of extrapolation techniques for long-term creep strain and creep life prediction based on equations designed to represent creep curve shape

K.R. Williams

Papers

Microstructural instability of 0.5Cr0.5Mo0.25V creep-resistant steel during service at elevated temperatures