Evaluation of transformation latent heat in C-Mn steels

Two sublattice thermodynamic model and DTA technique were adopted to determ ine the latent heat of alpha + Fe3C-->gamma transformation in C-Mn steels. The calculated latent heat varied with the transformation route. Supercooli ng resulted in higher latent heat than superheating. By splitting enthalpy change into components of specific heat and latent heat, the equilibrium la tent heat was calculated and proven to be a constant indpendent of cooling or heating. The latent heat determined using DTA agreed very well with calc ulation as carbon content was higher than 0.45wt%, while it was significant ly lower than calculation at lower carbon content. The inconsistency was at tributed to that lower carbon steels had a wide transformation temperature range, but DTA only detected heat change over part of the range. Molar frac tion of pearlite, latent heat absorbed per unit temperature, and the temper ature range of transformation were found to be the three main factors affec ting latent heat. Carbon and manganese additions increased latent heat by i ncreasing molar fraction of pearlite, while silicon addition increased late nt heat by expanding temperature range of transformation.