评价回火引起的硬度分布的变化.pdf

Evaluation of tempering induced changes in the hardness profileof case-carburised EN36 steel using magnetic Barkhausennoise analysisV. Moorthy*, B.A. Shaw, J.T. EvansDesign Unit, University of Newcastle, Newcastle Upon Tyne NE1 7RU, UKReceived 27 June 2002; revised 16 September 2002; accepted 16 September 2002AbstractThe effect of tempering on the magnetic Barkhausen noise (MBN) signal profile was studied in case-carburised EN36 steel using a range ofmagnetic excitation frequencies and a number of frequency ranges for analysis of the MBN signal. The MBN level generally increases withtempering due to coarsening of the microstructure. With higher values of excitation frequency, fEX, the MBN profile exhibits a single peak,but with low values of fEX, double peaks are observed. The MBN peak obtained with higher fEXwas found to correlate well with hardnesschanges in a region, down to 100 mm below the surface. The analysis of the MBN signal produced with low fEX, in narrow frequency rangesselected 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 differentanalysing frequency ranges were found to correlate well with hardness variations at different depths down to 425 mm below the surface. Anempirical 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 profile1. IntroductionIt 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 theirreversible movement of magnetic domain walls during acyclic magnetisation process. Magnetic domain wall move-ment is strongly influenced by microstructural features suchas dislocations, grain boundaries and second phase precipi-tates. In addition, the magnitude and sign of any macroscopicstresses (applied or residual) have a strong influence on MBN[1–4]. Thus, at constant macroscopic stress, the MBN signalchanges with changes in the microstructure induced by heattreatments so that comparative measurements can be used formonitoring purposes.The steels used for the manufacture of high performancegears are case hardened in order to improve the near-surfacemechanical properties. Often, the subsurface properties arefound to be very important, particularly with respect tocontact fatigue. To optimise mechanical properties, mostgear steels are case-carburised and then quenched andtempered to the required hardness level. Because the case-depth and the hardness profile within the case depth stronglyinfluence the contact fatigue properties, there is growinginterest in evaluating the hardness-depth profile by non-destructive testing (NDT). The MBN technique is consideredhere as a candidate NDT method for such applications.A number of previous publications have demonstratedthe high sensitivity of the MBN technique in detectingchanges produced by heat treatment in a number offerromagnetic alloys [4–12]. In the present work, theresults of a study of quenched and tempered case-carburisedEN36 gear steel is presented. The near-surface hardness andthe variation in the hardness values down to about 500 mmbelow the surface were correlated with MBN measure-ments, made using a commercially available system. It isshown that the information that can be obtained from MBNcan be increased if the frequency of magnetic excitation isvaried and appropriate frequency filtering is employed inanalysing the signal.0963-8695/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.PII: S0 96 3 -8 69 5 (0 2) 00 0 70 -1NDT 1972. p. 292–3.[2] Tebble RS, Craik DJ. Magnetic material. New York: Wiley; 1969. p.369–411.[3] Mikheev MN, Gorkunov ES. Relationship of magnetic properties tothe structural condition of a substance—the physical basis of magneticstructure analysis (review). Sov J NDT 1981;17(8):579–92.[4] Tiitto S. On the influence of microstructure on magnetizationtransitions in steel. Acta Polytech Scand, Appl Phys Ser 1977;119:1–80.[5] Moorthy V, Vaidyanathan S, Jayakumar T, Raj B. Microstructuralcharacterization of quenched and tempered 0.2% carbon steel usingmagnetic Barkhausen noise. J Magn Magn Mater 1997;171:179–89.[6] Moorthy V, Vaidyanathan S, Jayakumar T, Raj B. On the influence oftempered microstructures on magnetic Barkhausen emission in ferriticsteels. Phil Mag A 1998;77(6):1499–514.[7] Kameda J, Ranjan R. Non-destructive evaluation of steels usingacoustic and magnetic Barkhausen signals—I. Effect of carbideprecipitation and hardness. Acta Metall 1987;35(7):1515–26.[8] Karjalainen LP, Moilanen M, Rautioaho R. Influence of tensile andcyclic loading upon Barkhausen noise in a mild steel. Mater Eval1979;37(9):45–51.[9] Hyde TR, Evans JT, Shaw BA. Effect of stress and heat treatment onMBN in case-carburised steels. Mater Eval 2000;58(8):985–90.[10] Vaidyanathan S, Moorthy V, Jayakumar T, Raj B. Evaluation ofcarburization depth in service exposed ferritic steel using magneticBarkhausen noise analysis. Mater Eval 1998;56(3):449–52.[11] Moorthy V, Choudhary BK, Vaidyanathan S, Jayakumar T, BhanuSankara Rao K, Raj B. An assessment of low cycle fatigue damageusing magnetic Barkhausen emission in 9Cr–1Mo ferritic steel. Int JFatigue 1999;21:263–9.[12] Vaidyanathan S, Moorthy V, Jayakumar T, Raj B. Evaluation ofinduction hardened case depth through microstructural characteriz-ation using magnetic Barkhausen emission technique. Mater SciTechnol 2000;16(2):202–8.[13] Chikazumi S. Physics of magnetism. New York: Wiley; 1964. p. 322.[14] Woolman J, Mottram RA, The mechanical and physical properties ofthe British standard EN steels, vol. 2. New York: Pergamon Press;1966. p. 390, EN21–39.[15] Moorthy V, Shaw BA, Evans JT. Effect of magnetizing frequency andanalysing frequency range on the magnetic Barkhausen noise profile.IEEE Trans Magnetics, submitted for publication.Fig. 15. Schematic of the procedure to obtain the hardness profiles fortempered case-carburised EN36 steel from the MBN measurement atfEX¼ 4 Hz.V. Moorthy et al. / NDT&E International 36 (2003) 43–49 49