Evaluation of tempering induced changes in the hardness profile of case-carburised EN36 steel using magnetic Barkhausen noise analysis V. Moorthy * , B.A. Shaw, J.T. Evans Design Unit, University of Newcastle, Newcastle Upon Tyne NE1 7RU, UK Received 27 June 2002; revised 16 September 2002; accepted 16 September 2002 Abstract The effect of tempering on the magnetic Barkhausen noise (MBN) signal profile was studied in case-carburised EN36 steel using a range of magnetic excitation frequencies and a number of frequency ranges for analysis of the MBN signal. The MBN level generally increases with tempering due to coarsening of the microstructure. With higher values of excitation frequency, f EX , the MBN profile exhibits a single peak, but with low values of f EX , double peaks are observed. The MBN peak obtained with higher f EX was found to correlate well with hardness changes in a region, down to 100 mm below the surface. The analysis of the MBN signal produced with low f EX , in narrow frequency ranges selected by software frequency filtering, showed variations in the extent of changes in the relative height of the two MBN peaks in the profile. After taking into account the skin depth-frequency relation for the MBN signal, variations in the values of the two MBN peaks in different analysing frequency ranges were found to correlate well with hardness variations at different depths down to 425 mm below the surface. An empirical relationship has been established between the hardness-depth profile and the MBN measurements. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Magnetic Barkhausen noise; Carburised EN36 steel; Tampering; Hardness profile 1. Introduction It is well established that magnetic Barkhausen noise (MBN) measurements can be used as a sensitive non- destructive testing technique for characterizing ferromag- netic materials. The MBN signal is attributed to the irreversible movement of magnetic domain walls during a cyclic magnetisation process. Magnetic domain wall move- ment is strongly influenced by microstructural features such as dislocations, grain boundaries and second phase precipi- tates. In addition, the magnitude and sign of any macroscopic stresses (applied or residual) have a strong influence on MBN [1–4]. Thus, at constant macroscopic stress, the MBN signal changes with changes in the microstructure induced by heat treatments so that comparative measurements can be used for monitoring purposes. The steels used for the manufacture of high performance gears are case hardened in order to improve the near-surface mechanical properties. Often, the subsurface properties are found to be very important, particularly with respect to contact fatigue. To optimise mechanical properties, most gear steels are case-carburised and then quenched and tempered to the required hardness level. Because the case- depth and the hardness profile within the case depth strongly influence the contact fatigue properties, there is growing interest in evaluating the hardness-depth profile by non- destructive testing (NDT). The MBN technique is considered here as a candidate NDT method for such applications. A number of previous publications have demonstrated the high sensitivity of the MBN technique in detecting changes produced by heat treatment in a number of ferromagnetic alloys [4–12]. In the present work, the results of a study of quenched and tempered case-carburised EN36 gear steel is presented. The near-surface hardness and the variation in the hardness values down to about 500 mm below the surface were correlated with MBN measure- ments, made using a commercially available system. It is shown that the information that can be obtained from MBN can be increased if the frequency of magnetic excitation is varied and appropriate frequency filtering is employed in analysing the signal. 0963-8695/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0963-8695(02)00070-1 NDT&E International 36 (2003) 43–49 www.elsevier.com/locate/ndteint * Corresponding author. E-mail address: v.moorthy@ncl.ac.uk (V. Moorthy).