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Utilization of Barkhausen noise magnetizing sweeps for case-depth detection from hardened steel
NDT&E International 52 (2012) 95–102
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Utilization of Barkhausen noise magnetizing sweeps for case-depth detection from hardened steel Suvi Santa-aho a,n, Minnamari Vippola a, Aki Sorsa b, Kauko Leiviska¨ b, Mari Lindgren c, Toivo Lepisto¨ a a
Tampere University of Technology, Department of Materials Science, Tampere, Finland University of Oulu, Control Engineering Laboratory, Finland c Outotec (Finland) Oy, Finland b
a r t i c l e i n f o
abstract
Article history: Received 13 January 2012 Received in revised form 15 May 2012 Accepted 17 May 2012 Available online 28 May 2012
One of the current topics of Barkhausen noise method development is its application to case-depth measurements of hardened components. Usually Barkhausen noise (BN) measurements for case-depth determination are based on the difference in the magnetic properties between the hardened case and the soft base material core. The measurement is done using low magnetizing frequencies. This enables deeper penetration of magnetic field to the ferromagnetic samples compared to the conventional high frequency BN measurements, typically used for grinding burn detection. However, due to the eddy current damping, the penetration depth is limited, depending on the material, to certain distance from the surface. To enable case-depth measurements, the Barkhausen noise measuring device was utilized to obtain data from magnetizing voltage sweeps (MVS). The sweeps were analysed and compared to the case-depths determined with conventional means. A series of hardened samples was investigated including induction and carburizing case-hardened samples. All samples were characterized with X-ray diffraction to study the residual stress state of the surface. Finally destructive characterization was used to verify the actual hardening depth of the studied samples. It was noticed that by studying the MVSs it was possible to evaluate the case-depth values more effectively than in earlier studies. The analysis used in this study utilized the slopes of the MVS sweeps. Furthermore, it was noticed that the ratio of the maximum MVS slopes at different frequencies indicating the case-depth values led to good results. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Barkhausen noise Case-depth measurements Induction hardening Carburizing case-hardening
1. Introduction The detection of hardening layer thickness with non-destructive methods has been studied widely in the recent years. Nondestructive methods enable the detection of layer thickness left for re-grinding and the verification of case-depths of hardening heat treatments. The utilization of Barkhausen noise (BN) [1–7], ultrasonic inspection [8,9] and other methods such as magnetoacoustic emission (MAE) have been studied earlier [10–12]. Usually the test specimens are either induction hardened or nitrided because of their sharp transition layer between the hardened case and the soft core. This layer is different when compared to the evenly changing layer produced in the carburizing case-hardening. The hardening effect of the surface in carburizing and nitriding is due to carbon or nitrogen diffusion to the steel surface from a carbonaceous or nitrous atmosphere. In carburizing, this is followed by a quench where the transformation of martensite occurs in the surface layer. This creates a hardness gradient and also
n
Corresponding author. E-mail address: suvi.santa-aho@tut.fi (S. Santa-aho).
0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2012.05.005
compressive residual stresses on the sample surface [13]. The determination of case-depth is one way to evaluate the composition gradients, microstructure gradients and also the quality of the hardening treatment. The need to utilize these measurements comes from the quality control in machine industry. The non-destructive detection of hardened layer thickness could be useful in the detection of the layer left for re-grinding and verification of case-depths of hardening heat treatments. In consequence, some commercial systems [8,9] that operate with ultrasound for case-depth detection have been introduced. The problem with ultrasonic waves is that the determination of thin hardened layers, smaller than 1.5 mm, is difficult. In addition, the ultrasonic inspection is applicable for hardened layers with a steep hardness gradient such as formed in induction hardening. Thus, there is a need for non-destructive method for case-depth determination also to the carburized components. So far, no system based only on BN to detect case-depths is available commercially. One of the current topics of BN method development is its application to case-depth measurements of hardened gears. BN measurement in case-depth determination is based on the difference in the magnetic properties between the hardened case and
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the soft base material core of the component. The hard martensite case with high coercivity and high remanence values requires stronger magnetic fields to be magnetized than the soft ferrite interior [14]. The BN based case-depth measurements are usually performed using low magnetizing frequencies. This enables deeper penetration of magnetic field to the ferromagnetic samples compared to the conventional high frequency BN measurements used in the grinding burn detection [15]. The BN signal measurement depth depends on the permeability and conductivity of the tested material and the magnetizing frequency used. The problem in casedepth studies is that the BN signal, created at certain depth below surface corresponding to the magnetizing frequency, is attenuated exponentially as a function of the distance travelled inside the material due to the eddy current damping which in turn depends on the frequency of the magnetized signal [16]. According to the present knowledge, case-depths can be accurately predicted down to 1 mm by BN measurements [5]. Usually the case-depth studies using the BN method rely on two-peaked behaviour of the BN signal envelope. This is due to soft core and hard martensite case that both generate an own peak [1–3,5]. The first peak at low applied magnetic field strength corresponds to the softer interior and the second peak corresponds to the harder case. Also the filtering of the BN signal to exclude the signal of the soft interior has been studied [1,2]. The ratio of these two peaks can be calculated and has been shown to correlate with the case depth [1,3,5]. Magnetic hysteresis properties have also been studied for the case-depth measurements [6,7,17]. Some parameters that can be calculated from the hysteresis measurements have also been noticed to change as a function of the case-depth value. Lo et al. [7] found that the coercivity and hysteresis loss increases with the increasing case-depths in induction hardened AISI 1045 steel samples up to 1.9 mm hardening layer thickness. On the other hand, the maximum permeability was noticed to decrease as the case-depth increased. This phenomenon can be explained by the fact that as the case-depth increases, more domain wall reversals are occurring in the martensitic case. Martensite is known to require quite high magnetic field to unpin the domain walls and this contributes to the increase in coercivity and hysteresis loss values. The study concluded that the case-depths can be estimated as weighted sum of signals from case and core. The samples that Lo et al. [7] used were induction hardened but there was not any notation about the residual stress state of the samples. Zhang et al. [6] studied the hysteresis loop parameters for case-depth determination and noticed that variations occurred in the hysteresis loop shapes for induction hardened samples. The differential permeability mr calculated from the hysteresis loop was noticed to change along with the case-depth value for induction hardened samples and to a much lesser extent for case-carburizing samples due to the smoother hardness slope. Also saturation magnetization value Ms, depending on the individual magnetic moments and their interactions with each other, was found to correlate with the case-depth values on both induction and carburized samples. The saturation magnetization varied in the hardened surface layer and in the soft bulk due to the differences in the magnetic properties. The change in the hardened layer thickness affects to the overall saturation magnetization and the case-depth can be determined by calculation of volume-weighted sum of saturation magnetization of the core and case [6]. Also this article lacks the information about the residual stress state of the sample material. The study of Kai et al. [17] for carburized ring cut out samples concluded that the measurement and numerical analysis of magnetic flux density B and magnetic field strength H can be used to estimate the case-depth of hardened samples. They
noticed that the coercivity, relative permeability and also the saturation magnetization were dependent on the case-depth. A need has emerged to use other BN related measurements along with the case-depth measurements to determine the surface and subsurface state of the sample. For the high frequency measurements, the magnetizing voltage sweep (MVS) and magnetizing frequency sweep (MFS) are measurements that provide the relationship between the voltage or the frequency and the magnetoelastic parameter (mp). These sweeps are used to find the optimal parameters for the high frequency BN measurements. The BN equipment changes the magnetizing voltage keeping the magnetizing frequency constant or vice versa. Thomas and Fix [18] studied the use of MVS and MFS for maximizing the sensitivity of the BN measurements. They claimed that the highest magnetizing amplitude, not too high to saturate the specimen, needs to be chosen because the saturation of the sample decreases the sensitivity of the measurements. They determined also sensitivity ratio maps where the highest sensitivity for particular BN measurement can be verified. This paper presents results on the utilization of Rollscan 300 equipment to case-depth analysis based on producing magnetizing voltage sweeps on hardened samples. These magnetizing voltage sweeps show the applied ascending magnetic field value while increasing magnetizing voltage. The slope of the MVS curve at different voltages is calculated and the maximum slope is determined. The MVS curve measured this way can thus be correlated to the hysteresis curve and the MVS slope value can be correlated to the permeability or susceptibility value calculated from the hysteresis loop. Permeability is the calculated inclination of the magnetic flux density B as a function of the applied magnetic field H. Susceptibility can be characterized as the inclination of the magnetization M as a function of the magnetic field H [14]. Both of these calculated inclination values characterize material properties which respond to the applied magnetic field in a specific way depending on the microstructural state. The permeability is stated to be influenced by the magnetic domain coupling and also by the domain density [19]. The pinning site density and energy influence the coercivity value determined from the hysteresis loop [19]. Initial permeability and coercivity is known to have a reciprocal relationship [20]. The reciprocal value of the differential susceptibility was also found to have a linear correlation to the stress value [21]. Similarly the calculated slope from magnetizing voltage sweep can give information about the sample with composition gradients and microstructure gradients related to the casedepth value.
