Characterization of surface hardness, hardened layer depth and residual stress in laser-hardened steel using high-spatial-resolution magnetic incremental permeability sensor

Laser hardening is a critical surface strengthening technique for metallic components, and accurate evaluation of the hardened region is essential for quality assurance. However, key mechanical properties, including hardness, residual stress, and hardened layer depth, undergo complex changes during the hardening process. Furthermore, the precision-hardened zones and their transition regions are often sub-millimetre in scale, posing significant challenges for conventional characterisation methods. In this study, magnetic incremental permeability (MIP) with high spatial resolution is employed to characterise multiple mechanical properties within the laser-hardened region. A double-sided polished MIP sensor, featuring a magnetic gap with both width and length on the micrometre scale, has been developed. Its sub-millimetre spatial resolution is experimentally validated using defect-mimicking specimens. MIP measurements are then performed on various strips of laser-hardened Grade 50 samples, revealing strong linear correlations between MIP magnetic parameters and the three mechanical properties. Based on these findings, a multivariate linear regression model is constructed for simultaneous characterisation, achieving a goodness of fit of 0.955. These results demonstrate the feasibility and effectiveness of the high-resolution MIP method for non-destructive, multi-parameter evaluation of laser-hardened components.