Effect of pearlitic and bainitic initial microstructure on cementite spheroidization in rail steels

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.