Power-generating plants require materials that can withstand service conditions up to 100 000 h. It is well established that creep properties are controlled by the microstructure and that the microstructure changes during creep service. Accelerated aging was used to obtain a series of simulated microstructures similar to those observed in specimens after long-time service. Electron microscopy was used to monitor the microstructural changes in the specimens. Energy-dispersive X-ray analysis was performed on carbon extraction replicas in a transmission electron microscope. The analysis was combined with an in situ electron diffraction identification of the crystal structure of the carbides present. The authors concluded that accelerated thermal aging can produce simulated service microstructures and that it is possible to distinguish between the cubic carbide M6C and M23C6 using the X-rays generated in these crystals by the electron beam. Thus a comprehensive study of carbide formation and composit...

Analysis of carbides formed during accelerated aging of 2·25Cr–1Mo steel

The rejuvenation procedures to recover the creep properties of nickel-base superalloys by atmospheric pressure heat treatment and hot isostatic pressing techniques have been reviewed in detail It is very important that such treatments be applied at an optimum stage in the service life of a turbine blade In other words, the rejuvenation procedures must be applied early enough to prevent catastrophic failures or irreparable damage and late enough to give a cost-effective benefit The optimum stage at which to undertake a rejuvenation procedure to extend the creep lives of superalloys is immediately prior to the tertiary stage By using these techniques it is not possible to extend the creep lives of superalloys indefinitely because of the accumulation of some “permanent” damage incurred during service conditions

Rejuvenation procedures to recover creep properties of nickel-base superalloys by heat treatment and hot isostatic pressing techniques

Damage accumulation and microstructural changes occurring during the creep of a 214%Cr1%Mo steel

Ferrite morphologies and carbide precipitate dispersions in a ½Cr-Mo-V creep-resisting steel have been studied after isothermal transformation at a number of subcritical temperatures in the ferrite and bainite ranges, and also after rapid quenching to room temperature to produce martensite. It has been found that VC precipitates by the interphase mechanism during direct transformation to ferrite, and towards the latter stages of the transformation, some fibres of Mo2C also form. Bainite morphologies ranged from granular at the higher transformation temperatures to lath-like at lower temperatures. Tempering of bainite and martensite at 700°C resulted in VC precipitation on dislocations and the interparticle spacing tended to decrease as the initial transformation temperature in the bainite range was increased. However, the best resistance to overageing during tempering was provided by ferrite produced by direct transformation at 700°C. These differences in dispersion stability are explained in term...

Ferrite morphologies and carbide precipitation in a Cr-Mo-V creep-resisting steel

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.

L.M. Wyatt

Papers

Materials technology in power generation in the U.K.