Analysis of phase transformations during continuous cooling by the first derivative of dilatation in low carbon steels

Transformation behaviors during continuous cooling were analyzed by dilatation using the lever rule, which provides the start and finish temperatures of the overall transformation. For a precise understanding of the transformation behaviors of each phase, we analyzed dilatations with optical micrographs, and analyzed dilatations and their first derivatives with temperature as well as the calculated equilibrium temperatures of each phase. Because the first derivatives of dilatations represent the transformation behavior during continuous cooling, as with dilatation, the transformation characteristics of each phase were analyzed by these curves. By taking the first derivatives of dilatations with temperature and overlapping their curves, we observed three types of peaks related to the characteristics of the transformed phases on the curves; single peak, superimposed double peaks and separated double peaks. Because dilatometric experiments provide data on transformations as well as noise, the dilatations were revised by Fast Fourier Transformation (FFT) and Wavelet Transformation (WT) for a more accurate analysis of transformation behaviors.