The identification of phase by overlapping of first derivative of dilatation in low carbon steels multi-phase presenting

Abstract

The phase transformation in steels has been widely measured by dilatometer using the lever rule. However, the concept of lever rule is no longer applied in case of multi-phase present. Furthermore, it is quite difficult to differentiate a low temperature phase from the others due to the small fraction change (10-3 - 10-4 fraction of original length) and the plastic deformation during transformation. The overlapping of first derivatives of LVDT of several continuous cooling dilatations could be better way to identify and to analyze low temperature phases. In addition, the length change is simulated by considering the lattice parameter changes due to the temperature, composition and phase as well as decomposition kinetics of austenite in order to verify the method suggested. By comparing the simulated length change with the measured, the first derivative of dilatation interfered could be separated for each phase. As a result, the start, finish and peak temperature and the amount of each phase are determined. The method is also confirmed by OM and SEM.