INFLUENCE OF MICROSTRUCTURAL CHANGES DUE TO TEMPERING AT 923 K AND 1,0123 K ON MAGNETIC BARKHAUSEN NOISE BEHAVIOR IN NORMALIZED 2.25CR-1MO FERRITIC STEEL

Magnetic Barkhausen noise analysis has been used to characterize the m icrostructural changes in normalized and tempered 2.25Cr-1Mo steel. It is observed that tempering at 923 K shows a single peak behavior up t o 20 h and tempering at 1,023 K shows a two peak behavior. This has be en ex plained on the basis of the two stage process of irreversible do main wall movement during magnetization, associated with two major obs tacles to domain wall movement: namely lath/grain boundaries and secon dary phase precipitates. At lower fields, existing reverse domain wall s at lath/grain boundaries overcome the resistance offered by the grai n boundaries and move to a distance before they are pinned by the prec ipitates. Then, at higher field, they overcome these precipitates. The se two processes occur over a range of critical field strengths with s ome mean values. If these two mean values are close to each other, the n a single peak in the rms voltage of the magnetic Barkhausen noise, w ith the associated changes in its shape, is observed. On the other han d, if the mean values of the critical fields for these two barriers ar e widely separated, then a two peak behavior is the clear possibility. The effect of the microstructural changes due to tempering for differ ent durations at 923 K and 1,023 K in 2.25Cr-1Mo ferritic steel on mag netic Barkhausen noise is explained based on these two stage processes . The influence of high dislocation density in bainitic structure, dis sociation of bainite, and precipitation of different carbides such as Fe3C, Mo2C, Cr7C3, M(23)C(6), etc., on magnetic Barkhausen noise behav ior has been analyzed in this study.