Detection of Iron Oxide Layer in Quenched and Tempered Gear Steel Using

Mechanics, Materials Science & Engineering, July 2016

ISSN 2412-5954

Detection of Iron Oxide Layer in Quenched and Tempered Gear Steel Using Magnetic Barkhausen Noise M. M. Blaow 1, a and M. M. Sawalem 2, b

1 Department of Materials Science and Engineering, Faculty of Engineering, University of Misurata, Aljazera Street, Misurata, Libya 2 Department of Mechanical Engineering, Faculty of Engineering, University of Misurata, The Coase Road, Misurata, Libya a

mblaow@yahoo.co.uk

b

ferretti355@gmail.com DOI 10.13140/RG.2.1.1027.5444

Keywods: magnetic Barkhausen noise profile, tempering, oxidation, hysteresis.

ABSTRACT. This paper deals with the non-destructive evaluation of surface oxidation of gear steel using magnetic Barkhausen noise profiles analysis. Martensitic specimens are subjected to tempering at various temperatures in a muffle furnace. Tempering induced changes result in Barkhausen profile height increase and peak centers shift to lower profiles are seen with The utilization of MBN method for this purpose is based on the difference in the inherent magnetic properties between the degraded surface layer and the sub-surface unaffected bulk. The observations are discussed in the light of established models of Barkhausen noise.

Introduction. The manufacturing processes for gear components include heat treatment operations to achieve surface characteristics for components to increase wear resistance. Heat treatments include carburizing and induction hardening which introduce a hard layer at the surface and maintain a soft interior. Recently Ovako 667 steel was developed for low cost manufacture of wear-resistant elements. This type of steel could be fully hardened by air-cooling from the austenite region. Another feature is that the material is resistant to over-tempering.

When a ferromagnetic material is magnetized by a varying magnetic field, the local changes in the magnetization induces voltage pulses in a search coil placed on the surface which are known as magnetic Barkhausen noise [1]. Magnetic Barkhausen noise (MBN) is mainly associated with the irreversible domain wall movement and refers to the abrupt discontinues changes in the magnetization rate that result from domain walls overcoming various types of obstacles in their path. Obstacles include grain boundaries, voids and precipitates [2]. The sensitivity of MBN to microstructural inhomogeneities makes it potentially useful as a non-destructive testing technique.

Also, the assessment of microstructure and mechanical properties after initial heat treatment as a quality control measurement and their subsequent degradation during service such as exposure to high temperature [3]. In steels, microstructural defects like grain boundaries, inclusions and dislocations promote both mechanical and magnetic hardening, increasing the area of hysteresis curve and reducing permeability. This happens because the same defects which pin dislocations

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Mechanics, Materials Science & Engineering, July 2016

ISSN 2412-5954

In case carburized and decarburized steels a hardness gradient is present [5-9]. This can cause two MBN intensity peaks to appear at different field strengths. The reason is that each peak originates in a material layer of different hardness. The variation in ferromagnetic material properties can be correlated to different parameters derived from the MBN signal profile generated during the magnetization cycle [10]. The aim of the study is to investigate the magnetic Barkhausen noise response from iron oxide layer formed at the surface of martensite tempered at elevated temperatures as a function of weeping magnetic field.

Materials and Method. The composition of the stock material is shown in Table 1. In the present surface oxidation and decarburization of the specimens. Martensitic specimens were tempered in a muffle furnace at 500, 600 and 700o C for 1 hour to produce different degrees of oxidation. The details of the MBN apparatus is shown elsewhere [13]. In this experiment, the magnetizing frequency used was 1 Hz to enhance the MBN signal to noise ration and maintain a low magnetization rate.

Table 1. Composition of Ovako 677 steel Element

C

Mn

Ni

Cr

Mo

Si

S

P

Wt %

0.67

1.48

0.11

1.03

0.25

1.46

0.007

0.016

Results. Figure 1 shows half-cycles MBN profiles of a quenched specimen and quenched and

Fig. 1. Half-magnetizing cycle Barkhausen profile from specimen tempered at 500 o C.

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Fig. 2. Half-magnetizing Barkhausen profile from the specimen tempered at 600 o C.

overlapping peaks (Figs. 2 and 3). It seems reasonable to assume that the MBN profiles reflect the composition gradient at the skin depth of the specimens, which are the iron oxide at the surface and the steel at the subsurface. Figure 4 shows that the second peak at higher field from the oxidized specimen fits with MBN profile of the cleaned specimen which implies that the oxide layer is thin and does not attenuate the Barkhausen emission from the bulk material. After removal of the oxide layer, the specimen tempered at 700 compared to that from the oxidized surface.

