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

Abstract

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 terms of the varying dislocation densities and the numbers of interfaces in the structures.