2. Experimental procedure Hardened rod samples with varying hardened layer thicknesses were prepared by induction heating and carburizing case-hardening. A sample set of different induction hardened gear tooth samples was also studied. For induction hardening, two steel grades, 34CrNiMo6 and 42CrMo4, were used. The length of all rod shaped induction hardened samples was 300 mm and the diameter was 45 mm. The samples were induction hardened to varying case-depths from 0.65 mm to 2.2 mm with material 34CrNiMo6 and from 0.35 mm to 1.9 mm with material 42CrMo4. Some of the induction hardened samples, referred as turned, were turned and finally ground to smaller rod diameter of 39 mm in order to produce shallower case-depth. Table 1 shows the nominal chemical composition of all the studied steels. Five different sets of induction hardened samples were prepared. Sets 1 and 4 were manufactured at Takoma Gears (Finland) with onefrequency hardening machine. Set 2 was also manufactured by Takoma Gears with multi frequency hardening machine.
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Table 1 Nominal chemical composition of the studied steels. Wt%
C
Cr
Si
Mo
Mn
Ni
P
S
Ca
Cu
34CrNiMo6 42CrMo4 18CrNiMo7–6
0.36 0.41 0.21
1.36 1.07 1.57
0.28 0.31 0.20
0.17 0.17 0.27
0.71 0.73 0.65
1.31 – 1.45
0.024 0.017 0.016
0.029 0.03 0.029
– 0.0036 0.0044
– – 0.18
Table 2 Description of the sample sets.
Hardening method Material Hardening layer thickness variations [mm]
Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
Induction hardening 34CrNiMo6 Rht 0.65–2.2
Induction hardening 34CrNiMo6 Rht 0.825–1.85
Induction hardening 34CrNiMo6 Rht 0.65–1.45
Induction hardening 42CrMo4 Rht 0.35–1.9
Carburizing casehardening 18CrNiMo7-6 CHD 1.2–3.5
Induction hardening
The sample manufacturing procedure was similar in sets 1 and 4, only the hardening machine was different. For all these samples the induction coil was a ring shaped copper tube with 2 mm air gap between the rod and the induction coil. Rod samples were rotated simultaneously as the induction coil scanned throughout the sample height to ensure homogeneous hardening. The induction frequency was 10 kHz. The different hardening depths were achieved by varying the induction coil input power and the scanning speed values. The quench was done with water sprays. Sample set 3 was induction hardened by Bodycote V¨armebehandling AB (Malm¨o, Sweden) to hardened layer thicknesses (Rht) varying from 0.65 mm to 1.45 mm. Sample set 5 was manufactured from 18CrNiMo7-6 steel by carburizing case-hardening by Moventas Wind Oy (Finland) and Moventas Santasalo Oy (Finland). The length of rod samples was 300 mm and the diameter was 42 mm. The carburizing casehardening was performed to varying case-depths (CHD) from 1.2 mm to 3.5 mm. Table 2 presents the detailed description of the sample sets. In addition, a sample set with induction hardened gear tooth samples, referred to as set 6, was manufactured by Takoma Gears Oy (Finland). The set consisted of samples manufactured from 34CrNiMo6 steel with hardened layer thicknesses (Rht) 2.8 mm and 2.9 mm and samples manufactured from 42CrMo4 steel with hardened layer thicknesses 3 mm and 4 mm. All rod samples were turned prior the heat treatments. The visible oxide layer formed during the heat treatments was removed in a 2% citric acid bath from the samples before the measurements. The rod samples were studied first with Rollscan 300 BN measurement device using a specially designed sensor for rod shaped samples. The tooth samples were studied with gear teeth BN sensor having suitable geometry for curved gear surfaces. Also X-ray diffraction residual stress measurements were performed along with XStress3000 for all sample surfaces in sets 1–5. Finally a destructive verification of case-depths was done to these samples. 2.1. Barkhausen noise measurements with Rollscan 300 The samples were studied after the hardening with Rollscan 300 Barkhausen noise analyzer manufactured by Stresstech Oy (Finland) to detect the hardened surface condition. The BN measurements were recorded with MicroScan software, yielding the RMS value of the Barkhausen noise amplitude. The magnetizing frequency was 125 Hz and the magnetizing voltage was 7 Vpp (voltage from peak to peak). A special sensor for round shapes was used for rod samples. The experimental setup is shown in Fig. 1a. BN measurements were done in the circumferencial direction of the
34CrNiMo6, 42CrMo4 Rht 2.8, 2.9 (34CrNiMo6) Rht 3, 4 (42CrMo4)
rod. In addition, the magnetizing voltage sweeps (MVS) were studied with Rollscan 300 device. These sweeps were recorded with ViewScan software, which is shown in Fig. 1b. MVS was performed by increasing voltage from zero to maximum value of 16 Vpp as the frequencies: 20 Hz, 30 Hz, 60 Hz and 125 Hz, were kept constant. The recorded BN value is plotted as a function of the voltage. These sweeps are typically utilized for determining the optimum parameters for the actual BN measurements. Then the optimum measurement parameters are chosen so that the BN slope has the highest value. A couple of examples of the magnetizing voltage sweeps (MVS) made with Rollscan to sample sets are presented in Fig. 2. The curves present the response of magnetic parameter (mp) to increasing voltage value up to 16 Vpp while the frequency is kept constant. Fig. 2a shows the different magnetizing voltage behaviour with tensile and compressive residual stress values. Fig. 2b shows the change in the sweeps according to different X-ray diffraction peak full-width at half-maximum (FWHM) values. The microstresses occurring in the samples change the measured FWHM values. A correlation between the peak width and hardness can be established [22]. 2.2. Material characterization The surface residual stresses (RS) and FWHM (full-width at half-maximum) of the diffraction peak values were measured from the samples with X-ray diffractometer (XRD) XStress 3000 manufactured by Stresstech Oy (Finland). X-ray diffraction measurements were performed using CrKa radiation and the modified chi method with totally eight tilt angle degrees for both the longitudinal and circumferential direction of the rod samples [23]. Current 6.7 mA and voltage 30 kV was used in the measurements. After the non-destructive measurements, the rod samples were sectioned into smaller sections with Discotom cut-off machine and cross-sectional samples were prepared to determine the actual case-depth of the samples. The cross-sectional samples were then ground with silicon carbide grinding paper and polished with diamond suspension. The polished samples were etched with Nital 2% in order to examine the microstructure and to perform the hardness depth profiles with Duramin-300 hardness tester to verify the actual case-depths. The verification of the hardened layer thickness was done with hardness depth profile according to standards [24,25]. For the induction hardened samples the value of case-depth (Rht) is defined as the depth below the surface at which the Vicker’s hardness value drops to 400 HV [24]. For carburizing case-hardened samples the case-hardening depth
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Fig. 1. (a) Experimental arrangement of Rollscan 300 measurements for rod samples and (b) magnetizing voltage sweep recorded with Viewscan.
