Analysis of phase transformation in Fe-C alloys using differential scanning calorimetry

Differential scanning calorimetry (DSC) is a well suited technique for studying phase transformation kinetics provided the transformation involves some kind of heat effect. The diffusional transformation in Fe-C alloys and low alloy steels upon cooling from the austenitic state involve both heat of transformation and changes in heat capacity. In principle, it can therefore be analysed using DSC techniques. Particular complications in the analysis of the heat effects occurring on this transformation are the strong temperature dependence of the heat capacity of the ferrite and the strong temperature dependence of the enthalpy of formation of ferrite from austenite. A simple mathematical method is presented, wh ich allows for the non-linear temperature dependence of heat capacities a nd enthalpy of formation. As a consequence, the method can be used to analyse the simultaneous formation of pearlite. Using this method, the fractions of both pro-eutectoid ferrite and pearlite as a function of the temperature are determined from heat capacity measurements for several Fe-C alloys. Data for the enthalpy of formation of pearlite are presented. The method presented is generally applicable for analysing reactions or transformations occurring over a wide temperature interval where the heat effects are temperature dependent.