Effect of Deformation during Austempering on Bainite Transformation and Retained Austenite in a Medium‐Carbon Bainitic Steel

Carbide-free bainitic steels present microstructures containing bainitic ferrite, carbon-enriched retained austenite (RA), and may contain martensite. The effects of deformation during austempering on the subsequent bainite transformation and the final volume fraction of RA are not reported. Herein, compressive true strains of 20% and 40% are applied on two specimens during isothermal holding. For comparison, a specimen is treated with pure isothermal holding. The results show that the deformation during isothermal holding enables the subsequent transformation of the carbon-enriched austenite into bainite. The bainite fraction formed subsequent to the deformation increases with the applied strain level. Due to the mechanical stabilization of austenite, the final fraction of RA increases with the strain level. Moreover, the martensite/austenite constituent in the final microstructure is clearly refined by deformation. For comparison, a specimen is deformed before isothermal holding. The results indicate a higher total bainite fraction compared with the specimens deformed during austempering. This effect correlates well with the chemical stabilization of the untransformed austenite in the specimens deformed during austempering.