Fig. 2. (a) Voltage sweep made with Rollscan to two 34CrNiMo6 samples with different residual stress states and (b) voltage sweeps made to two different sample places with different XRD FWHM values; HH09_1 with FWHM 5 and HH09_2 FWHM 3.
800
3. Results and discussion 3.1. Material characterization of samples Table 3 presents the measured RMS values of the BN signal from different sample sets 1–5 measured with frequency 125 Hz and voltage 7 Vpp. Sample set 6 was measured with frequency 45 Hz and voltage 4.3 Vpp. The surface residual stress values to the same direction as the BN was measured and are also shown in Table 3. The RMS values for the induction hardened sample sets 1 and 4 had some variation, because the turned samples had increased RMS values, up to 100. The induction hardened sample sets 1–4 had average RMS values; 89, 38, 61 and 76, respectively. Set 5, which contained carburized samples had the average value of RMS 108. All induction hardened samples had compressive residual stress state on the surface. However, many carburizing case-hardened samples had tensile residual stresses on the surface. The tensile residual stress on the surface of some carburized samples may be due to some residual oxide layer on the sample surface or to a fluctuation of heat treatment quality for these samples. However, there was some minor variation in the hardness depth profiles for these carburized samples as shown in Fig. 3. The sample geometry for set 6 samples did not allow measuring the residual stress values for these samples. In addition, residual stress depth profiles were carried out to some of the induction hardened samples in set 1. The residual stress depth distributions were compressive residual stress on the
Hardness [HV1]
(CHD) is determined to be the distance where hardness drops to 550 HV (50 HRC) measured with HV0.1, HV1 or HV5 [25]. An example of hardness profiles for 18CrNiMo7–6 carburizing casehardened rod specimens of set 5 is shown in Fig. 3.
HH09
HH20
750
HH11
HH25
700
HH14
HH30 HH17
650 600 550 500 450 400 0
0.5
1 1.5 2 2.5 3 3.5 Distance from the surface [mm]
4
4.5
Fig. 3. Hardness depth profiles for set 5 of carburizing case-hardened samples.
surface layer, decreasing and changing into tensile residual stress in the depth equal to the hardened layer thickness. 3.2. Results of Barkhausen noise signal analysis A new approach for the BN case-depth studies was performed with measuring MVS from hardened samples. The calculated slope of the MVS curve changed as the magnetizing voltage was increased referring to increase in the value of the applied magnetic field. The slopes were calculated for each voltage sweep
S. Santa-aho et al. / NDT&E International 52 (2012) 95–102
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Table 3 The measured RMS and X-ray diffraction residual stress values for different sample sets. Set 1
Set 2
Set 3
Sample
Rht [mm]
RMS [MP]
RS [MPa]
Sample
Rht [mm]
RMS [MP]
RS [MPa]
Sample
Rht [mm]
RMS [MP]
RS [MPa]
S52 S48 S51 CD37 CD36 CD39 S55 CD38 S57 S56
0.65 0.75 0.85 1.12 1.15 1.25 1.55 1.75 1.8 2.2
113 118 126 53 52 54 111 61 113 111
320 370 350 440 390 400 330 390 380 390
TG1 TG2 TG3 TG4 TG5
0.825 0.875 1.1 1.45 1.85
32 31 41 43 45
520 540 480 500 510
B8 B9 B7 B6 B5 B4 B3 B2 B1
0.65 0.7 0.75 0.9 0.95 1.05 1.15 1.3 1.45
49 50 57 53 68 66 62 76 69
200 200 230 240 230 260 250 240 300
89
376
38
510
61
239
Average Set 4
Set 5
Set 6
Sample
Rht [mm]
RMS [MP]
RS [MPa]
Sample
CHD [mm]
RMS [MP]
RS [MPa]
Sample
Rht [mm]
RMS [MP]
S20 IND5 IND10 S23 S25 IND9 IND8 S27 IND11
0.35 1 1 1.05 1.25 1.25 1.3 1.3 1.9
98 35 60 105 101 63 56 104 62
110 380 430 140 260 400 450 320 400
HH09 HH11 HH14 HH17 HH20 HH25 HH30
1.2 1.5 1.8 2 2.5 2.9 3.5
55 145 132 124 155 67 78
90 þ 310 þ 190 þ 130 þ 100 190 130
#504 #509 #478 #522
2.8 2.9 3 4
54 79 56 70
76
321
Average
Fig. 4. The slope calculated from MVS as a function of voltage.
