Hardness and microstructure of interstitial free steels in the early stage of high-pressure torsion

The hardness and microstructure distributions in interstitial free (IF) steel disks processed via the high-pressure torsion (HPT) process with an early stage (up to 1 turn) are investigated using experimental and simulation approaches. The results indicate that the deformation in the HPT-processed IF steel disk is inhomogeneous, providing almost linearly increasing hardness from the center to the edge regions. In particular, near the surface of the disk is a soft region that shrinks with increasing numbers of revolutions. Compared with the compression-only disks by the HPT die, there is a hardness hill in the center region of the HPT-processed disk. The hardness distributions in the HPT disks indicate that the deformation proceeds gradually from the edge to the center with the degree of revolutions. In addition, as the degree of revolutions increases, the strain in the center region increases and the plastic deformation becomes uniform along the radial direction. The finite element analyses strongly support the conclusions of the experimental results.