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Microstructural characterization of quenched and tempered 0.2% carbon steel using magnetic Barkhausen noise analysis
ELSEVIER
Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
~ H Journalof magnetism ~ i and magnetic ~ i ~ materials
Microstructural characterization of quenched and tempered 0.2% carbon steel using magnetic Barkhausen noise analysis V. Moorthy, S. Vaidyanathan, T. J a y a k u m a r , B a l d e v R a j * Division fi)r PIE and NDT Development, lndira Gandhi Centre.]'or Atomic Research, Kalpakkam 603 102, India Received 23 July 1996; received in revised form 4 February 1997
Abstract Magnetic Barkhausen noise (MBN) has been used to characterize the microstructures in quenched and tempered 0.2% carbon steel. It has been observed that tempering at 873 K shows a single peak MBN behaviour after 0.5 h and a slope change indicating the development of two peak behaviour after 1 h. After 5 h of tempering, MBN shows a clear two peak behaviour. A two stage process of irreversible domain wall movement during magnetization is proposed considering the 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 a slope change in MBN behaviour, appears. On the other hand, if the mean values of the critical fields of these two barriers are widely separated, then a two peak behaviour appears. The effect of microstructural changes on MBN is explained based on these two stage processes. The influence of dissolution of martensite and the precipitation of cementite (Fe3C) on MBN are explained.
Keywords. Microstructure; Steel; Barkhausen noise; Domain wall motion
1. Introduction Microstructural characterization of materials by non-destructive evaluation (NDE) techniques is essential for the assessment of initial heat treatment and subsequent degradation in microstructure and mechanical properties under service environment. Exposure to high temperature, static and cyclic
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loading, etc. results in microstructural degradation, creep and fatigue damage. Conventional microscopy techniques do not give global information about the changes in the whole component. Techniques like transmission electron microscopy (TEM), etc. which give finer details are more time consuming and are also not amenable for on-line assessment of components. Advanced N D E techniques such as ultrasonic attenuation and velocity measurements, positron annihilation techniques, acoustic microscopy, magnetic Barkhausen noise (MBN) measurements, etc. have proved their
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~ Moorthv et al. /Journal o['Magnetism and Magnetic Materials ] 71 (1997) 179 189
viability for characterization of microstructures, creep and fatigue damage. MBN measurement is considered as a valuable NDE technique for microstructural characterization of ferromagnetic materials [1 9]. Earlier studies [10-14] have shown that MBN is also sensitive to internal stress/strain and can be applied to evaluate in-service, creep and fatigue damage. When a component is subjected to creep and/or fatigue damage, the internal stress/strain of the material varies significantly in addition to the simultaneous variation in microstructure such as phase transformation, second phase precipitation, etc. Therefore, it is essential to understand the influence of different microstructures on MBN for better insight towards the effect of creep/fatigue damage on the behaviour of MBN. When a ferromagnetic material is subjected to a varying magnetic field, the discrete changes in flux density during magnetization induces voltage pulses in the pick up coil. This phenomenon, called MBN is attributed to the discrete movement of domain walls overcoming obstacles and is sensitive to microstructural variation and strain in the material. Even though several studies [1 9] have been done on microstructural characterization using MBN, still, a clear understanding on the individual influence of different microstructural parameters such as grain size, precipitation of second phase particles, etc. on the generation of MBN has not been well established. Buttle et al. [2] observed a three peak behaviour in MBN activity profile in pure iron, whereas Gatelier-Rothea et al. [4] observed only a single peak for pure iron. Similarly, Ranjan et al. [6] observed a two peak behaviour for pure nickel, whereas Hill et al. [7] observed a three peak behaviour for pure nickel. Also, in a carbon steel with cementite precipitation, Gatelier-Rothea [4] and Kameda et al. [5] have observed a single peak behaviour, whereas Buttle et al. [3] observed a three peak behaviour in MBN activity for Incoloy 904 with coherent spherical precipitates. The difference in their observations could also be due to different magnetizing methods, in addition to changes in microstructural features. Some investigators [2, 3, 5] have used a solenoid for magnetization where the demagnetizing field is very high, and others [7, 9] have used a closed circuit electromagnetic yoke for magnetization to reduce the
demagnetizing field. For the mechanisms of MBN generation also, some investigators [2, 3, 5] have suggested that the domain nucleation process would be the major contribution to MBN and other researchers [6 -8] have suggested the irreversible domain wall movement as the major contribution to the MBN generation. It is generally observed that, on tempering a quenched or normalised ferritic steel such as carbon steel, C r - M o steel, etc., there is reduction in dislocation density associated with recovery, recrystallization and coarsening of ferrite laths or grains, in addition to the precipitation and growth of second phase particles at different stages. Hence, there is a continuous variation in lath/grain size and precipitation of one or more types of second phase particles with different size, size distribution and shape. The interaction of domain walls with the grain boundaries and the precipitates is expected to be significantly different. Hence, it should be possible to distinguish the effect of lath/grain size variation and second phase precipitation on the magnetization process. With this aim, in this study, an attempt has been made to understand the effect of tempering of martensite and precipitation of carbide particles on M BN generation. The influence of tempering on MBN generation in a simple system like 0.2% carbon steel, which on tempering after quenching, results in the dissolution of martensite and precipitation and spheroidization of only cementite particles, is discussed in this paper.