by fitting a first order polynomial to 25 consecutive data points. An example of the MVS slope changes in one sweep is shown in Fig. 4. The maximum values of the MVS slopes were recorded and used in later analyses. The MVS curve created this way can thus be correlated to the hysteresis curve and the slope value calculated from the MVS curve to the permeability or susceptibility value calculated from the hysteresis loop. Both of these values, permeability and susceptibility, characterize the material magnetic behaviour when the ferromagnetic specimen is exposed to an applied magnetic field. According to the literature, the reciprocal initial susceptibility has a linear behaviour with respect to the applied tensile stress [21]. The permeability calculated from the slope is found to be influenced by the magnetic domain coupling and also to the domain density [19]. The magnetizing frequency value determines partly the depth where the measurement information comes from [12]. Thus the MVS performed with different frequencies can give information from different depths. The use
108
65
of two different magnetizing frequencies in the MVS gives information about the compositional gradient layer influenced by the microstructural features and also about residual stresses. The MVS behaviour showed a systematic change with respect to surface hardness values. This was shown in Fig. 2b. Also the surface residual stresses change the MVS appearance as shown in Fig. 2a when all other characteristics of the samples are similar. Thus, it is reasonable to assume that the calculated slope from the MVS curve can characterize the material properties. Some papers [6,7] indicate that the maximum permeability decreases as the case-depth increases. Only the carburizing case-hardened samples in our studies, having tensile residual stresses on the surface, showed this behaviour. However, the studies [6,7,17] did not give the information about the residual stress for their samples and the studied sample sets were also quite small. Fig. 5 presents the relationship between the MVS slopes obtained from the 60 Hz MVS and case-depths. Correlations were linear within all sample sets. However, the residual stress state affected the direction of the correlation. With compressive residual stresses the correlation was positive and with tensile residual stresses the correlation was negative. This behaviour can be seen clearly in set 5 including carburizing case-hardened samples with either tension or compression residual stress on the surface. The MVS slope values for different surface residual stress values were noticed to change. The magnetic hysteresis behaviour is known to vary because the magnetostriction coefficients change according to the stresses from magnetic field exposure of ferromagnetic material. The tensile stress causes the magnetostrictive coefficient to increase more slowly in the magnetization [27,28]. Thus the different calculated slope values for samples with compressive or tensile stresses were noticed to change. However, no conclusions can be drawn from the MVS slope values at one magnetizing frequency value comparing to case-depths. Thus, the information in the MVS slope was refined to take into account the effect of residual stresses and to get more detailed information about the case-depths. The slopes at different frequencies were subtracted
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and divided with each other in the analysis. Also the reciprocal of the maximum slopes was calculated. To analyse the sweeps, correlation coefficients between the case-depths and the refined slope information were calculated. Correlation is a statistical measure describing the relationship of two variables. Correlation coefficient varies between 1 and 1 so that 1 refers to a perfect correlation and 1 to a perfect negative correlation [26]. Also the reciprocal of the maximum slopes was calculated. Table 4 shows these correlation coefficients between the sample sets and the case-depths. The samples were divided into two groups according to the residual stress being tension or compression. Maximum correlation for samples with compressive residual stress on the surface, 0.86, was measured with division of MVS slopes for frequencies 20 Hz and 125 Hz. This frequency range was the widest frequency range studied. Slope division also produced higher correlations in all studied frequency ranges than subtracted sweeps. Higher correlations were observed if they were calculated only for one set at the time. For example to division 20 Hz/125 Hz for set 3, the correlation value was 0.98. The differences between the sample sets may be due to the different manufacturing equipment and process parameters. Within each sample set, the manufacturing process was similar. The maximum correlation for carburizing case-hardened samples with tensile residual stress on the surface was noticed to be
60
Set 1 Set 2 Set 3 Set 4 Set 5, + RS Set 5, -RS Set 6
Slope (60 Hz)
50 40 30 20 10 0
0
0.5
1
1.5 2 2.5 3 Hardening depth [mm]
3.5
4
4.5
Fig. 5. The hardening depth for different sample series as a function of the calculated slope for 60 Hz voltage sweeps. RS denotes compressive residual stresses and þ RS tensile residual stresses.
higher, 0.97, than to the induction and carburized samples having compressive residual stresses on the surface, 0.86. This was calculated for carburized samples with division of sweep slopes for frequencies 30 Hz and 20 Hz. The induction hardened teeth series had maximum correlation, 0.81, in slope division between 20 Hz and 125 Hz and also the same correlation in subtraction of 30 Hz slope and 125 Hz slope. The decreased correlation may be due to the differences in the samples, the materials and sample sizes, referring to tooth module, within set 6 varied. The differences in the best frequency ranges for induction and tensile carburizing samples indicates that the change in the magnetic properties occurs more gradually over a long distance in the induction hardened samples and thus needs larger frequency range to be taken into account. Zhang et al. [6] noticed also different behaviour of saturation magnetization as a function of case-depth for induction and carburizing hardened samples. They noticed that carburized samples produced larger changes in the ratio between the saturation magnetization and the case-depth values. An example of the dependency between the reciprocal of MVS subtraction and the case-depths is given in Fig. 6. The calculated values give linear relationship in shallower hardening depths up to 2.5 mm. They also distinguish the samples according to their surface residual stress values being tension or compression into two separate groups that both have linear relationship as a function of hardened layer thickness up to 2.5 mm. The best calculated correlation for division of two MVS slopes is given in Fig. 7 for magnetizing frequencies 20 Hz and 125 Hz. The slope ratio is correlated to the hardening depth in all sample sets, including both the tensile and compressive residual stress carburizing case-hardened samples. The calculated inclinations between slope ratio of 20 Hz and 125 Hz and case-depths for different sets are presented in Table 5. For sample sets 3 and 5 with lower compressive residual stress values the inclinations are larger. However, for the carburizing case-hardened sample set with tensile residual stress on the surface the inclination is smaller than for the carburized samples with compressive residual stress. Turned samples were separated from sets 1 and 4 and produced a group with lowest inclination. The sample sets with residual stress values from 300 MPa to 400 MPa had inclinations between these two groups. The inclination seems to be affected highly by the residual stress of the surface (Table 5). Table 5 shows that when the inclinations are compared to the surface residual stresses, generally decrease is observed with increasing compressive residual stress. However, set 3 deviates from this observation. The different manufacturing equipment may explain this divergent behaviour for set 3.
Table 4 Calculated correlations for different frequency sweeps for induction and carburizing case-hardened samples with compression residual stress on the surface and calculated correlations for case-hardened sample with tensile residual stress. Highest absolute value of correlations shown in bold.
