Abstract
The influences of prolonged service on microstructure evolution and mechanical properties of NiCrMoV steel welded joint in an ex-service welded steam turbine rotor were investigated. The welded rotor had been operated for 22 years since 1991. The specimens for the present study were taken from the location where the temperature was as high as 230°C. The optical microscope (OM) showed that even after long-term service, there were no obvious defects such as creep cavities, cracks found in the microstructure of the whole welded joint after such a long term service. The microstructure was uniform and no obvious grain coarsening was observed. However some black strip-shaped zones were found in base metal and heat affected zone (HAZ). The distribution of hardness across the welded joints showed no anomalies. The results of tensile strength and fracture toughness tests demonstrated that the welded joint still exhibited excellent. Mechanical performance after long-term service, indicating that the welding process of Shanghai Turbine Plant was reliable and stable. With the improvement of forging and welding qualities and improved heat treatment furnaces with more accurately controlled temperature, it is reasonable to assume that the current large low-pressure (LP) welded rotors are definitely safe to operate under similar service conditions for designed life.
Highlights
Steam turbine rotor is the key component in fossil fired power plants and nuclear power plants, and its reliability and stability have a decisive effect on the safety and service life of steam turbine
Several investigations have been carried out on evolution of microstructure and mechanical properties of CrMo and CrMoV steels for high-pressure (HP) and intermediate-pressure (IP) steam turbine rotors after prolonged service exposure . [1]-[4] The results revealed that the degree of performance degradation is related to service temperature, service duration and stress level, among which the influence of service temperature and service duration played roles more significant
Specimens sampled from welded joint including base metal, heat affected zone (HAZ) and weld metal were used for conventional optical metallography study
Summary
Steam turbine rotor is the key component in fossil fired power plants and nuclear power plants, and its reliability and stability have a decisive effect on the safety and service life of steam turbine. Operation at elevated temperatures for long terms may lead to deterioration in mechanical properties due to changes in microstructures, such as grain coarsening, precipitates coarsening and coalition, dislocation annihilation and so on. Several investigations have been carried out on evolution of microstructure and mechanical properties of CrMo and CrMoV steels for high-pressure (HP) and intermediate-pressure (IP) steam turbine rotors after prolonged service exposure . Investigations by Joarder and Cheruvu showed that service exposure at 288°C for 23 years resulted in no changes in microstructure and mechanical properties in CrMoV steel[2,3]. With the increase of service temperature between 350°C and 527°C after long-term service exposure, service embrittlement or softening behavior, coarsening and coalition of the precipitates occurred in CrMoV rotor steel[2,3]. All above investigations focused on forged rotors, little work has been conducted to characterize comprehensive performance evolution of welded joints after long-term service for the welded rotor
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