To improve the performance of the rails and wheels for the railway industry, new microstructures need to be researched. Nevertheless, the soft region observed in the weld heat-affected zone (HAZ), caused by the spheroidization of cementite, leads to localized loss of hardness. A methodology for calculating the percentage of cementite spheroidization is not well defined in the literature. Accordingly, a new calculating method for determining the spheroidized cementite fraction was proposed through image analysis. This calculation method observes the cementite area/morphology, not the spheroidized cementite gross area. Two starting microstructures (pearlite and lower bainite) were compared in their spheroidization tendency by divorced eutectoid transformation (DET) during a simulated HAZ thermal cycle. The phase transformation kinetics through the heat treatments was also analyzed using high-resolution dilatometry tests. The results showed that the lower bainite has a higher tendency for cementite spheroidization than the pearlite due to its initial configuration of carbides. Furthermore, it was confirmed that the higher the cooling rates, the lower the spheroidization tendency.