Utilization of Barkhausen noise magnetizing sweeps for case-depth detection from hardened steel

One of the current topics of Barkhausen noise method development is its application to case-depth measurements of hardened components. Usually Barkhausen noise (BN) measurements for case-depth determination are based on the difference in the magnetic properties between the hardened case and the soft base material core. The measurement is done using low magnetizing frequencies. This enables deeper penetration of magnetic field to the ferromagnetic samples compared to the conventional high frequency BN measurements, typically used for grinding burn detection. However, due to the eddy current damping, the penetration depth is limited, depending on the material, to certain distance from the surface. To enable case-depth measurements, the Barkhausen noise measuring device was utilized to obtain data from magnetizing voltage sweeps (MVS). The sweeps were analysed and compared to the case-depths determined with conventional means. A series of hardened samples was investigated including induction and carburizing case-hardened samples. All samples were characterized with X-ray diffraction to study the residual stress state of the surface. Finally destructive characterization was used to verify the actual hardening depth of the studied samples. It was noticed that by studying the MVSs it was possible to evaluate the case-depth values more effectively than in earlier studies. The analysis used in this study utilized the slopes of the MVS sweeps. Furthermore, it was noticed that the ratio of the maximum MVS slopes at different frequencies indicating the case-depth values led to good results.

► Barkhausen noise magnetizing voltage sweeps (MVS) were measured from hardened samples.
► The MVSs measured with different frequencies were analysed.
► Features from the MVSs correlated linearly with the case-depth values.
► Case-depth variation can be estimated from regression line with confidence limits.