A steel plate having hardened and non-hardened regions in one plate is promising material in automobile field owing to its high performance. This steel plate has to be evaluated whether the non-hardened region was prepared properly at the desired position with desired hardness. However, the conventional evaluation method of hardness measurement is destructive testing and requires a long inspection time. Therefore, a reliable and fast nondestructive inspection method is required. In this study, the hardened and non-hardened regions were evaluated using an eddy current testing (ECT) to distinguish each region. The measured resistivity and permeability at each region were different and these differences were found to be caused by the crystal structure change of heat treatment. Therefore, the magnetic response between the hardened and non-hardened regions was different and this difference was correlated with the hardness of each region. These results indicate that the hardened and non-hardened regions can be distinguished using ECT.
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