Fig. 3. Half-magnetizing Barkhausen profile from the specimen tempered at 700 C.

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Mechanics, Materials Science & Engineering, July 2016

ISSN 2412-5954

This indicates that the oxide layer is able to attenuate the MBN signals from the bulk material.

Barkhausen profiles of the quenched and tempered specimens without oxides are shown in Fig. 4.

Fig. 4. MBN profiles showing tempering induced changes.

Discussion. Although MBN has been attributed to a number of mechanisms, most current thinking associates it with the irreversible movement of domain walls. Theoretical models highlighting the connection MBN and the irreversible component of magnetisation Mirr have been reviewed by Jiles 14 . A basic assumption is that the intensity of emission is proportional to the differential susceptibility irr= dMirr/dH, where H is the magnetic field. This is illustrated schematically in Fig. 5, where the Mirr H hysteresis loop is shown in relation to MBN emission for a complete magnetisation cycle. The amplitude of emission is greatest when the slope of the Mirr H curve is a maximum, and smallest at points approaching saturation. The MBN characteristics observed for the different microstructures (Fig. 4) are consistent with the theory. If the hysteresis loop becomes narrower, with steeper sides, the peak intensity of emission will increase and the position of the peak will shift towards zero field. The converse will occur if the hysteresis loop becomes broader with a smaller maximum slope. In the experiments, peak position (Fig. 4) shifted to lower values as the microstructure changes from martensite to ferrite-cementite microstructure.

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Fig. 5. Magnetic hysteresis loops and the corresponding MBN signals [14].

It is widely accepted that the Barkhausen noise signal is strongly dependent on the number of pinning obstacles met by domain walls during the magnetization process. The peak amplitude is thus strongly sensitive to the phase proportion, whereas the position of the peak is usually linked to the nature and the strength of the obstacles. The discontinuous jumps of Bloch walls are due to their local pinning by different obstacles such as inclusions, precipitates, grain boundaries, and dislocation tangles. The MBN increases with the number of these pinning obstacles. A shift of the peak to the high value of the magnetic field is also observed when the influence of these obstacles on Bloch walls increases. Because the magnetic structure is directly linked to the nature of metallurgical state and hence its differential susceptibility, each phase has its inherent magnetisation saturation and hence a distinctive MBN response [15, 16]. This is consistent with the present observations on the oxidized and non oxidized specimens. The differential susceptibility of iron oxide layer is different from that of ferrite and cementite structure and hence this results in two magnetization responses (MBN) relative to the applied sweeping field .

Summary 1. and the bulk material.

2. Iron oxide layer at the surface of the specimens attenuates the MBN signals.

3. Magnetic Barkhausen noise technique is very sensitive to microstructural gradient at the surface and the subsurface bulk.

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[4] D. C. Jiles, "Dynamics of domain magnetization and the Barkhausen effect, Czechoslovak Journal of Physics, 50, 893, 2000.

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[5] J. Alessandra, , L. Gunther, and F. Gerhardt, Magnetic Barkhausen Noise Profile Analysis: Effect of Excitation Field Strength and Detection Coil Sensitivity in Case Carburized Steel, Materials Sciences and Applications, Vol. 16, pp.1015-1019, 2013, doi: 10.1590/S151614392013005000095.

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[10] D. C. Jiles, L. B. Sipahi, and G. J. Williams, J. App. Phys Vol. 73, p. 5830, 1993. ation of Dual-Phase Steels Using Magnetic Barkhausen Noise Technique, Journal of Nondestructive Evaluation, Vol. 26, pp.79-87, 2007, doi: 10.1007/s10921-007-0022-0.

[12] M. J. Sablik, J Appl Phys, Vol. 74, pp.5898, 1993 [13] M. M. Blaow, J. T. Evans and B. A. Shaw, J. Magn. Magn. Mat., Vol.303, pp.153, 2006.

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[16] M. M. Blaow and B.A. Shaw, Magnetic Barkhausen Noise Profile Analysis: Effect of Excitation Field Strength and Detection Coil Sensitivity in Case Carburized Steel, Materials Sciences and Applications, Vol. 5 pp.258-266, 2014, doi: 10.4236/msa.2014.55030.

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