Insight into the microstructural characterization of ferritic steels using micromagnetic parameters

The influence of tempering-induced microstructural changes on the micromagnetic parameters such as magnetic Barkhausen emission (MBE), coercive force (Hc), residual induction (Br), and maximum induction (Bmax) has been studied in 0.2 pct carbon steel, 2.25Cr-1Mo steel, and 9Cr-1Mo steel. It is observed that, after short tempering, the micromagnetic parameters show more or less linear correlation with hardness, which is attributed to the reduction in dislocation density, but long-term tempering produces nonlinear behavior. The variation in each of these parameters with tempering time has been explained based on the changes in the size and distribution of ferrite laths/grains and precipitates. It has been shown that the individual variation in the microstructural features such as size and distribution of laths/grains and precipitates during tempering can be clearly identified by the MBE parameters, which is not possible from the hysteresis loop parameters (Hc and Br). It is also shown that the MBE parameters can not only be used to identify different stages of tempering but also to quantify the average size of laths/grains and second-phase precipitates