Retained Austenite in the C.G.H.A.Z. of 0.1 wt% C Si-alloyed Steels

This investigation deals at first with the existence of retained austenite in the coarse-grained heat-affected zone (CGHAZ) of 0.1 wt% C steels, and subsequently with the mechanism of deterioration of the CGHAZ toughness by retained austenite. The study is achieved by X-ray diffractometry, Mössbauer spectrometry, transmission electron microscopy, instrumented Charpy V, CTOD and impact tensile tests. The work demonstrates the general increase of the concentration of carbon, and therefore of the stability of retained austenite with the 0.05 to 0.5 wt% silicon content of only Ti-microalloyed steels. These correlations are explained by the hindering effect of silicon on the formation of carbides. Otherwise, and according on the one hand to the test temperature, and on the other hand to their morphology, strength and localization, the particles of retained austenite are more or less completely transformed into martensite during an impact because of their stress raising power. The influence of the attendant stress concentrations on the fracture mechanism is further demonstrated with the consequence of the interpretation of the deleterious effect of retained austenite on the CGHAZ toughness. Finally, the whole of these results lead to the choice of an optimum chemical composition that is proved to ensure a favourable microstructure for the CGHAZ toughness of 0.1 wt% C microalloyed steels