Effect of torsional deformation on crystallographic evolution of carbide-free nano-bainitic steels prepared at two different austempering temperatures

Abstract

The current study investigates the nature and extent of crystallographic change after torsional deformation in nano-structured bainite. This is extremely useful as nano-structured bainite is not stable under mechanical deformation and the evolved structure under torsion is finally going to determine other mechanical responses like fracture and fatigue. Two blocks of nano-bainitic steels were prepared after austenitization at 950°C and austempering at 250°C (NB250) and 350°C (NB350) respectively. It was found that there was a decrease in the bainitic ferrite (BF) and retained austenite (RA) length scale with a reduction in austempering temperature along with reduction in the RA content. Specimens from the undeformed two blocks (NB250-UD and NB350-UD) were subjected to torsional deformation till failure (NB250-T and NB350-T) and the crystallographic characteristics of the torsion-tested and undeformed specimens were obtained using electron back-scattered diffraction (EBSD). The orientation relationship (OR) between BF and RA for NB250-UD and NB350-UD was close to Kurdjumov-Sachs (K-S) which changed to predominantly Nishiyama Wassermann (NW) after torsion deformation (NB250-T and NB350-T). Significant refinement in packet and block was observed as a result of torsional deformation. Variant selection was stricter for NB350-UD than NB250-UD but torsional deformation led to weakening of variant selection. The extent of lowest misorientation V1-V4 variant pairing was higher for NB350-UD but it did not change much with torsion in case of NB250-T whereas NB350-T displayed a reduction. Fractography revealed that the dimple size corresponded to block size range between the undeformed and torsion-tested sample for the respective austempering condition.