Evaluation of a Nitrided Case Depth by the Magnetic Barkhausen Noise

Applicability of magnetic methods for non-destructive evaluation of plasma nitrided cases was investigated comprehensively. Nitrided layers of 60-800 mu m thicknesses were produced on a 16MnCr5 steel surface varying the plasma temperature, the nitriding time and atmosphere. Low-frequency Barkhausen noise was measured by a unique laboratory system to obtain physically grounded data. The laboratory measurements were performed at the triangular waveforms of the magnetizing voltage and the surface magnetic field. A typical rms envelope with a single peak positioned near the hysteresis coercive field was obtained for the unnitrided homogeneous sample. All nitrided samples with the hardened surface layer demonstrated an additional envelope peak at higher fields. Deeper nitridation suppressed both envelope peaks equally making the classical rms parameter sensitive to variation of the nitrided case depth. An industrial method of detection of the Barkhausen noise was tested for comparison using a commercial Rollscan device. The industrial measurements gave a single-peak envelope for all studied samples and a lower depth sensitivity of the rms parameter. Bulk testing methods, the magnetic hysteresis and the magneto-acoustic emission, were not able to detect the nitrided cases.