Mechanisms and kinetics of tempering in weldments of 9Cr–1Mo steel

Abstract The microstructural mechanisms and the kinetics of the tempering process in an MMA welded 9Cr–1Mo steel have been studied in detail. Based on the microstructural studies of the various regions of the weldment tempered at different temperatures, three distinct mechanisms could be identified for the gradual softening observed in the weldments. A classification scheme has been proposed based on which the temperature regimes over which tempering proceeds through different mechanisms, have been rationalised. The kinetics of the tempering process have been studied using the temperature dependence of the rate of softening. The apparent activation energy of the tempering process is evaluated using an Arrhenius analysis and the corresponding rate-controlling process is identified as the diffusion of carbon in α-ferrite.