Model based laser-ultrasound determination of hardness gradients of gas-carburized steel |
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| Affiliation: | 1. Department of Civil Engineering, University of Malaya, Kuala Lumpur, Malaysia;2. Department of Physics, University of Malaya, Kuala Lumpur, Malaysia;1. School of Traffic and Transportation Engineering, Central South University, Changsha 410075, China;2. Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, Quanzhou 362200, China;3. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China;1. National Key Laboratory for Remanufacturing, Academy of Army Armored Forces, Beijing 100072, China;2. 92601 Troops, People’s Liberation Army Navy, Zhanjiang 524009, China;3. College of Material Science and Engineering, Shanghai University, Shanghai 200444, China;4. Key Lab of Nondestructive Testing Ministry of Education, Nanchang Hangkong University, Nanchang 330063, China |
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| Abstract: | Gas carburizing is a common industrial process utilized for case hardening of low carbon steels. However, there is a lack of non-destructive evaluation systems for the measurement of hardness-depth profiles. We propose a novel measurement method for the determination of hardness-depth profiles of two-step gas carburized steel specimens. The method is based on the measurement of broadband laser excited Rayleigh waves. Rayleigh waves were generated by a pulsed Nd: YAG laser in the thermoelastic regime and measured with a heterodyne Mach-Zehnder interferometer in the near-field. From two measurements with different source to receiver distances the dispersion diagrams were calculated by means of the phase spectral analysis method. In order to simulate the observed dispersive behavior of the Rayleigh waves, first the two-step gas carburizing process was simulated using solutions of the diffusion equation. The resulting continuous hardness profile was then discretized into up to 100 layers. Thereafter the Rayleigh wave dispersion diagram was calculated from the discretized stack of layers using a delta-matrix formulation of the Thomson-Haskell transfer matrix method. In order to obtain best fitting hardness profiles, the simulated dispersion diagrams were fitted to measurements with a curve fitting algorithm. Comparison of the Rayleigh wave inversion method with destructively obtained Vickers hardness profiles shows good quantitative agreement. |
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| Keywords: | Surface acoustic waves Laser-ultrasound Case hardening Gas carburizing |
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