2. Experimental The chemical composition (wt%) of the material used in this study is: 0.22 C, 0.003 P, 0.045 S, 0.015 Si, 0.02 Ni, 0.02 Mn, 0.005 AI, balance Fe. The carbon steel rods (150 mm long and 12 mm diameter) and 12 mm diameter disc samples were solutionised at 1223K for l h followed by water quenching and then, tempered at 873 K for 0.5, 1, 5, 15, 25 and 100h. After the heat treatment, the diameter of the rods was reduced to 10mm by machining and the rods were polished with 600 grit emery paper and subsequently electropolished to remove work hardened surface, if any. The disc samples were polished and etched using 2% Nital
K Moorthv et al. Journal o[Magnetism and Magnetic Materials l 7l (1997) 179 189
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and observed under optical and scanning electron microscopes. Block diagram of the experimental set up for measuring magnetic parameters is shown in Fig. 1. The rods were subjected to continuously varying cyclic magnetic field with a period of 10 s/cycle in an electromagnetic yoke. The current from a sweep controller circuit is fed to a bipolar high current generator to generate a symmetrical bipolar triangular field. The applied magnetic field, HA was measured at the centre of the yoke using a Hall probe (Walker Scientific Inc.) connected to a Gaussmeter (MG-50 Walker Scientific Inc.) in the absence of specimen. HA was varied between _+ 12000 A/m. This field corresponds to a current of _+0.7 A applied to the yoke. HA was calibrated with respect to the current applied to the yoke. The tangential magnetic field, HT is also measured on
181
the sample surface. The MBN signal was acquired using differential coil having 4000 turns (each coil) and the signal was amplified to 74 dB using a low noise amplifier with 100 Hz 100 kHz bandwidth. The RMS voltage of the MBN signal was measured using a RMS meter having a time constant of 0.25 s. The magnetic flux density was measured using a 50 turn coil wound on the sample connected to Fluxmeter (MF-5DP Walker Scientific Inc.). The output voltage signal of all parameters were conditioned suitably for PC based data acquisition. The applied current and the RMS voltage of the MBN were acquired for half of the magnetization cycle i.e. from negative maximum of the field to positive maximum of the field. The parameters were acquired by a PC-AT through a 16 channel, 12 bit A/D converter card having a maximum sampling rate of 100 kHz and using a data acquisition software. Photomicrographs from optical microscope were used for grain size measurement and photomicrographs from scanning electron microscope were used for determining the average size and size distribution of carbides. The image was grabbed using a CCD camera (Astracam, UK). The image analysis was done using VISILOG image processing software. Thresholding and border killing operation were performed for optimizing the grabbed image. The average size and size distribution were determined from the projected area.
3. Results and discussion
Fig. 2a and Fig. 2b show the variation in the RMS voltage of the MBN as a function of the current applied to the yoke for different samples. Figs. 3 and 4 show, respectively, the variation in the peak height and peak position of the MBN as a function of tempering time. In Figs. 3 and 4, the two peak points in the I h tempered sample correspond to the sharp slope changes, one at lower field {Peak 1) and the other at higher field (Peak 2) in the MBN profile shown in Fig. 2a. Table 1 shows the average lath/grain size and average size of carbides for different samples. Fig. 5a Fig. 5e show the size distribution of carbide particles for different tempered samples.
182
K Moorthv et al. ,"Journal o f Magnetism and Magnetic Materials 171 (1997) 179 189
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Fig. 2. (a) Variation in the RMS voltage of the MBN signal as function of current applied to the yoke for half of the magnetization cycle for as quenched sample and, for samples tempered at 873 K for 0.5, 1 and 5 h. (b) Variation in the RMS voltage of the MBN signal as a function of current applied to the yoke for half of the magnetization cycle for samples tempered at 873 K for 15, 25 and 100 h.
Earlier studies [1-9] have shown that M B N signal depends on the number of domain walls moving at a given instant and the mean free path of the domain wall displacement, which are strongly influenced by the microstructural features such as dislocation density, grain size, second phase precipitate size, morphology and its number density and size distribution etc. It is well known that the
magnetization involves various processes such as domain nucleation, domain wall movement and domain rotation [15]. But the major contribution to magnetization comes from irreversible movement of domain walls which causes high rate of change of magnetization. This is because of the fact that fresh domain nucleation is not involved unless ideal saturation is achieved which is difficult in
K Moorthy et al. ,/'Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
Table 1 Average lath/'grain size and average size of carbides for different tempered samples
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practice. It has been observed that reverse or transverse spike domains are associated with pores, large inclusions and grain boundaries in order to reduce its magnetic free pole energy [-15-17].