20 Hz, 30 Hz 20 Hz, 60 Hz 20 Hz, 125 Hz 30 Hz, 20 Hz 30 Hz, 60 Hz 30 Hz, 125 Hz 60 Hz, 20 Hz 60 Hz, 30 Hz 60 Hz, 125 Hz 125 Hz, 20 Hz 125 Hz, 30 Hz 125 Hz, 60 Hz
Maximum slope for sweep subtraction Correlation 1 ( RS)
Maximum slope for sweep division Correlation 2 ( RS)
Maximum slope for 1/ Sweep slope sweep subtraction subtraction Correlation 3 ( RS) Correlation 1 (þRS)
Maximum slope for sweep division Correlation 2 (þRS)
1/ Sweep slope subtraction Correlation 3 (þRS)
0.10 0.28 0.50 0.10 0.38 0.56 0.28 0.38 0.54 0.50 0.56 0.54
0.66 0.82 0.86 0.59 0.79 0.79 0.69 0.71 0.66 0.76 0.69 0.60
0.07 0.51 0.74 0.07 0.70 0.18 0.51 0.70 0.10 0.74 0.18 0.10
0.96 0.92 0.88 0.97 0.86 0.84 0.94 0.87 0.66 0.86 0.81 0.62
0.87 0.83 0.80 0.87 0.80 0.33 0.83 0.80 0.57 0.80 0.33 0.57
0.92 0.87 0.86 0.92 0.84 0.82 0.87 0.84 0.53 0.86 0.82 0.53
S. Santa-aho et al. / NDT&E International 52 (2012) 95–102
Sample set 6 was left out from the examination in Fig. 7 because the RS data was not available. In general, the deviation in all the results may be due to the different manufacturing equipments because all of the samples were not manufactured with same induction hardening machine. Also the changes in the process parameters can have influence on the samples. The hardened layer thickness value range was the narrowest for sample set 3. There were totally 9 samples within a hardened layer thickness range of 0.8 mm. Thus, in Table 5 the calculated inclination for this sample set can be misleading because of the smaller range of hardened layer thicknesses compared to other sample sets. Fig. 8 presents the regression line together with the calculated upper and lower confidence limits for the same data set as shown in
1/slope (125 Hz)-slope (20 Hz)
0.7
Set 5, + RS Set 2,3,5, -RS
0.6
0.4 0.3 0.2 0.1
0
0.5
1
1.5 2 2.5 Hardening depth [mm]
3
3.5
4
Fig. 6. The hardening depth for different sample series as a function of the calculated slope for reciprocal of subtraction of 20 Hz and 125 Hz slopes. RS denotes compressive residual stresses and þRS tensile residual stresses.
1
Slope 20 Hz / slope 125 Hz
0.9 0.8 0.7 0.6 0.5 0.4
Set 1 Set 2 Set 3 Set 4 Set 5, + RS Set 5, -RS
0.3 0.2 0.1 0
Fig. 7. The confidence interval can be used to indicate the reliability of the regression line. A linear regression analysis was used for the MVS slope division data by utilizing the least squares method calculated for the data sets 1–5. The confidence level of 95% was used. Thus, the confidence limits show the range where the estimated values are located with a probability of 95%. We can estimate that with certain measured slope division values the hardened layer thickness variations are within a range of certain limits with 95% probability. For example with MVS slope division value of 0.3 the hardened thickness layer is within a range of 0.65–0.95 mm. The calculated inclinations from the MVS slope division for different sample sets seemed to be affected highly by the residual stress of the surface. The higher the compressive stress the smaller the changes are in the MVS slope ratio. This is probably due to the decreased Barkhausen noise activity with higher compressive stresses [29].
4. Conclusions
0.5
0
101
0
0.5
1
1.5 2 2.5 Hardening depth [mm]
3
3.5
4
Fig. 7. The division of 20 Hz MVS slope with 125 Hz MVS slope for sample sets 1–5. RS denotes compressive residual stresses and þ RS tensile residual stresses.
One important research area of the Barkhausen noise development is its application to case-depth measurements of hardened components. Since the ultrasound based measurement is not applicable to the carburized samples, the applicability of some other measurement technique such as magnetic Barkhausen noise measurement is worthwhile to be studied. The different hardening processes and process parameters create differences in hardened layer thickness, but produce also a compositional and microstructural gradient with different magnetic properties affected by residual stresses, too. The surface hardness variations can be noticed as altered RMS values of the samples. In this study, the residual stress state of the induction hardened samples was quite uniform so the RMS values are mostly influenced by the hardness. However, some of the studied carburizing case-hardened samples had tensile residual stresses on the surface that was produced in the hardening process or due to some residual oxide layer left on the sample surface. The previous BN studies concerning the case-depth measurements have concluded that the case-depth can be reliably estimated down to 1 mm. Here it was noticed, that by studying the magnetizing voltage sweeps it is possible to evaluate the compositional gradients referring to the case-depth values even deeper. It was also concluded that any MVS slope with certain frequency does not explain the changes in the case-depth values on their own. Thus, the ratio of the maximum MVS slopes at different frequencies needed to be calculated. When comparing the ratios within one sample set, the correlation was close to 1 indicating a linear behaviour. Even taking into account many different sample sets the MVS slope ratios led to high correlations, 0.86, when comparing these ratios to the case-depth values. Also the estimation of the hardened layer thickness variations with certain measured slope division values could be performed with calculating the regression line together with the confidence limits. The current needs for the non-destructive case-depth determination can consist of layer thickness estimation e.g. for regrinding of hardened component. The studied sweep method can be applicable to carburized samples also in such case that a calibration series of that particular material is manufactured.
Table 5 Calculated inclinations from Fig. 7 for MVS slope 20 Hz/125 Hz with different sample sets.
Inclination 20 Hz/125 Hz Residual stress average [MPa]
Set 1, ground
Set 1, turned
Set 2
Set 3
Set 4, ground
Set 4, turned
Set 5, RS
Set 5, þ Rs
0.15 410
0.06 360
0.2 510
0.38 240
0.13 410
0.05 210
0.21 140
0.1 þ180
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S. Santa-aho et al. / NDT&E International 52 (2012) 95–102
1 0.9
Slope 20 Hz / slope 125 Hz
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Set 1 0
Regression line
0.25 0.5 0.75
1
Lower 95 % conf. limit
Upper 95 % conf. limit
1.25 1.5 1.75 2 2.25 2.5 2.75 Hardening depth [mm]
3
3.25 3.5 3.75
Fig. 8. The 95% confidence limits calculated for division of 20 Hz and 125 Hz MVS slope for sample sets.
With such samples, a regression line with confidence limits can be performed to be used in the non-destructive testing of hardened samples. For now, the overall regression line gives information about the limits between what the layer thickness lies. The challenge of the utilization of this method is still to increase the accuracy and confidence of the method. For this reason more actual hardened components need to be tested verifying the applicability of the method.
Acknowledgement Support from TEKES for the NOVEBARK Research Project, Foundation of Walter Ahlstrom and Graduate School of TUT is gratefully acknowledged. The authors would also like to thank personnel of Stresstech group, Moventas Wind and Santasalo (Finland), Takoma ¨ Sweden), Katsa Oy (Finland) for Gears (Finland), Bodycote (Malmo, their technical support. Research assistants at the Laboratory of Materials Characterization (Tampere University of Technology) are thanked for sample preparation. References [1] Dubois M, Fiset M. Evaluation of case depth on steels by Barkhausen noise measurements. Mater Sci Technol 1995;11(3):264–7. [2] Bach G, Goebbels K, Theiner WA. Characterization of hardening depth by Barkhausen noise measurements. Mater Eval 1988;46:1576–80. [3] Vaidyanathan A, Moorthy V, Jayakumar T, Raj B. Evaluation of induction hardened case depth through microstructural characterization using magnetic Barkhausen emission technique. Mater Sci Technol 2000;16:202–8. [4] Moorthy V, Shaw BA, Hopkins P. Surface and subsurface stress evolution in case-carburised steel using high and low frequency magnetic Barkhausen emission measurements. J Magn Magn Mater 2006;299(2):363–75. [5] Moorthy V, Shaw BA, Brimble K. Testing of case depth in case carburized steels using magnetic Barkhausen emission technique. Mater Eval 2004;62(5):523–7. [6] Zhang C, Bowler N, Lo C. Magnetic characterization of surface-hardened steel. J Magn Magn Mater 2009;321:3878–87. [7] Lo CCH, Kinser ER, Melikhov Y, Jiles DC. Magnetic non-destructive characterization of case depth in surface-hardened steel components. In: Thompson DO, Chimenti DE, editors. Review of progress in quantitative non-destructive evaluation 25B. AIP Conference Proceedings 2006;820:1253–60. [8] Internet: /http://www.qnetworld.com/case_testing/p3121.htmlS, 12.12.2011.
