MICROSTRUCTURAL CHARACTERIZATION OF QUENCHED AND TEMPERED 0.2-PERCENTCARBON-STEEL USING MAGNETIC BARKHAUSEN NOISE-ANALYSIS

Magnetic Barkhausen noise (MEN) has been used to characterize the micr ostructures in quenched and tempered 0.2% carbon steel. It has been ob served 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 behav iour after 1 h. After 5 h of tempering, MBN shows a clear two peak beh aviour. A two stage process of irreversible domain wall movement durin g magnetization is proposed considering the grain boundaries and secon d phase precipitates as the two major obstacles to domain wall movemen t. 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 si ngle 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 bas ed on these two stage processes. The influence of dissolution of marte nsite and the precipitation of cementite (Fe3C) on MBN are explained.