Understanding basic mechanism to optimize and predict in service properties of nanobainitic steels (MECBAIN) : final report

The present project, which involved seven industrial or academic partners (Ascometal, Bosch, CNRS, CSIC-CENIM, GERDAU I+D EUROPA, Ruhr Universität Bochum, Technische Universität Kaiserslautern AWP), was concerned with understanding the ductility and the fatigue properties of 'carbide-free' low temperature bainite as obtained through isothermal transformation at 220-250 °C. The tensile and fatigue properties of six different materials were investigated, with different heat-treatment conditions. Efforts to understand tensile ductility focussed on retained austenite stability and were successful in relating tensile ductility with retained austenite mechanical stability. It is shown that poor stability leads to brittle behaviour (rupture before UTS), and that the onset of fracture in these conditions may be related to the fraction of martensite formed. Once sufficient stability was achieved, tensile elongation improved constantly with retained austenite stability. From a microstructural point of view, it is shown that poor stability is associated with a higher fraction of blocky retained austenite and/or a lesser carbon enrichment of these features. For at least one of the materials, fatigue performances were shown to be 10-20% superior to the reference material 100Cr6 (heat-treated to a UTS of 2300 MPa). Detailed investigation of fatigue specimens suggested that the critical microstructural feature governing fatigue strength was the block size. Finally, modelling helped explain the supersaturation of bainitic ferrite, and explore the potential role of Si in that respect. It was shown that higher carbon solubility in bainitic ferrite could be expected if the latter retained its tetragonality. Estimation of the different interface strengths were carried out, and the influence of alloying elements such as Si and C on the latter was estimated. It was shown that in the presence of C, differences in strength between the different interfaces tended to level out. Finally, the results suggest that addition of Si and C, as they increase reduce the tendency for mechanical destabilisation of austenite, which is consistent with many of the experimental results obtained.