Goodenough [16] has shown that the grain boundaries are the preferred sites for reverse domain formation and inclusions would generate only closure domains [-17]. Also Goodenough [16] and Cullity [18] have shown that closure domains around inclusions do not, in general, contribute to the 180 '~ wall which moves irreversibly to reverse the flux. Hence, the M B N can be considered to be associated with the irreversible movement of already existing reverse domain walls as compared to domain nucleation or rotation phenomenon. It can be considered that, in polycrystalline heat treated ferromagnetic alloys, the magnetic free poles (i) at the grain/lath boundaries and (ii) at the interface between matrix and inclusions or second phase precipitates are the two major barriers to domain wall motion compared to individual dislocations, fluctuation in matrix chemical composition, etc. However, high dislocation density affects the domain wall mobility. It is well known that the presence of inclusions/precipitates increases the coercivity of the material which is attributed to the pinning of the domain walls by the carbides [16, 19]. Therefore, it can be considered that the process of demagnetization and remagnetization in the opposite direction involves two stages: (i) the irreversible movement of already existing reverse domain walls overcoming resistance offered by the grain boundary free poles, small obstacles such as very fine second phase particles, dislocations, compositional fluctuations, etc., before they are strongly pinned by the larger precipitates or by the spike domain walls associated with the large sized precipitates and (ii) overcoming the stronger obstacles
V. Moorthy et al. /' Journal of Mognetism and Magnetic Malerials 171 (1997) 179 189
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K Moorthy et al. /Journal o['Magnetism and Magnetic Materials 171 (1997) 179 189
such as large size inclusions/precipitates at higher field. If we take Hgb as the critical field strength for overcoming the resistance offered by the grain boundary and Hop as the critical field strength for overcoming the resistance offered by carbide precipitate, these two processes occur over a range of critical field strengths characteristic of barriers, namely, AHgb and AHcp with some mean values Hgh for grain boundaries and Hempfor carbide precipitates. If AHgb and AHcp are overlapping and their two mean values are very close to each other, then the RMS voltage of the MBN can be expected to give a single peak or peak broadening, or a slope change indicating the two stage process can appear in the RMS voltage profile. On the other hand, if the mean values of these two processes are widely separated, then the RMS voltage of the MBN gives a two peak behaviour each corresponding to the mean values of these two processes. Based on the above consideration, the observed influence of different tempered microstructures on the MBN can be explained as follows. The tempering of martensite in low carbon steels has been well studied [20-23]. Caron et al. [23] studied, in particular, the tempering of Fe-0.2%C lath martensite. These studies showed that, tempering of carbon steels with carbon content greater than 0.2% and at temperatures greater than 673 K causes the recovery of martensite into an acicular ferrite, recrystallization, growth of ferrite grains, spheroidization of cementite etc. at different stages. Fig. 6a Fig. 6d shows the optical micrograph of samples tempered at 0.5, 5, 15, 100h, respectively. Fig. 6 and Table 1 clearly show the increasing grain size and growth of cementite with tempering time. It can be expected that the variation in grain size of ferrite and the size of cementite are the major parameters to influence the MBN signal. The quenched specimen shows a very low MBN activity (Fig. 2a) and is attributed to the presence of high dislocation density in the martensite structure. After 0.5 h of tempering, the RMS voltage still shows a single peak behaviour. This is attributed to the incomplete dissolution of the martensite (Fig. 6a) and high number density of fine cementite particles which would result in the overlapping of AHg~, and AHcp. The increase in peak height is
185
attributed to the reduction in dislocation density. After 1 h of tempering, sharp slope changes can be observed, one at lower field and the other at higher field indicating the development of two peak behaviour (Fig. 2a). After 5 h of tempering, a distinct two peak behaviour appears. This clearly shows gradual separation of the two mean values Hg% associated with the dissolution of martensite, transforming into ferrite grains (peak 1) and H~c],associated with the spheroidization and growth of cementites (peak 2). The magnetic free poles at the lath or grain boundary would depend on the boundary dislocation density and the orientation of the magnetization vector in the adjacent laths/grains. Recrystallization and grain growth would always have the tendency to reduce the grain boundary surface pole energy density. But, the magnetic free poles at the matrix precipitate interface depend on the magnetic properties of both matrix and the precipitate and the size of the precipitate (the demagnetization energy is proportional to the radius of the precipitate) [16]. On tempering, the dissolution of martensite, the recrystallization and grain growth, etc. would reduce the boundary dislocation density and the grain boundary energy and hence, reduces the field required for unpinning of domain wall from the grain boundary. But the growth of cementite particles would increase the free pole density at the matrix-second phase particle interface and would require higher field for wall movement. Hence, the peak 1 at lower field strength is attributed to the irreversible movement of domain walls existing at the ferrite lath/grain boundaries overcoming the grain boundary surface free pole energy. The peak 2 at higher field is attributed to the irreversible movement of domain walls overcoming cementite particles (Fig. 2a and Fig. 2b). The width of peak 1 indicates the AHgb and that of peak 2 indicates AHcp. The position of peak 1 indicates the mean value Hg'], and that of peak 2 indicates Hpp. Fig. 7a and Fig. 7b show the variation in RMS voltage of the MBN, applied field (HA) (measured without the specimen), tangential field (HT), and flux density as a function of current applied to the yoke for samples tempered at 873 K for 25 h and 100 h, respectively. It can be observed from Fig. 7
186
~ Moorthy et al. /Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
Fig. 6. Optical micrograph of samples tempered at (a) 0.511, (b) 5 h, (c) 15 h and (d) 100 h.
