Magnetic Barkhausen noise: the influence of microstructure and deformation in bending

The effect of microstructure and bending deformation on the characteristics of magnetic Barkhausen noise profiles were examined in carbon steel of the type used for wear-resistant engineering components. Microstructures associated with magnetic softness produced the largest profile peaks and the lowest peak positions. Multi-peak profiles were observed in compression in spherodised cementite specimens and to a much smaller extent in the martensite tempered at 400 °C. In the spherodised cementite, the change from a sharp, single peak to a broad, three-peak profile, with a greatly reduced height was precipitous and occurred over a small strain increment. Where single-peak profiles were obtained, strain affected emission but did not alter the ranking imposed by microstructure. The most magnetically soft microstructure (spheroidal cementite) seems to confer little scope for a positive increase in emission with increasing strain. The most magnetically hard (martensite) showed the greatest relative strain sensitivity. All the profiles observed were reversible with respect to loading and unloading in the elastic regime. The onset of plasticity did not induce any discontinuous change in the profiles whilst under load but it did induce irreversibility on unloading, due, in part, to the residual stresses induced by the inhomogeneous deformation in bending. The observations are discussed in the light of established models of Barkhausen noise.