Partial induction hardening realizes the different strengths in the longitudinal direction of the steel bar. This study investigate the material property of the strength transition region, which is located between the two different strength regions. The strength transition region connecting the regions with the target yield strengths of 800 and 1200 MPa, shows a smooth strength variation without any metal phase changes. However, the strength transition region connecting the 600 and 1200 MPa-regions is tempered martensite outside but ferrite-pearlite inside. It results from insufficient core temperature during the heat treatment, and eventually makes it the origin of the tensile fracture.
