Eddy Current Mapping Technology for Residual Stress and Surface Hardness Evaluation in Laser Hardened Steels

Abstract

Laser quenching can achieve surface hardening with narrow heat affected zone while without affecting the surface roughness of the material. Therefore, the laser quenching technique is increasingly applied for surface hardening in key components such as precision gear, guide rail, etc. Phase transformation and thermal diffusion occur in the process of laser quenching, resulting in change of microstructures and residual stress distribution in the surface layer material. To evaluate the quality of the laser-hardened material of 45and 30Cr steel, eddy current mapping technology is employed in this study to characterize the residual stress and surface hardness. First, C-scan eddy current inspection was performed to the laser hardened region. The mapping results of the impedance parameters clearly show the boundary between the hardened zone and the matrix. Second, X-ray diffraction system and Vickers hardness tester were alternatively used to analyze the distribution of surface hardness and residual stress. Third, the correlation between the individual eddy current parameter and the surface hardness (or residual stress) is investigated by simple linear regression tool. The impedance amplitude demonstrates good linear dependencies on both the surface hardness and residual stress. Therefore, the proposed eddy current mapping technology is capable of mapping the distribution of surface hardness (related with microstructures) or the residual stress in laser hardened ferromagnetic components.