Deformation mechanisms in nanostructured bainitic steels under torsion

A clean high carbon steel was made using vacuum induction melting and three distinct nanostructures were obtained by austempering it at 250, 300 and 350 degrees C respectively after a similar austenitization treatment at 950 degrees C. A decrease in austempering temperature causes refinement of bainitic ferrite (BF) as well as retained austenite (RA). Moreover, the volume fraction of BF increases with a reduction in austempering temperature. The mechanical properties under tension and torsion at ambient temperature were investigated. The results demonstrated that the slenderness of BF plates as well as their enhanced volume fraction plays a major role in enhancing the tensile as well as shear strength. This came with a concomitant loss in ductility of the specimens austempered at lower temperatures. However, the hardness measurements on the gauge section were able to highlight the important differences in the deformation behavior between torsion and uniaxial tension. It was found that hardness increase of the specimen after torsion was much higher than uniaxial tension. The ductility is controlled by the content and stability of RA, BF as well as the newly formed martensite. X-ray diffraction studies showed that more RA was transformed under torsion than uniaxial tension. Steel specimens with the highest content of RA showed the highest ductility under torsion while the specimens with the highest BF showed the highest strength.