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that the variation in HA is linear, but the variation in H-r shows a sharp slope change c o r r e s p o n d i n g to the steep p o r t i o n of flux density. The slope change in HT is m a i n l y due to internal d e m a g n e t i z i n g field (H~d) arising from the magnetic free poles associated
with the grain b o u n d a r i e s , precipitates, etc. [15]. The c o n t r i b u t i o n from external d e m a g n e t i z i n g field can be taken as m i n i m u m , as the experiment was done in closed loop electromagnet [15]. The decrease in difference between HT a n d HA at higher
188
K Moorthv et al. /Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
current also supports this fact. It is known that the external demagnetizing field is proportional to magnetization 1-15]. Hence, if the external demagnetization is dominating, the difference between HT and HA would not decrease again after reaching maximum, as the magnetization approaches saturation at higher field. The difference between HT and Ha should remain constant after reaching maximum at higher current also. In addition, the maximum flux density is more or less same for these two samples and the sample dimensions are kept constant. Hence, the difference in H r between these two samples can be considered to be mainly due to the difference in the internal demagnetization factors. It is well known that, when the domain wall intersect the inclusion/wecipitate, the interfacial demagnetizing energy is reduced [15, 18]. It can be observed from Fig. 7a and Fig. 7b that, after the first peak, the difference between HT and Ha decreases. This supports the above-mentioned first stage phenomenon that, the reverse domain walls moving away from the grain boundaries are stopped by the precipitates, thereby reducing the interracial demagnetizing energy and Hid. After the reversal of field to positive direction, again the difference between HT and HA goes through a maximum value where exactly the second peak occurs. This indicates the second stage phenomenon that, the domain walls are moving away from the precipitates thereby increasing the interfacial demagnetizing energy and the associated demagnetizing field H~d. The decrease in difference between HT and Ha at higher current is attributed to the reduction in magnetic free poles associated with the tendency for the interfacial regions to saturate under increasing applied field. The MBN peak height depends on the number of domain walls moving at a given instant and the mean free path of the domain wall displacement. Initially, the dissolution of lath martensite is associated with the recovery and coarsening of ferrite laths. This would increase the number of reverse domain walls and the mean free path of the domain wall displacement. Hence, the first peak shows higher value after 5 h of tempering (Fig. 2a and Fig. 3). On prolonged tempering, the growth of ferrite grains (Fig. 6c, Fig. 6d and Table 1) would
decrease the effective grain boundary area, which in turn, reduces the number of domain walls. This results in the decrease in the first peak height after 15 h of tempering. The recrystallization and grain growth is associated with the decrease in the grain boundary dislocation density and the boundary energy, which in turn, reduces the grain boundary magnetic free pole energy. This reduces the grain boundary resistance to domain wall pinning and hence, shifts the first peak position to lower field (Fig. 2b and Fig. 4). This agrees with the observation of Gatelier-Rothea et al. [4] that, in pure iron, the MBN peak height decreases and the MBN peak position shifts to lower field as the grain size increases. The peak 2 height and its position depend on the average size and the size distribution of cementite particles. The maximum value of peak 2 corresponds to a narrow size distribution of spheroidized cementite particles (Figs. 3 and 5c). The broadening of carbide size distribution associated with coarsening of precipitates {Fig. 5c Fig. 5e) causes the decrease in the height of peak 2 beyond 15 h of tempering. This is also evident from the broadening of RMS voltage profile with increase in tempering time (Fig. 2b). Beyond 15 h of tempering, the shift in the peak 2 position to higher field with increase in tempering time (Fig. 4) is attributed to the increase in the average size of the cementite which is evident from Table 1.
4. Conclusions
This study shows the remarkable correlation of tempered microstructure of 0.2% carbon steel with MBN signal. It has been observed that MBN generation is strongly influenced by the dissolution of martensite and precipitation of cementite particles. At longer duration, the effect of both grain boundaries and the carbides can be observed distinctly by a two peak behaviour. The variation in grain size and the cementite particle size could be clearly observed from the variation in the MBN peak height and peak position values. This study shows that MBN measurements could be used to evaluate different stages of tempering in ferritic steels.
V. Moorth3' el al.
Journal q f Magnelism and Magnetic Materials 17l (1997) 179 189
Acknowledgements A u t h o r s a r e t h a n k f u l t o Shri. C. B a b u R a o a n d S m t . S. S o s a m m a for t h e i r h e l p m i m a g e a n a l y s i s . A u t h o r s a r e a l s o t h a n k f u l to Shri. P. K a l y a n a s u n d a r a m , H e a d , D i v i s i o n for P I E a n d N D T D e v e l o p ment and Dr. Placid Rodriguez. Director, Indira G a n d h i C e n t r e for A t o m i c R e s e a r c h , K a l p a k k a m for t h e i r c o n s t a n t e n c o u r a g e m e n t a n d s u p p o r t d u r ing t h i s s t u d y .
References [1] S. Tiitlo. Acta Polytech. Stand. Helsinki Appl. Phys. Ser. 119 I1977) 1. [2] D.J. Buttle, C.B. Scrnby. J.P. Jakubovies. G.A.D. Briggs, Phil. Mag. A 55 16){19871 717. [3] D.J. Bnttle, C.B. Scruby, J.P. Jakubovics, G A D . Briggs, Phil. Mag. A 55 16) (1987) 735. [4] C. Gatelier-Rothea, P. Fleiscl:mann. J. Chicois, R, Vougeres. Nondestr. Test. Eval. 8 9 (1992) 591. [51] J. Kameda, R. Ranjan, Acta Metall, Mater. 35 {7) (1987) 1515. [6] R. Ranjan, D.C. Jiles, O. Buck, R.B. Thompson, J. Appl. Phys. 61 18) (1987) 3199. [7] R. Hill, R.R.S. Geng, A. Cowking, J.W. Mackersic. NDT Int. 24 (4i 11991) 179.
