Study of microstructure, mechanical properties, and magnetization process in low carbon steel bars by Barkhausen emission

Correlation between Barkhausen emission (BE) signal, microstructure and mechanical properties of a series of low carbon steel bars after receiving different heat treatments are studied for the purpose of further developing the nondestructive BE technique to monitor the stress-strain characteristics of steel components. It is found that the BE signal, which is proportional to the number of unpinning events of domain walls (DW), is more intense in samples with small grains which have been annealed at lower temperatures, than in those with large grains which have been annealed at higher temperatures. The small-grain samples have larger BE signals, because they have large fractional volume of grain boundaries that act as sites for the DW unpinning. A qualitative model is proposed to describe the BE signal obtained from these ferritic steel samples. BE measurements are also made in-situ on each sample while it is under increasing tension. The BE signal increases in sample under tension. In the case of small-grain sample, the increase is mainly due to the re-orientation of domain configurations that facilitates the magnetization process. In the case of large-grain sample, tensile stress re-orients the domain configurations as well as creating new DWs that increase the number of unpinning events. In this work, it is also found that the BE-strain plots of the steel bars annealed at temperatures ranging from room temperature to 600 °C exhibit behaviors which are similar to their corresponding stress-strain curves.