Characterisation of thermal stability and phase transformation energetics in tempered 9Cr–1Mo steel using drop and differential scanning calorimetry

The thermodynamic stability and phase transformation energetics of a tempered 9Cr–1Mo steel have been studied using drop and isochronal differential scanning calorimetry techniques. The measured enthalpy reveals a steep increase with temperature, especially for temperatures exceeding 650 °C. It is found that the emergence of various phases as a function of temperature has been clearly marked by the presence of distinct inflections in the measured enthalpy increment versus temperature curve, obtained using drop calorimetry. The isochronal differential scanning calorimetry experiments yielded precise values for the transformation arrest points. The ferromagnetic to paramagnetic transformation temperature is found to be about 740 ± 5 °C. The austenite start (Ac1) and finish (Ac3) temperatures recorded during continuous heating are found to be strongly dependent on the heating rate and the extrapolated, so called zero scan rate values are found to be 830 and 870 ± 5 °C, respectively, for Ac1 and Ac3. The process of cooling from austenite field yielded martensite for the cooling rates 5–40 °C min−1 employed in the present study. The martensite start temperature is fairly independent of the cooling rate and is found to be 370 ± 5 °C. By adopting a phenomenological model for calculating the enthalpy of metastable ferrite at temperatures higher than its known range of stability, the enthalpy change associated with austenitisation (α-ferrite + carbide → γ-austenite) transformation has been estimated as a function of temperature. It is found that in the temperature range 625–1200 °C, the variation of the enthalpy with temperature due to phase change can be considered to be linear, which in effect renders a constant value for the effective transformation specific heat. This value is found to be of the order of 15.4 J mol−1 K−1 (±8%) in the temperature interval, 650–1200 °C.