Model based laser-ultrasound determination of hardness gradients of gas-carburized steel
Gas carburizing is a common industrial process utilized for case hardening of low carbon steels. However, thereis a lack of non-destructive evaluation systems for the measurement of hardness-depth profiles. We propose anovel measurement method for the determination of hardness-depth profiles of two-step gas carburized steelspecimens. The method is based on the measurement of broadband laser excited Rayleigh waves. Rayleighwaves were generated by a pulsed Nd: YAG laser in the thermoelastic regime and measured with a heterodyneMach-Zehnder interferometer in the near-field. From two measurements with different source to receiverdistances the dispersion diagrams were calculated by means of the phase spectral analysis method. In order tosimulate the observed dispersive behavior of the Rayleigh waves,first the two-step gas carburizing process wassimulated using solutions of the diffusion equation. The resulting continuous hardness profile was thendiscretized into up to 100 layers. Thereafter the Rayleigh wave dispersion diagram was calculated from thediscretized stack of layers using a delta-matrix formulation of the Thomson-Haskell transfer matrix method. Inorder to obtain bestfitting hardness profiles, the simulated dispersion diagrams werefitted to measurementswith a curvefitting algorithm. Comparison of the Rayleigh wave inversion method with destructively obtainedVickers hardness profiles shows good quantitative agreement.