189
[8] L. Clapham. C. Jagadish, D.L. Atherton, Acta. Metall. Mater. 39 (7) (1991) 1555. [9] D.K. Bhanacharya, T. Jayakumar, V. Moorthy, S. Vaidyanathan, Baldev Raj, NDT&E lnt. 26 (3j (1993) 141. [10] D.J. Buttle, C.B. Scruby, G.AD. Briggs, J. Jakubovics, Proc. R. Soc. Lond. A 414 (1987) 469. [11] R Rautioaho, L.P. Karjalainen, M. Moilanen, J. Magn. Magn. Mater. 73 (1988) 96. [12] L.P. KarjalainemM. Moilanen. NDTInt. 12 (21(1979) 51. [13] K. Titto, Nondestr. Test. Eval. 5 (1989) 27. [14] H. Willems. T. Jayakumar, T.D. Koble, W. Theiner, Application of ultrasonic and micromagnetic testing methods for creep damage assessment, 2nd Annual report on Project D4. European committee on special testing Cost 501/1I WP 5(7, 1989 1990, IzfF'. Saarbri_icken, Germany. [15] R.S. Tebble, D.J. Craik, Magnetic Materials, Wiley Interscience. New York, 1969, pp. 369 41 I. [16] J.B. Goodenough, Phys. Rev. 95 (4) (1954) 917. [17] L.F. Bates, D.H. Martin. Proc. Phys. Soc. 6912-B) (19551 145. [18] B.D. Cullity, Introduction to Magnetic Materials, AddisonWesley. Reading, MA, 1972, p. 292. [19] A.T. English, Acta Metall. Mater. f5 (1967) 1573. [20] G.R. Spcich, Trans. Metall. Soc. AIME 245 (1969) 2553. [21] C.J. Roberts, B.L. Averbach, M. Cohen, Trans. ASM 45 (19531 576. [22] H.W. King, S.G. Glover. J. Iron Steel lnst. 193 (1959) 123. [23] R.N. Caron, G. Krauss. Mctall. Trans. 3(1972) 2381.
Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
~ H Journalof magnetism ~ i and magnetic ~ i ~ materials
Microstructural characterization of quenched and tempered 0.2% carbon steel using magnetic Barkhausen noise analysis V. Moorthy, S. Vaidyanathan, T. J a y a k u m a r , B a l d e v R a j * Division fi)r PIE and NDT Development, lndira Gandhi Centre.]'or Atomic Research, Kalpakkam 603 102, India Received 23 July 1996; received in revised form 4 February 1997
Abstract Magnetic Barkhausen noise (MBN) has been used to characterize the microstructures in quenched and tempered 0.2% carbon steel. It has been observed that tempering at 873 K shows a single peak MBN behaviour after 0.5 h and a slope change indicating the development of two peak behaviour after 1 h. After 5 h of tempering, MBN shows a clear two peak behaviour. A two stage process of irreversible domain wall movement during magnetization is proposed considering the 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 a slope change in MBN behaviour, appears. On the other hand, if the mean values of the critical fields of these two barriers are widely separated, then a two peak behaviour appears. The effect of microstructural changes on MBN is explained based on these two stage processes. The influence of dissolution of martensite and the precipitation of cementite (Fe3C) on MBN are explained.
Keywords. Microstructure; Steel; Barkhausen noise; Domain wall motion
1. Introduction Microstructural characterization of materials by non-destructive evaluation (NDE) techniques is essential for the assessment of initial heat treatment and subsequent degradation in microstructure and mechanical properties under service environment. Exposure to high temperature, static and cyclic
*Corresponding author. Tel.: +91 + 91 4114 40301.
4114 40356; fax:
loading, etc. results in microstructural degradation, creep and fatigue damage. Conventional microscopy techniques do not give global information about the changes in the whole component. Techniques like transmission electron microscopy (TEM), etc. which give finer details are more time consuming and are also not amenable for on-line assessment of components. Advanced N D E techniques such as ultrasonic attenuation and velocity measurements, positron annihilation techniques, acoustic microscopy, magnetic Barkhausen noise (MBN) measurements, etc. have proved their
0304-8853/97/$17.00 ~(;: 1997 Elsevier Science B.V. All rights reserved PH S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 0 4 9 - 8
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~ Moorthv et al. /Journal o['Magnetism and Magnetic Materials ] 71 (1997) 179 189
viability for characterization of microstructures, creep and fatigue damage. MBN measurement is considered as a valuable NDE technique for microstructural characterization of ferromagnetic materials [1 9]. Earlier studies [10-14] have shown that MBN is also sensitive to internal stress/strain and can be applied to evaluate in-service, creep and fatigue damage. When a component is subjected to creep and/or fatigue damage, the internal stress/strain of the material varies significantly in addition to the simultaneous variation in microstructure such as phase transformation, second phase precipitation, etc. Therefore, it is essential to understand the influence of different microstructures on MBN for better insight towards the effect of creep/fatigue damage on the behaviour of MBN. When a ferromagnetic material is subjected to a varying magnetic field, the discrete changes in flux density during magnetization induces voltage pulses in the pick up coil. This phenomenon, called MBN is attributed to the discrete movement of domain walls overcoming obstacles and is sensitive to microstructural variation and strain in the material. Even though several studies [1 9] have been done on microstructural characterization using MBN, still, a clear understanding on the individual influence of different microstructural parameters such as grain size, precipitation of second phase particles, etc. on the generation of MBN has not been well established. Buttle et al. [2] observed a three peak behaviour in MBN activity profile in pure iron, whereas Gatelier-Rothea et al. [4] observed only a single peak for pure iron. Similarly, Ranjan et al. [6] observed a two peak behaviour for pure nickel, whereas Hill et al. [7] observed a three peak behaviour for pure nickel. Also, in a carbon steel with cementite precipitation, Gatelier-Rothea [4] and Kameda et al. [5] have observed a single peak behaviour, whereas Buttle et al. [3] observed a three peak behaviour in MBN activity for Incoloy 904 with coherent spherical precipitates. The difference in their observations could also be due to different magnetizing methods, in addition to changes in microstructural features. Some investigators [2, 3, 5] have used a solenoid for magnetization where the demagnetizing field is very high, and others [7, 9] have used a closed circuit electromagnetic yoke for magnetization to reduce the
demagnetizing field. For the mechanisms of MBN generation also, some investigators [2, 3, 5] have suggested that the domain nucleation process would be the major contribution to MBN and other researchers [6 -8] have suggested the irreversible domain wall movement as the major contribution to the MBN generation. It is generally observed that, on tempering a quenched or normalised ferritic steel such as carbon steel, C r - M o steel, etc., there is reduction in dislocation density associated with recovery, recrystallization and coarsening of ferrite laths or grains, in addition to the precipitation and growth of second phase particles at different stages. Hence, there is a continuous variation in lath/grain size and precipitation of one or more types of second phase particles with different size, size distribution and shape. The interaction of domain walls with the grain boundaries and the precipitates is expected to be significantly different. Hence, it should be possible to distinguish the effect of lath/grain size variation and second phase precipitation on the magnetization process. With this aim, in this study, an attempt has been made to understand the effect of tempering of martensite and precipitation of carbide particles on M BN generation. The influence of tempering on MBN generation in a simple system like 0.2% carbon steel, which on tempering after quenching, results in the dissolution of martensite and precipitation and spheroidization of only cementite particles, is discussed in this paper.
