A study of nonisothermal tempering kinetics in plain 9Cr 1Mo steel

The present study elaborates the behavior of tempering reaction of nuclear grade plain 9Cr 1Mo martensitic steel investigated under a constant heating rate imposed by a high temperature heat flux differential scanning calorimeter (DSC). The heating rate imposed is varied from 1 to 30 K min −1 . The inflection in the baseline which occurs during the tempering reaction namely, martensite (α′) to ferrite (α) + chromium carbide (M 23 C 6 ) around 873 K is identified in the DSC profile as the typical tempering onset temperature. The DSC profiles are used to characterize the extent of carbide precipitation under different heating rates. The analysis of the kinetics of tempering reaction is performed using Kolmogorov-Johnson-Mehl-Avrami (KJMA) model to extract the kinetic parameters. In addition, the Kissinger method of obtaining apparent activation energy (Q eff ) is also attempted. The value of the Q eff obtained using both of the models are in good agreement and suggest that bulk diffusion of chromium in iron lattice could be the rate controlling mechanism in the tempering of 9Cr-steels. The effect of tempering reaction is also supported by metallographic studies.