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

An attempt was made to evaluate nondestructively the degradation of thermally aged 2.25Cr-1Mo steel by electrical resistivity measurement. Artificial aging was performed to simulate the microstructural changes in 2.25Cr-1Mo steel arising from long time exposure at 540 C. Microstructural parameter (amount of dissolved Mo in the matrix), mechanical property (ductile-brittle transition temperature) and electrical resistivity were measured to investigate the relationship among these parameters. Both the amount of dissolved Mo and the electrical resistivity decreased rapidly in the initial 1000 hours of aging and then changed little thereafter. On the other hand, the ductile-brittle transition temperature (DBTT) increased rapidly in the initial stage of aging and then saturated afterward. Electrical resistivity was found to have linear correlation with the amount of dissolved Mo and DBTT, respectivley. Electrical resistivity was suggested as a potential nondestructive evaluation parameter for assessing DBTT of the thermally degraded 2.25Cr-1Mo steel.

Nondestructive Evaluation of Thermally Degraded 2.25Cr-1Mo Steel by Electrical Resistivity Measurement

Temper embrittlement of a 2.25Cr1Mo steel is evaluated by use of small punch testing with disc specimens. Clearly, a ductile-to-brittle transition temperature of the steel may be determined by this test, but the value obtained is much lower than that determined by the standard Charpy test. The steel exhibits some temper embrittlement when tempered at 400°C for 86 days.

Temper embrittlement of a CrMo low-alloy steel evaluated by means of small punch testing

Bolts made of2
 1/4 CrMoV steel became gradually embrittled and exhibited an increasing fraction of intercrystalline fracture after long exposure (more than 25,000 h) at 540 °C. This has been found to be caused by the segregation of phosphorus to the prior austenite grain boundaries. This was accompanied by the depletion of molybdenum in the ferrite down to about 0.3 pct owing to the formation of a molybdenum-containing carbide Fe3Mo3C(M6C). Reheating of the embrittled bolts to 680 °C removed the segregation of phosphorus at the grain boundaries and therefore also removed the intercrystalline embrittlement. However, the molybdenum content of the ferrite was lowered further to about 0.2 pct; hence the scavenging of phosphorus by molybdenum was further reduced, and therefore the embrittling tendency of these bolts during reuse was increased. Even after reaustenitizing, quenching and tempering, the service life of these bolts was found to be shorter than in the original condition; this is presumed to be due to an inhomogeneity of molybdenum in the austenite not removed by the reaustenitizing treatment.

K. H. Kuo

Papers

Embrittlement of 2 1/4 CrMoV steel bolts after long exposure at 540 °C