ON THE INFLUENCE OF TEMPERED MICROSTRUCTURES ON MAGNETIC BARKHAUSEN EMISSION IN FERRITIC STEELS

Magnetic Barkhausen emission (MBE) has been used to characterize vario us microstructures in tempered 0.2 wt% C steel, 2.25Cr-1Mo steel and 9 Cr-1Mo steel samples (where the composition is in approximate weight p er cent). A two-stage process of irreversible magnetic domain wall mov ement during magnetization is proposed considering the lath or grain b oundaries and second-phase precipitates as the two major obstacles to domain-wall movement. The domain walls overcome these two major obstac les over a range of critical field strengths with some mean values, ch aracteristic of the obstacles. If these two mean values are close to e ach other, then a single peak, sometimes associated with slope changes , appears in the MBE behaviour. On the other hand, if the mean values are widely separated, then two-peak MBE behaviour appears to indicate the influence of these two major obstacles separately. Based on this, the influence of the dissolution of martensite and/or bainite and the precipitation and growth of the second phase precipitates with differe nt sizes and morphologies on the MBE has been explained. It is observe d that the presence of two or more different types of carbide in the C r-Mo steels significantly reduces the MBE compared with the growth of single carbides in the carbon steel. This study establishes the viabil ity of the MBE technique for identifying different stages of tempering in ferritic steels.