2. Experimental The chemical composition (wt%) of the material used in this study is: 0.22 C, 0.003 P, 0.045 S, 0.015 Si, 0.02 Ni, 0.02 Mn, 0.005 AI, balance Fe. The carbon steel rods (150 mm long and 12 mm diameter) and 12 mm diameter disc samples were solutionised at 1223K for l h followed by water quenching and then, tempered at 873 K for 0.5, 1, 5, 15, 25 and 100h. After the heat treatment, the diameter of the rods was reduced to 10mm by machining and the rods were polished with 600 grit emery paper and subsequently electropolished to remove work hardened surface, if any. The disc samples were polished and etched using 2% Nital
K Moorthv et al. Journal o[Magnetism and Magnetic Materials l 7l (1997) 179 189
_ _
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and observed under optical and scanning electron microscopes. Block diagram of the experimental set up for measuring magnetic parameters is shown in Fig. 1. The rods were subjected to continuously varying cyclic magnetic field with a period of 10 s/cycle in an electromagnetic yoke. The current from a sweep controller circuit is fed to a bipolar high current generator to generate a symmetrical bipolar triangular field. The applied magnetic field, HA was measured at the centre of the yoke using a Hall probe (Walker Scientific Inc.) connected to a Gaussmeter (MG-50 Walker Scientific Inc.) in the absence of specimen. HA was varied between _+ 12000 A/m. This field corresponds to a current of _+0.7 A applied to the yoke. HA was calibrated with respect to the current applied to the yoke. The tangential magnetic field, HT is also measured on
181
the sample surface. The MBN signal was acquired using differential coil having 4000 turns (each coil) and the signal was amplified to 74 dB using a low noise amplifier with 100 Hz 100 kHz bandwidth. The RMS voltage of the MBN signal was measured using a RMS meter having a time constant of 0.25 s. The magnetic flux density was measured using a 50 turn coil wound on the sample connected to Fluxmeter (MF-5DP Walker Scientific Inc.). The output voltage signal of all parameters were conditioned suitably for PC based data acquisition. The applied current and the RMS voltage of the MBN were acquired for half of the magnetization cycle i.e. from negative maximum of the field to positive maximum of the field. The parameters were acquired by a PC-AT through a 16 channel, 12 bit A/D converter card having a maximum sampling rate of 100 kHz and using a data acquisition software. Photomicrographs from optical microscope were used for grain size measurement and photomicrographs from scanning electron microscope were used for determining the average size and size distribution of carbides. The image was grabbed using a CCD camera (Astracam, UK). The image analysis was done using VISILOG image processing software. Thresholding and border killing operation were performed for optimizing the grabbed image. The average size and size distribution were determined from the projected area.
3. Results and discussion
Fig. 2a and Fig. 2b show the variation in the RMS voltage of the MBN as a function of the current applied to the yoke for different samples. Figs. 3 and 4 show, respectively, the variation in the peak height and peak position of the MBN as a function of tempering time. In Figs. 3 and 4, the two peak points in the I h tempered sample correspond to the sharp slope changes, one at lower field {Peak 1) and the other at higher field (Peak 2) in the MBN profile shown in Fig. 2a. Table 1 shows the average lath/grain size and average size of carbides for different samples. Fig. 5a Fig. 5e show the size distribution of carbide particles for different tempered samples.
182
K Moorthv et al. ,"Journal o f Magnetism and Magnetic Materials 171 (1997) 179 189
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Earlier studies [1-9] have shown that M B N signal depends on the number of domain walls moving at a given instant and the mean free path of the domain wall displacement, which are strongly influenced by the microstructural features such as dislocation density, grain size, second phase precipitate size, morphology and its number density and size distribution etc. It is well known that the
magnetization involves various processes such as domain nucleation, domain wall movement and domain rotation [15]. But the major contribution to magnetization comes from irreversible movement of domain walls which causes high rate of change of magnetization. This is because of the fact that fresh domain nucleation is not involved unless ideal saturation is achieved which is difficult in
K Moorthy et al. ,/'Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
Table 1 Average lath/'grain size and average size of carbides for different tempered samples
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practice. It has been observed that reverse or transverse spike domains are associated with pores, large inclusions and grain boundaries in order to reduce its magnetic free pole energy [-15-17].
