Magneto-Acoustic Emission and Magnetic Barkhausen Emission for Case Depth Measurement in En36 Gear Steel

There is a need in industry to supply safe, economical, and reliable techniques to characterize surface treatments such as case hardening and peening of steel components and structures, both at the manufacturing stage and in service. Magnetic Barkhausen emission (MBE) has proved successful for these tasks, but has severe limitations in terms of measurement depth of the technique and deeper treatments such as laser peening and case hardening often fall outside the scope of MBE inspection. The domain wall motion that generates MBE also causes a release of elastic energy known as magneto-acoustic emission (MAE), which has a much greater measurement depth, and so offers a complementary technique to extend the measurement depth for the characterization of surface treatments in steel. In this paper, comparative MBE and MAE results from case hardened En36 gear steels are presented in the form of signal profiles and correlations are drawn between MBE and MAE profile features and domain activity within the soft core and the case hardened layer. The results show that the overall amplitudes for both MBE and MAE exhibit a good correlation with case depth, but profile analysis for MAE is ambiguous, so possible interpretations of the MAE profile are discussed.