[9] Internet: /http://www.sonix.com/learning/files/UMA_Breakthrough.pdfS, 12.12.2011. [10] O’Sullivan D, Cotterell M, Cassidy S, Tanner DA, Meszaros I. Magneto-acoustic emission for the characterization of ferritic stainless steel microstructural state. J Magn Magn Mater 2004;271:381–9. [11] Fulton JP, Wincheski B, Namkung MA. Probabilistic model for simulating magnetoacoustic emission responses in ferromagnets. In: Thompson DO, Chimenti DE, editors. Review of progress in quantitative nondestructive evaluation. New York: Plenum Press; 1994. [12] Wilson JW, Tian GY, Moorthy V, Shaw BA. Magneto-acoustic emission and magnetic Barkhausen emission for case depth measurement in En36 gear steel. IEEE Trans Magn 2009;45(1):177–83. [13] Totten GE, editor. Steel heat treatment: metallurgy and technologies. New York: Deklor; 1997. [14] Chikazumi S. Physics of magnetism.John Wiley & Sons, Inc.; 1964. [15] Santa-aho S, Vippola M, Sorsa A, Latokartano J, Lindgren M, Leiviska¨ K, et al. Development of Barkhausen noise calibration blocks for reliable grinding burn detection. J Mater Process Technol 2012;212:408–16. [16] Jiles D. Introduction to magnetism and magnetic materials. Chapman & Hall; 1991. [17] Kai Y, Tsuchida Y, Enokizono M. Numerical analysis for non-destructive evaluation of hardening steel taking into account measured magnetic properties depending on depth. In: Takahashi S, Kikuchi H, editors. Electromagnetic nondestructive evaluation. Amsterdam: IOS Press; 2007. [18] Thomas J, Fix RM. Optimizing magnetization parameters of magnetic Barkhausen noise for more accurate detection of residual stresses and variations in microstructure. In: Proceedings of 7th international conference on Barkhausen noise and micromagnetic testing, July 15–16. Aachen, Germany; 2009. [19] Jiles DC, Atherton DL. Theory of ferromagnetic hysteresis. J Magn Magn Mater 1986;61:48–60. [20] Jiles DC. Recent advances and future directions in magnetic materials. Acta Mater 2003;51:5907–39. [21] Jiles DC. The effect of stress on magnetic Barkhausen activity in ferromagnetic steels. IEEE Trans Magn 1989;25(5):3455–7. [22] Cullity BD. Elements of X-ray diffraction. 2nd ed.Addison-Wesley Publishing Company Inc.; 1978. [23] SFS-EN 15305. Non-destructive testing—test method for residual stress analysis by X-ray diffraction; 2008. [24] SFS-EN 10328. Iron and steel. Determination of the conventional depth of hardening after surface heating; 2005. [25] SFS-EN ISO 2639. Steels. Determination and verification of the depth of carburized and hardened cases; 2002. [26] Hays WL. Statistics. 5th ed.Harcourt Brace College Publishers; 1994. [27] Stewart DM, Stevens KJ, Kaiser AB. Magnetic Barkhausen noise analysis of stress in steel. Curr Appl Phys 2004;4:308–11. [28] Stevens KJ. Stress dependence of ferromagnetic hysteresis loops for two grades of steel. NDT&E Int 2000;33(2):111–21. [29] Lindgren M, Lepisto¨ T. Effect of prestraining on Barkhausen noise vs. stress relation. NDT&E Int 2001;34(5):337–44.
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NDT&E International journal homepage: www.elsevier.com/locate/ndteint
Utilization of Barkhausen noise magnetizing sweeps for case-depth detection from hardened steel Suvi Santa-aho a,n, Minnamari Vippola a, Aki Sorsa b, Kauko Leiviska¨ b, Mari Lindgren c, Toivo Lepisto¨ a a
Tampere University of Technology, Department of Materials Science, Tampere, Finland University of Oulu, Control Engineering Laboratory, Finland c Outotec (Finland) Oy, Finland b
a r t i c l e i n f o
abstract
Article history: Received 13 January 2012 Received in revised form 15 May 2012 Accepted 17 May 2012 Available online 28 May 2012
One of the current topics of Barkhausen noise method development is its application to case-depth measurements of hardened components. Usually Barkhausen noise (BN) measurements for case-depth determination are based on the difference in the magnetic properties between the hardened case and the soft base material core. The measurement is done using low magnetizing frequencies. This enables deeper penetration of magnetic field to the ferromagnetic samples compared to the conventional high frequency BN measurements, typically used for grinding burn detection. However, due to the eddy current damping, the penetration depth is limited, depending on the material, to certain distance from the surface. To enable case-depth measurements, the Barkhausen noise measuring device was utilized to obtain data from magnetizing voltage sweeps (MVS). The sweeps were analysed and compared to the case-depths determined with conventional means. A series of hardened samples was investigated including induction and carburizing case-hardened samples. All samples were characterized with X-ray diffraction to study the residual stress state of the surface. Finally destructive characterization was used to verify the actual hardening depth of the studied samples. It was noticed that by studying the MVSs it was possible to evaluate the case-depth values more effectively than in earlier studies. The analysis used in this study utilized the slopes of the MVS sweeps. Furthermore, it was noticed that the ratio of the maximum MVS slopes at different frequencies indicating the case-depth values led to good results. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Barkhausen noise Case-depth measurements Induction hardening Carburizing case-hardening
1. Introduction The detection of hardening layer thickness with non-destructive methods has been studied widely in the recent years. Nondestructive methods enable the detection of layer thickness left for re-grinding and the verification of case-depths of hardening heat treatments. The utilization of Barkhausen noise (BN) [1–7], ultrasonic inspection [8,9] and other methods such as magnetoacoustic emission (MAE) have been studied earlier [10–12]. Usually the test specimens are either induction hardened or nitrided because of their sharp transition layer between the hardened case and the soft core. This layer is different when compared to the evenly changing layer produced in the carburizing case-hardening. The hardening effect of the surface in carburizing and nitriding is due to carbon or nitrogen diffusion to the steel surface from a carbonaceous or nitrous atmosphere. In carburizing, this is followed by a quench where the transformation of martensite occurs in the surface layer. This creates a hardness gradient and also
n
Corresponding author. E-mail address: suvi.santa-aho@tut.fi (S. Santa-aho).
