Magnetic Barkhausen Noise Measurements to Assess Temper Embrittlement in HY-80 Steels

Magnetic Barkhausen noise (MBN) can result from abrupt motion of domain walls due to their interaction with pinning sites during magnetization of ferromagnetic steel. However, understanding domain structure interactions with pinning site density is limited. In this article, the effect of density of pinning sites within grains of HY80 steel, on generation of MBN, was investigated. Pinning site density was modified by a heat treatment that produces temper embrittlement. Temper embrittlement refers to the reduction of fracture toughness of alloy steels when heated or slowly cooled through the embrittling temperature range and arises due to migration of impurity elements to grain boundaries. Samples, machined from a casting, were held at a constant temperature (525 °C) for different holding times, to generate varying degrees of temper embrittlement. MBN signal response was observed to decrease exponentially as a function of holding time with decay constants (relaxation time) in the range of 160-190 h. A reduction in the MBN signal along each sample's easy axis was attributed to different densities of impurity elements, which act as pinning sites for domain wall movement within grains. Samples were also characterized using a scanning electron microscope (SEM), hardness testing, and impact toughness testing. Hardness was observed to increase with holding time and MBN energy decreased linearly with increasing hardness. Furthermore, a linear relationship between impact energy and MBN energy was observed. This implies that MBN signal analysis can be used as an indirect measure of the change in material properties of the sample.