RETAINED AUSTENITE IN THE CGHAZ OF 0.1 WT-PERCENT C SI-ALLOYED STEELS

This investigation deals at first with the existence of retained auste nite in the coarse-grained heat-affected zone (CGHAZ) of 0.1 wt% C ste els, and subsequently with the mechanism of deterioration of the CGHAZ toughness by retained austenite. The study is achieved by X-ray diffr actometry, Mossbauer spectrometry, transmission electron microscopy, i nstrumented Charpy V, CTOD and impact tensile tests. The work demonstr ates the general increase of the concentration of carbon, and therefor e of the stability of retained austenite with the 0.05 to 0.5 wt% sili con content of only Ti-microalloyed steels. These correlations are exp lained by the hindering effect of silicon on the formation of carbides . Otherwise, and according on the one hand to the test temperature, an d on the other hand to their morphology, strength and localization, th e particles of retained austenite are more or less completely transfor med into martensite during an impact because of their stress raising p ower. The influence of the attendant stress concentrations on the frac ture mechanism is further demonstrated with the consequence of the int erpretation of the deleterious effect of retained austenite on the CGH AZ toughness. Finally, the whole of these results lead to the choice o f an optimum chemical composition that is proved to ensure a favourabl e microstructure for the CGHAZ toughness of 0.1 wt% C microalloyed ste els.