Goodenough [16] has shown that the grain boundaries are the preferred sites for reverse domain formation and inclusions would generate only closure domains [-17]. Also Goodenough [16] and Cullity [18] have shown that closure domains around inclusions do not, in general, contribute to the 180 '~ wall which moves irreversibly to reverse the flux. Hence, the M B N can be considered to be associated with the irreversible movement of already existing reverse domain walls as compared to domain nucleation or rotation phenomenon. It can be considered that, in polycrystalline heat treated ferromagnetic alloys, the magnetic free poles (i) at the grain/lath boundaries and (ii) at the interface between matrix and inclusions or second phase precipitates are the two major barriers to domain wall motion compared to individual dislocations, fluctuation in matrix chemical composition, etc. However, high dislocation density affects the domain wall mobility. It is well known that the presence of inclusions/precipitates increases the coercivity of the material which is attributed to the pinning of the domain walls by the carbides [16, 19]. Therefore, it can be considered that the process of demagnetization and remagnetization in the opposite direction involves two stages: (i) the irreversible movement of already existing reverse domain walls overcoming resistance offered by the grain boundary free poles, small obstacles such as very fine second phase particles, dislocations, compositional fluctuations, etc., before they are strongly pinned by the larger precipitates or by the spike domain walls associated with the large sized precipitates and (ii) overcoming the stronger obstacles
V. Moorthy et al. /' Journal of Mognetism and Magnetic Malerials 171 (1997) 179 189
184
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K Moorthy et al. /Journal o['Magnetism and Magnetic Materials 171 (1997) 179 189
such as large size inclusions/precipitates at higher field. If we take Hgb as the critical field strength for overcoming the resistance offered by the grain boundary and Hop as the critical field strength for overcoming the resistance offered by carbide precipitate, these two processes occur over a range of critical field strengths characteristic of barriers, namely, AHgb and AHcp with some mean values Hgh for grain boundaries and Hempfor carbide precipitates. If AHgb and AHcp are overlapping and their two mean values are very close to each other, then the RMS voltage of the MBN can be expected to give a single peak or peak broadening, or a slope change indicating the two stage process can appear in the RMS voltage profile. On the other hand, if the mean values of these two processes are widely separated, then the RMS voltage of the MBN gives a two peak behaviour each corresponding to the mean values of these two processes. Based on the above consideration, the observed influence of different tempered microstructures on the MBN can be explained as follows. The tempering of martensite in low carbon steels has been well studied [20-23]. Caron et al. [23] studied, in particular, the tempering of Fe-0.2%C lath martensite. These studies showed that, tempering of carbon steels with carbon content greater than 0.2% and at temperatures greater than 673 K causes the recovery of martensite into an acicular ferrite, recrystallization, growth of ferrite grains, spheroidization of cementite etc. at different stages. Fig. 6a Fig. 6d shows the optical micrograph of samples tempered at 0.5, 5, 15, 100h, respectively. Fig. 6 and Table 1 clearly show the increasing grain size and growth of cementite with tempering time. It can be expected that the variation in grain size of ferrite and the size of cementite are the major parameters to influence the MBN signal. The quenched specimen shows a very low MBN activity (Fig. 2a) and is attributed to the presence of high dislocation density in the martensite structure. After 0.5 h of tempering, the RMS voltage still shows a single peak behaviour. This is attributed to the incomplete dissolution of the martensite (Fig. 6a) and high number density of fine cementite particles which would result in the overlapping of AHg~, and AHcp. The increase in peak height is
185
attributed to the reduction in dislocation density. After 1 h of tempering, sharp slope changes can be observed, one at lower field and the other at higher field indicating the development of two peak behaviour (Fig. 2a). After 5 h of tempering, a distinct two peak behaviour appears. This clearly shows gradual separation of the two mean values Hg% associated with the dissolution of martensite, transforming into ferrite grains (peak 1) and H~c],associated with the spheroidization and growth of cementites (peak 2). The magnetic free poles at the lath or grain boundary would depend on the boundary dislocation density and the orientation of the magnetization vector in the adjacent laths/grains. Recrystallization and grain growth would always have the tendency to reduce the grain boundary surface pole energy density. But, the magnetic free poles at the matrix precipitate interface depend on the magnetic properties of both matrix and the precipitate and the size of the precipitate (the demagnetization energy is proportional to the radius of the precipitate) [16]. On tempering, the dissolution of martensite, the recrystallization and grain growth, etc. would reduce the boundary dislocation density and the grain boundary energy and hence, reduces the field required for unpinning of domain wall from the grain boundary. But the growth of cementite particles would increase the free pole density at the matrix-second phase particle interface and would require higher field for wall movement. Hence, the peak 1 at lower field strength is attributed to the irreversible movement of domain walls existing at the ferrite lath/grain boundaries overcoming the grain boundary surface free pole energy. The peak 2 at higher field is attributed to the irreversible movement of domain walls overcoming cementite particles (Fig. 2a and Fig. 2b). The width of peak 1 indicates the AHgb and that of peak 2 indicates AHcp. The position of peak 1 indicates the mean value Hg'], and that of peak 2 indicates Hpp. Fig. 7a and Fig. 7b show the variation in RMS voltage of the MBN, applied field (HA) (measured without the specimen), tangential field (HT), and flux density as a function of current applied to the yoke for samples tempered at 873 K for 25 h and 100 h, respectively. It can be observed from Fig. 7
186
~ Moorthy et al. /Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
Fig. 6. Optical micrograph of samples tempered at (a) 0.511, (b) 5 h, (c) 15 h and (d) 100 h.
