Using the Barkhausen Noise Frequency Spectrum for Case Depth Estimates

We present an innovative method for case depth estimates, using Barkhausen noise, for AISI 1020 steel into which carbon had been diffused for various distances. Quenching after a short heat treatment resulted in a martensitic surface layer on top of a ferrite/pearlite nucleus. Case depths were measured by traditional destructive techniques. Barkhausen noise measurements were made and both the RMS Barkhausen pulse envelopes and the fast Fourier transforms (FFT) were obtained by numerical calculation. The areas of the Barkhausen pulses lend themselves to the construction of a calibration curve for determining the case depths. The FFT amplitudes were obtained as a function of frequency, and were associated with distance from the sample surface via the classical skin depth equation. Case depths can be estimated from a normalized power index, a quantity consistent with details of the sample microstructure. A knowledge of the material properties is especially important for determining the usefulness of this technique.