0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2012.05.005
compressive residual stresses on the sample surface [13]. The determination of case-depth is one way to evaluate the composition gradients, microstructure gradients and also the quality of the hardening treatment. The need to utilize these measurements comes from the quality control in machine industry. The non-destructive detection of hardened layer thickness could be useful in the detection of the layer left for re-grinding and verification of case-depths of hardening heat treatments. In consequence, some commercial systems [8,9] that operate with ultrasound for case-depth detection have been introduced. The problem with ultrasonic waves is that the determination of thin hardened layers, smaller than 1.5 mm, is difficult. In addition, the ultrasonic inspection is applicable for hardened layers with a steep hardness gradient such as formed in induction hardening. Thus, there is a need for non-destructive method for case-depth determination also to the carburized components. So far, no system based only on BN to detect case-depths is available commercially. One of the current topics of BN method development is its application to case-depth measurements of hardened gears. BN measurement in case-depth determination is based on the difference in the magnetic properties between the hardened case and
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the soft base material core of the component. The hard martensite case with high coercivity and high remanence values requires stronger magnetic fields to be magnetized than the soft ferrite interior [14]. The BN based case-depth measurements are usually performed using low magnetizing frequencies. This enables deeper penetration of magnetic field to the ferromagnetic samples compared to the conventional high frequency BN measurements used in the grinding burn detection [15]. The BN signal measurement depth depends on the permeability and conductivity of the tested material and the magnetizing frequency used. The problem in casedepth studies is that the BN signal, created at certain depth below surface corresponding to the magnetizing frequency, is attenuated exponentially as a function of the distance travelled inside the material due to the eddy current damping which in turn depends on the frequency of the magnetized signal [16]. According to the present knowledge, case-depths can be accurately predicted down to 1 mm by BN measurements [5]. Usually the case-depth studies using the BN method rely on two-peaked behaviour of the BN signal envelope. This is due to soft core and hard martensite case that both generate an own peak [1–3,5]. The first peak at low applied magnetic field strength corresponds to the softer interior and the second peak corresponds to the harder case. Also the filtering of the BN signal to exclude the signal of the soft interior has been studied [1,2]. The ratio of these two peaks can be calculated and has been shown to correlate with the case depth [1,3,5]. Magnetic hysteresis properties have also been studied for the case-depth measurements [6,7,17]. Some parameters that can be calculated from the hysteresis measurements have also been noticed to change as a function of the case-depth value. Lo et al. [7] found that the coercivity and hysteresis loss increases with the increasing case-depths in induction hardened AISI 1045 steel samples up to 1.9 mm hardening layer thickness. On the other hand, the maximum permeability was noticed to decrease as the case-depth increased. This phenomenon can be explained by the fact that as the case-depth increases, more domain wall reversals are occurring in the martensitic case. Martensite is known to require quite high magnetic field to unpin the domain walls and this contributes to the increase in coercivity and hysteresis loss values. The study concluded that the case-depths can be estimated as weighted sum of signals from case and core. The samples that Lo et al. [7] used were induction hardened but there was not any notation about the residual stress state of the samples. Zhang et al. [6] studied the hysteresis loop parameters for case-depth determination and noticed that variations occurred in the hysteresis loop shapes for induction hardened samples. The differential permeability mr calculated from the hysteresis loop was noticed to change along with the case-depth value for induction hardened samples and to a much lesser extent for case-carburizing samples due to the smoother hardness slope. Also saturation magnetization value Ms, depending on the individual magnetic moments and their interactions with each other, was found to correlate with the case-depth values on both induction and carburized samples. The saturation magnetization varied in the hardened surface layer and in the soft bulk due to the differences in the magnetic properties. The change in the hardened layer thickness affects to the overall saturation magnetization and the case-depth can be determined by calculation of volume-weighted sum of saturation magnetization of the core and case [6]. Also this article lacks the information about the residual stress state of the sample material. The study of Kai et al. [17] for carburized ring cut out samples concluded that the measurement and numerical analysis of magnetic flux density B and magnetic field strength H can be used to estimate the case-depth of hardened samples. They
noticed that the coercivity, relative permeability and also the saturation magnetization were dependent on the case-depth. A need has emerged to use other BN related measurements along with the case-depth measurements to determine the surface and subsurface state of the sample. For the high frequency measurements, the magnetizing voltage sweep (MVS) and magnetizing frequency sweep (MFS) are measurements that provide the relationship between the voltage or the frequency and the magnetoelastic parameter (mp). These sweeps are used to find the optimal parameters for the high frequency BN measurements. The BN equipment changes the magnetizing voltage keeping the magnetizing frequency constant or vice versa. Thomas and Fix [18] studied the use of MVS and MFS for maximizing the sensitivity of the BN measurements. They claimed that the highest magnetizing amplitude, not too high to saturate the specimen, needs to be chosen because the saturation of the sample decreases the sensitivity of the measurements. They determined also sensitivity ratio maps where the highest sensitivity for particular BN measurement can be verified. This paper presents results on the utilization of Rollscan 300 equipment to case-depth analysis based on producing magnetizing voltage sweeps on hardened samples. These magnetizing voltage sweeps show the applied ascending magnetic field value while increasing magnetizing voltage. The slope of the MVS curve at different voltages is calculated and the maximum slope is determined. The MVS curve measured this way can thus be correlated to the hysteresis curve and the MVS slope value can be correlated to the permeability or susceptibility value calculated from the hysteresis loop. Permeability is the calculated inclination of the magnetic flux density B as a function of the applied magnetic field H. Susceptibility can be characterized as the inclination of the magnetization M as a function of the magnetic field H [14]. Both of these calculated inclination values characterize material properties which respond to the applied magnetic field in a specific way depending on the microstructural state. The permeability is stated to be influenced by the magnetic domain coupling and also by the domain density [19]. The pinning site density and energy influence the coercivity value determined from the hysteresis loop [19]. Initial permeability and coercivity is known to have a reciprocal relationship [20]. The reciprocal value of the differential susceptibility was also found to have a linear correlation to the stress value [21]. Similarly the calculated slope from magnetizing voltage sweep can give information about the sample with composition gradients and microstructure gradients related to the casedepth value.
2. Experimental procedure Hardened rod samples with varying hardened layer thicknesses were prepared by induction heating and carburizing case-hardening. A sample set of different induction hardened gear tooth samples was also studied. For induction hardening, two steel grades, 34CrNiMo6 and 42CrMo4, were used. The length of all rod shaped induction hardened samples was 300 mm and the diameter was 45 mm. The samples were induction hardened to varying case-depths from 0.65 mm to 2.2 mm with material 34CrNiMo6 and from 0.35 mm to 1.9 mm with material 42CrMo4. Some of the induction hardened samples, referred as turned, were turned and finally ground to smaller rod diameter of 39 mm in order to produce shallower case-depth. Table 1 shows the nominal chemical composition of all the studied steels. Five different sets of induction hardened samples were prepared. Sets 1 and 4 were manufactured at Takoma Gears (Finland) with onefrequency hardening machine. Set 2 was also manufactured by Takoma Gears with multi frequency hardening machine.
