The combining effects of ausforming and below-Ms or above-Ms austempering on the transformation kinetics, microstructure and mechanical properties of low-carbon bainitic steel

The isothermal transformation kinetics, microstructure and mechanical properties of a low-carbon bainitic steel, subjected to below-Ms/above-Ms austempering with or without prior ausforming, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that for all austempered samples, prior ausforming largely refines the bainitic laths and enhances the mechanical stability of supercooled austenite, the latter leaving more austenite untransformed at the finish of isothermal treatment. Nevertheless, divergent consequences on the final microstructure arise: for the above-Ms austempered samples, prior ausforming increases the amount of large, unwanted brittle martensite/austenite blocks and decreases the volume fraction of retained austenite at room temperature; however, for the below-Ms austempered samples, prior ausforming decreases the size and amount of martensite/austenite blocks and increases the volume fraction of retained austenite. Accordingly, the below-Ms austempered samples with prior ausforming exhibit a product of strength and ductility of ∼ 43 GPa% and impact toughness of ∼180 J/cm2, in sharp contrast with those of the above-Ms austempered samples with prior ausforming, ∼33 GPa% and ∼80 J/cm2, respectively. The present results clearly demonstrate a favorable effect of the combined process of ausforming and below-Ms austempering against the adverse combining effect of ausforming and above-Ms austempering. Keywords: Mechanical properties, Carbide-free bainite, Transformation kinetics, Isothermal treatment, Ausforming