Inconsistent effects of austempering time within transformation stasis on monotonic and cyclic deformation behaviors of an ultrahigh silicon carbide-free nanobainite steel

Effect of Multi-Step Austempering Treatment on the Microstructure and Mechanical Properties of a High Silicon Carbide-Free Bainitic Steel with Bimodal Bainite Distribution

“…Changes in length vs. temperature curves for multi-step treatments are then depicted in Figure 4. As expected, during isothermal holding an increase in specimen length is observed due to bainitic transformation [26,48,52]. For all the multistep heat treatments, no martensite formation is detected while cooling to room temperature.…”

“…Both images illustrate that the microstructure consists of bainitic ferrite plates (dark grey) separated by high carbon enriched austenite in form of films (R AF ) (light grey), as reported in the literature for similar steels [6,[55][56][57]. Moreover, martensitic islands (M) and retained austenite blocks (R AB ) are present in correspondence with the prior austenite grain boundaries, according to Zhao [52]. The formers are the product of partial martensite transformation of retained austenite, as supported by the dilatometric curves shown in Figure 3.…”

“…For all the multistep heat treatments, no martensite formation is detected while cooling to room temperature. This suggests that after the second stage bainitic transformation, the retained austenite was enriched in carbon, thus lowering the Ms below room temperature [48,52].…”

Influence of Austempering Temperature on Microstructure and Mechanical Properties of High‐Silicon Carbide‐Free Bainitic Steel

“…Thus, both are not able to sustain large elongations and suppress the cracks propagation that are forming at the interface between the different microstructural constituents. [ 56 ] Furthermore, the superior mechanical behavior both in terms of strength and elongation for the specimen treated at low temperatures should be ascribed to the thickness of the bainitic ferrite plates. As demonstrated by Garcia‐Mateo et al, [ 57 ] bainitic ferrite represent the strong phase with the CFB mictrostructure and provide a major contribute to the the high strength.…”

Effects of ausforming temperature on carbide-free bainite transformation and its correlation to the transformation plasticity strain in a medium C- Si-rich steel

“…This finding indicates that larger regions of untransformed austenite are partitioned (i.e., geometrically divided) by the formation of oriented bainitic ferrite plates during the Def_400 and Def_325 treatment. A desirable secondary effect of this blocky subdivision is that it enhances the presence of RAfilm, which is thermally and mechanically more stable, improving the ductility and toughness response of the microstructure [52][53][54].…”