Journal of Magnetism and Magnetic Materials 303 (2006) 153–159 Effect of hardness and composition gradients on Barkhausen emission in case hardened steel M. Blaow, J.T. Evans à , B.A. Shaw Design Unit, School of Mechanical Systems and Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK Received 3 December 2004; received in revised form 31 January 2005 Available online 28 November 2005 Abstract The effect of gradients in hardness, structure and composition in the surface layers on magnetic Barkhausen noise (MBN) was investigated in two widely used low alloy steels. One material was case hardened by induction hardening. The other was case carburised. Similar hardness profiles were produced by the two treatments. It was found that the variation in structure and hardness through the case in the induction-hardened steel had a minor effect on the MBN profile. In contrast, the inhomogeneity in the case-carburised material showed up clearly. This took the form of a broadened or two-peak MBN profile. When the surface layer containing the case was removed by etching, the broadened profile was replaced by a profile with a single peak. It is concluded that the shape and position of the MBN profile is significantly affected when a gradient in microstructure is induced by a gradient in carbon content. On the other hand, a gradient in microstructure induced by heat treatment with a constant carbon level has much less affect on the MBN profile for the induction-hardened steel. r 2005 Elsevier B.V. All rights reserved. Keywords: Magnetic Barkhausen noise; Case-depth; Inhomogeneity; Case-carburised steels; Induction-hardened steels 1. Introduction When a ferromagnetic material is magnetised by a varying magnetic field, local changes in the magnetisation induce voltage pulses in a search coil. This phenomenon is referred to as magnetic Barkhausen noise (MBN) or Barkhausen emission. It is principally associated with the irreversible movement of domain walls and discontinuous changes in the magnetisation rate that result when domain walls overcome obstacles such as grain boundaries and precipitates [1,2]. A number of features of MBN make it potentially useful in non-destructive testing of ferromagnetic materials. The root mean square value of MBN calculated over a complete magnetisation cycle gives a single parameter that can be useful for characterising the integrity of near-surface material [3]. For instance, grinding abuse in hardened steel components has been detected using this approach [4]. More recently, attention has focussed on the MBN profile, in which the intensity of Barkhausen emission is viewed as a function of the applied field. Measurement of the MBN profile opens the possibility of detecting differences in materials that might not be revealed using a single parameter such as the root mean square value. In particular, the MBN profile might be used to reveal gradients in structure and composition that occur in case carburised steel [5] or in components with a decarburised surface layer [6]. Carburisation is widely used for case hardening steel. Here, the gradient in carbon content produces a gradient in microstructure and hardness near the surface. Case hard- ening by induction hardening is also widely used in engineering applications. Here, the variation in hardness is produced by the variation in microstructure with no change in composition. Because of the technical require- ments of the two processes, the compositions of the alloy steels used in the two processes are different. It was the object of the present work to compare the MBN responses ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.07.034 à Corresponding author. Tel.: +44 (0)191 222 7911; fax: +44 (0)191 222 8600. E-mail address: j.t.evans@ncl.ac.uk (J.T. Evans).