Evaluation of transformation latent heat in C-Mn steels

Abstract

Two sublattice thermodynamic model and DTA technique were adopted to determine the latent heat of α + Fe3C → γ transformation in C-Mn steels. The calculated latent heat varied with the transformation route. Supercooling resulted in higher latent heat than superheating. By splitting enthalpy change into components of specific heat and latent heat, the equilibrium latent heat was calculated and proven to be a constant independent of cooling or heating. The latent heat determined using DTA agreed very well with calculation as carbon content was higher than 0.45 wt%, while it was significantly lower than calculation at lower carbon content. The inconsistency was attributed to that lower carbon steels had a wide transformation temperature range, but DTA only detected heat change over part of the range. Molar fraction of pearlite, latent heat absorbed per unit temperature, and the temperature range of transformation were found to be the three main factors affecting latent heat. Carbon and manganese additions increased latent heat by increasing molar fraction of pearlite, while silicon addition increased latent heat by expanding temperature range of transformation.