Morphological aspects of martensite–austenite constituents in intercritical and coarse grain heat affected zones of structural steels

Thermal simulation with Gleeble was performed in order to reproduce HAZ microstructures in steels with silicon content varying between 0.003 and 0.315%. Optical metallographic characterisation of the fine scale constituents was used to investigate the formation of martensite–austenite (M–A) constituents in coarse grain heat affected zones or intercritically reheated heat affected zones. It was found that silicon increased the density of M–A constituents, the proportion of the large and massive constituents, and favoured their formation on a wider range of temperature. Compared with the coarse grain heat affected zone, a further intercritical peak increases notably the proportion of large and massive M–A constituents. Experiments showed that a subsequent temper treatment contributes to the transformation of M–A into carbides, higher silicon delaying this transformation. It was shown that the change in M–A morphology during tempering cannot be described as a progressive “shrinkage” phenomenon resulting from carbon diffusion.