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

Given the transformation stasis after the incomplete transformation of carbide-free bainite, this paper aimed to study the effects of austempering time within transformation stasis on the bainite microstructure, monotonic deformation behavior, and especially cyclic deformation behavior using an ultrahigh silicon (2.59 wt%) steel. With increasing austempering time, the dislocation density of bainitic ferrite decreased, carbon content of retained austenite slight increased, and carbon distribution in retained austenite blocks gradually homogenized. The best combination of strength and ductility was obtained through a longer austempering time within transformation stasis. But the longer austempering time, indeed, did not result in longer fatigue life. The opposite trend could be explained by the fact that the primary factors affecting these mechanical properties were different. The lower density of mobile dislocation pre-existed in the starting microstructure of the samples austempered for a longer time was primarily responsible for its lower cyclic hardenability. Moreover, one retained austenite block with completely homogeneous carbon distribution was only transformed to one martensite grain, which increased the cyclic softening and degrade the fatigue life. By contrast, the deformation-induced martensite transformation from the retained austenite with higher mechanical stability in the samples austempered for a longer time enhanced strain hardenability at higher monotonic tensile strains and well delayed the necking, thus improving the combination of strength and ductility.