Barkhausen noise in plastically deformed low-carbon steels

We discuss Barkhausen noise experiments performed in interstitial-free low-carbon steel laminations, subjected to increasing deformation either by tensile straining or cold rolling. The spectral density of the noise, measured under a constant magnetization rate J˙(t) using both sensing coils and a soft ferrite probe, exhibits a defined dependence on the deformation εp. In particular, the low- and high-frequency portions of the spectra display opposite trends with εp, reflecting the behavior of remanent induction (permeability) and coercivity, respectively. Different roles are played by surface and bulk magnetization processes in noise buildup. Fast domain wall unpinning events in the outer lamination layer determine the spectra at high frequencies, while long-range propagation of the Barkhausen reversals in the bulk affects the low frequencies. By conventional measurements and noise analysis, one can thus achieve a consistent picture of the effect of plastic deformation on the magnetization process and provide a rationale for nondestructive steel testing by magnetic methods.