Abstract
Magnetic Barkhausen noise (MBN) has been used to characterize the microstructures in quenched and tempered 0.2% carbon steel. It has been observed that tempering at 873 K shows a single peak MBN behaviour after 0.5 h and a slope change indicating the development of two peak behaviour after 1 h. After 5 h of tempering, MBN shows a clear two peak behaviour. A two stage process of irreversible domain wall movement during magnetization is proposed considering the grain boundaries and second phase precipitates as the two major obstacles to domain wall movement. The domain walls overcome these two major obstacles over a range of critical field strengths with some mean values characteristic of the obstacles. If these two mean values are close to each other, then a single peak, sometimes associated with a slope change in MBN behaviour, appears. On the other hand, if the mean values of the critical fields of these two barriers are widely separated, then a two peak behaviour appears. The effect of microstructural changes on MBN is explained based on these two stage processes. The influence of dissolution of martensite and the precipitation of cementite (Fe 3C) on MBN are explained.
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