2025 Volume 89 Issue 5 Pages 188-194
S50C steel and SCM440 steel were produced with systematically varied hardening depths, and their microstructure, hardness, and magnetic hysteresis properties were investigated. In S50C steel, a positive correlation was confirmed between the hardening depth and coercivity. To explore the mechanism behind this correlation, dynamic magnetic domain structure observations were conducted using Lorentz microscopy to investigate the behavior of domain wall movement within the hardened and base metal regions when a magnetic field was applied to the sample. In the ferrite-pearlite structure of the base metal, the domain size in the ferrite phase was large, facilitating domain wall movement. In contrast, in the martensite of the hardened region, the domain size was small, and numerous block and packet boundaries hindered the movement of domain walls. This was identified as the cause of the positive correlation. This study demonstrates the usefulness of magnetic non-destructive evaluation methods for assessing the hardening depth of surface-hardened materials.
