Drop calorimetry studies on 9Cr-1W-0.23V-0.06Ta-0.09C reduced activation steel

Abstract

The temperature dependence of enthalpy increment ( H_T − H₂₉₈) of 9 mass% Cr-1 mass% W-0.23 mass% V-0.06 mass% Ta-0.09 mass% C reduced activation steel has been measured by inverse drop calorimetry in the temperature range 400 K to 1273 K. A critical comparison of present isothermal enthalpy measurements with the results of our previous dynamic calorimetry studies has been made to reveal clearly the occurrence of various diffusional phase transformations that occur at high temperature. These phase changes are marked by the presence of distinct inflections or cusps in an overall nonlinear variation of enthalpy values with temperature. The principal thermal relaxation step of the martensitic microstructure obtained through quenching from the high-temperature γ-austenite phase is observed around 793 K. The ferromagnetic-to-paramagnetic transition of the α-ferrite phase is found to occur at 1015 K. The equilibrium values of γ-austenite start (Ae₁) and finish (Ae₃) temperatures are found to be 1063 K and 1148 K, respectively. A value of 12J · g⁻¹ has been estimated for Δ° H^{α→ γ} the latent heat associated with the α→ γ transformation. The measured enthalpy increment variation of the α-ferrite phase with temperature has been fitted to a suitable empirical function to estimate the temperature-dependent values of the specific heat. A comparison of the drop calorimetry-based indirect estimate of the specific heat with the direct differential scanning calorimetry-based values revealed that the drop calorimetry estimates are systematically lower than its dynamic calorimetry counterpart. This difference is attributed to the fact that, under finite heating rate conditions that are typical of dynamic calorimetry, measurements are made under nonequilibrium conditions. Notwithstanding this limitation, there is a good overall agreement between the two C_p values and also among the phase transformation temperatures so that a reliable assessment of thermal properties and phase transformation characteristics of reduced activation steel can be determined by a combined analysis of the results of drop and differential scanning calorimetry.