Non-destructive method of characterizing nitrided layers in the 42CrMo4 steel using the amplitude-frequency technique of eddy currents

Abstract The aim of this work was to investigate the possibility of using the eddy current method, a technique for measuring voltage amplitude and resonant frequency, for non-destructive assessment of the thickness of the near-surface layer of iron nitrides in 42CrMo4 steel after gas nitriding. The scope of the work included preparation of test samples, chemical composition tests, surface roughness measurements, hardness distribution using the Vicker’s method and measurements of the thickness of nitrided layer on cross-sections, X-ray phase composition analysis, testing of nitrided layer using the eddy current method, analysis of the correlation of the results of destructive and non-destructive tests. The main research apparatus was the Wirotest M2 with the 25 kHz measuring head. Differences in electromagnetic parameters between the white layer and the rest of the nitrided material, as well as changes in the surface roughness of the layer, are factors influencing the eddy current signal, which allows indirect measurement of its thickness. The analysis of the voltage amplitude is more accurate, than the resonant frequency, in assessing the thickness of nitrides layer. With the increase in thickness of the nitrides layer, the voltage value of the signal of eddy currents increases. The research results also indicate the possibility of using the same measuring head to assess the roughness parameter Ra of the nitrided layer. The Wirotest M2 can be used in quality control of steel parts after nitriding.