187
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that the variation in HA is linear, but the variation in H-r shows a sharp slope change c o r r e s p o n d i n g to the steep p o r t i o n of flux density. The slope change in HT is m a i n l y due to internal d e m a g n e t i z i n g field (H~d) arising from the magnetic free poles associated
with the grain b o u n d a r i e s , precipitates, etc. [15]. The c o n t r i b u t i o n from external d e m a g n e t i z i n g field can be taken as m i n i m u m , as the experiment was done in closed loop electromagnet [15]. The decrease in difference between HT a n d HA at higher
188
K Moorthv et al. /Journal of Magnetism and Magnetic Materials 171 (1997) 179 189
current also supports this fact. It is known that the external demagnetizing field is proportional to magnetization 1-15]. Hence, if the external demagnetization is dominating, the difference between HT and HA would not decrease again after reaching maximum, as the magnetization approaches saturation at higher field. The difference between HT and Ha should remain constant after reaching maximum at higher current also. In addition, the maximum flux density is more or less same for these two samples and the sample dimensions are kept constant. Hence, the difference in H r between these two samples can be considered to be mainly due to the difference in the internal demagnetization factors. It is well known that, when the domain wall intersect the inclusion/wecipitate, the interfacial demagnetizing energy is reduced [15, 18]. It can be observed from Fig. 7a and Fig. 7b that, after the first peak, the difference between HT and Ha decreases. This supports the above-mentioned first stage phenomenon that, the reverse domain walls moving away from the grain boundaries are stopped by the precipitates, thereby reducing the interracial demagnetizing energy and Hid. After the reversal of field to positive direction, again the difference between HT and HA goes through a maximum value where exactly the second peak occurs. This indicates the second stage phenomenon that, the domain walls are moving away from the precipitates thereby increasing the interfacial demagnetizing energy and the associated demagnetizing field H~d. The decrease in difference between HT and Ha at higher current is attributed to the reduction in magnetic free poles associated with the tendency for the interfacial regions to saturate under increasing applied field. The MBN peak height depends on the number of domain walls moving at a given instant and the mean free path of the domain wall displacement. Initially, the dissolution of lath martensite is associated with the recovery and coarsening of ferrite laths. This would increase the number of reverse domain walls and the mean free path of the domain wall displacement. Hence, the first peak shows higher value after 5 h of tempering (Fig. 2a and Fig. 3). On prolonged tempering, the growth of ferrite grains (Fig. 6c, Fig. 6d and Table 1) would
decrease the effective grain boundary area, which in turn, reduces the number of domain walls. This results in the decrease in the first peak height after 15 h of tempering. The recrystallization and grain growth is associated with the decrease in the grain boundary dislocation density and the boundary energy, which in turn, reduces the grain boundary magnetic free pole energy. This reduces the grain boundary resistance to domain wall pinning and hence, shifts the first peak position to lower field (Fig. 2b and Fig. 4). This agrees with the observation of Gatelier-Rothea et al. [4] that, in pure iron, the MBN peak height decreases and the MBN peak position shifts to lower field as the grain size increases. The peak 2 height and its position depend on the average size and the size distribution of cementite particles. The maximum value of peak 2 corresponds to a narrow size distribution of spheroidized cementite particles (Figs. 3 and 5c). The broadening of carbide size distribution associated with coarsening of precipitates {Fig. 5c Fig. 5e) causes the decrease in the height of peak 2 beyond 15 h of tempering. This is also evident from the broadening of RMS voltage profile with increase in tempering time (Fig. 2b). Beyond 15 h of tempering, the shift in the peak 2 position to higher field with increase in tempering time (Fig. 4) is attributed to the increase in the average size of the cementite which is evident from Table 1.
4. Conclusions
This study shows the remarkable correlation of tempered microstructure of 0.2% carbon steel with MBN signal. It has been observed that MBN generation is strongly influenced by the dissolution of martensite and precipitation of cementite particles. At longer duration, the effect of both grain boundaries and the carbides can be observed distinctly by a two peak behaviour. The variation in grain size and the cementite particle size could be clearly observed from the variation in the MBN peak height and peak position values. This study shows that MBN measurements could be used to evaluate different stages of tempering in ferritic steels.
V. Moorth3' el al.
Journal q f Magnelism and Magnetic Materials 17l (1997) 179 189
Acknowledgements A u t h o r s a r e t h a n k f u l t o Shri. C. B a b u R a o a n d S m t . S. S o s a m m a for t h e i r h e l p m i m a g e a n a l y s i s . A u t h o r s a r e a l s o t h a n k f u l to Shri. P. K a l y a n a s u n d a r a m , H e a d , D i v i s i o n for P I E a n d N D T D e v e l o p ment and Dr. Placid Rodriguez. Director, Indira G a n d h i C e n t r e for A t o m i c R e s e a r c h , K a l p a k k a m for t h e i r c o n s t a n t e n c o u r a g e m e n t a n d s u p p o r t d u r ing t h i s s t u d y .
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