On the influence of tempered microstructures on magnetic Barkhausen emission in ferritic steels

Abstract

Magnetic Barkhausen emission (MBE) has been used to characterize various microstructures in tempered 0.2 wt% C steel, 2.25Cr-1Mo steel and 9Cr-1Mo steel samples (where the composition is in approximate weight per cent). A two-stage process of irreversible magnetic domain wall movement during magnetization is proposed considering the lath or 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 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 different sizes and morphologies on the MBE has been explained. It is observed that the presence of two or more different types of carbide in the Cr-Mo steels significantly reduces the MBE compared with the growth of single carbides in the carbon steel. This study establishes the viability of the MBE technique for identifying different stages of tempering in ferritic steels.