Investigation of bainite formation in a chemically inhomogeneous medium carbon steel

Abstract

The effect of chemical inhomogeneity on the isothermal bainite formation is investigated in medium-carbon low-silicon spring steel by dilatometry and microscopy. The analysis of the microstructure at different times during transformation shows that chemical segregation of substitutional alloying elements resulting from casting strongly affects the bainite formation by retarding the transformation kinetics and limiting the maximum achievable bainite fraction. The effect of prior austenite grain size and Cr-rich carbide precipitation in the segregation bands is investigated and compared with the findings in homogenized material. A physically based model is used to simulate bainite formation and the mechanisms of nucleation and growth are discussed. The calculated difference in nucleation rates between the enriched and the depleted areas is not by itself sufficient to explain the microstructures obtained and thus significant influence variations of growth on bainite formation is involved, which indicates that a diffusional transformation is taking place.