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Table 1 Nominal chemical composition of the studied steels. Wt%
C
Cr
Si
Mo
Mn
Ni
P
S
Ca
Cu
34CrNiMo6 42CrMo4 18CrNiMo7–6
0.36 0.41 0.21
1.36 1.07 1.57
0.28 0.31 0.20
0.17 0.17 0.27
0.71 0.73 0.65
1.31 – 1.45
0.024 0.017 0.016
0.029 0.03 0.029
– 0.0036 0.0044
– – 0.18
Table 2 Description of the sample sets.
Hardening method Material Hardening layer thickness variations [mm]
Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
Induction hardening 34CrNiMo6 Rht 0.65–2.2
Induction hardening 34CrNiMo6 Rht 0.825–1.85
Induction hardening 34CrNiMo6 Rht 0.65–1.45
Induction hardening 42CrMo4 Rht 0.35–1.9
Carburizing casehardening 18CrNiMo7-6 CHD 1.2–3.5
Induction hardening
The sample manufacturing procedure was similar in sets 1 and 4, only the hardening machine was different. For all these samples the induction coil was a ring shaped copper tube with 2 mm air gap between the rod and the induction coil. Rod samples were rotated simultaneously as the induction coil scanned throughout the sample height to ensure homogeneous hardening. The induction frequency was 10 kHz. The different hardening depths were achieved by varying the induction coil input power and the scanning speed values. The quench was done with water sprays. Sample set 3 was induction hardened by Bodycote V¨armebehandling AB (Malm¨o, Sweden) to hardened layer thicknesses (Rht) varying from 0.65 mm to 1.45 mm. Sample set 5 was manufactured from 18CrNiMo7-6 steel by carburizing case-hardening by Moventas Wind Oy (Finland) and Moventas Santasalo Oy (Finland). The length of rod samples was 300 mm and the diameter was 42 mm. The carburizing casehardening was performed to varying case-depths (CHD) from 1.2 mm to 3.5 mm. Table 2 presents the detailed description of the sample sets. In addition, a sample set with induction hardened gear tooth samples, referred to as set 6, was manufactured by Takoma Gears Oy (Finland). The set consisted of samples manufactured from 34CrNiMo6 steel with hardened layer thicknesses (Rht) 2.8 mm and 2.9 mm and samples manufactured from 42CrMo4 steel with hardened layer thicknesses 3 mm and 4 mm. All rod samples were turned prior the heat treatments. The visible oxide layer formed during the heat treatments was removed in a 2% citric acid bath from the samples before the measurements. The rod samples were studied first with Rollscan 300 BN measurement device using a specially designed sensor for rod shaped samples. The tooth samples were studied with gear teeth BN sensor having suitable geometry for curved gear surfaces. Also X-ray diffraction residual stress measurements were performed along with XStress3000 for all sample surfaces in sets 1–5. Finally a destructive verification of case-depths was done to these samples. 2.1. Barkhausen noise measurements with Rollscan 300 The samples were studied after the hardening with Rollscan 300 Barkhausen noise analyzer manufactured by Stresstech Oy (Finland) to detect the hardened surface condition. The BN measurements were recorded with MicroScan software, yielding the RMS value of the Barkhausen noise amplitude. The magnetizing frequency was 125 Hz and the magnetizing voltage was 7 Vpp (voltage from peak to peak). A special sensor for round shapes was used for rod samples. The experimental setup is shown in Fig. 1a. BN measurements were done in the circumferencial direction of the
34CrNiMo6, 42CrMo4 Rht 2.8, 2.9 (34CrNiMo6) Rht 3, 4 (42CrMo4)
rod. In addition, the magnetizing voltage sweeps (MVS) were studied with Rollscan 300 device. These sweeps were recorded with ViewScan software, which is shown in Fig. 1b. MVS was performed by increasing voltage from zero to maximum value of 16 Vpp as the frequencies: 20 Hz, 30 Hz, 60 Hz and 125 Hz, were kept constant. The recorded BN value is plotted as a function of the voltage. These sweeps are typically utilized for determining the optimum parameters for the actual BN measurements. Then the optimum measurement parameters are chosen so that the BN slope has the highest value. A couple of examples of the magnetizing voltage sweeps (MVS) made with Rollscan to sample sets are presented in Fig. 2. The curves present the response of magnetic parameter (mp) to increasing voltage value up to 16 Vpp while the frequency is kept constant. Fig. 2a shows the different magnetizing voltage behaviour with tensile and compressive residual stress values. Fig. 2b shows the change in the sweeps according to different X-ray diffraction peak full-width at half-maximum (FWHM) values. The microstresses occurring in the samples change the measured FWHM values. A correlation between the peak width and hardness can be established [22]. 2.2. Material characterization The surface residual stresses (RS) and FWHM (full-width at half-maximum) of the diffraction peak values were measured from the samples with X-ray diffractometer (XRD) XStress 3000 manufactured by Stresstech Oy (Finland). X-ray diffraction measurements were performed using CrKa radiation and the modified chi method with totally eight tilt angle degrees for both the longitudinal and circumferential direction of the rod samples [23]. Current 6.7 mA and voltage 30 kV was used in the measurements. After the non-destructive measurements, the rod samples were sectioned into smaller sections with Discotom cut-off machine and cross-sectional samples were prepared to determine the actual case-depth of the samples. The cross-sectional samples were then ground with silicon carbide grinding paper and polished with diamond suspension. The polished samples were etched with Nital 2% in order to examine the microstructure and to perform the hardness depth profiles with Duramin-300 hardness tester to verify the actual case-depths. The verification of the hardened layer thickness was done with hardness depth profile according to standards [24,25]. For the induction hardened samples the value of case-depth (Rht) is defined as the depth below the surface at which the Vicker’s hardness value drops to 400 HV [24]. For carburizing case-hardened samples the case-hardening depth
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Fig. 1. (a) Experimental arrangement of Rollscan 300 measurements for rod samples and (b) magnetizing voltage sweep recorded with Viewscan.
Fig. 2. (a) Voltage sweep made with Rollscan to two 34CrNiMo6 samples with different residual stress states and (b) voltage sweeps made to two different sample places with different XRD FWHM values; HH09_1 with FWHM 5 and HH09_2 FWHM 3.
800
3. Results and discussion 3.1. Material characterization of samples Table 3 presents the measured RMS values of the BN signal from different sample sets 1–5 measured with frequency 125 Hz and voltage 7 Vpp. Sample set 6 was measured with frequency 45 Hz and voltage 4.3 Vpp. The surface residual stress values to the same direction as the BN was measured and are also shown in Table 3. The RMS values for the induction hardened sample sets 1 and 4 had some variation, because the turned samples had increased RMS values, up to 100. The induction hardened sample sets 1–4 had average RMS values; 89, 38, 61 and 76, respectively. Set 5, which contained carburized samples had the average value of RMS 108. All induction hardened samples had compressive residual stress state on the surface. However, many carburizing case-hardened samples had tensile residual stresses on the surface. The tensile residual stress on the surface of some carburized samples may be due to some residual oxide layer on the sample surface or to a fluctuation of heat treatment quality for these samples. However, there was some minor variation in the hardness depth profiles for these carburized samples as shown in Fig. 3. The sample geometry for set 6 samples did not allow measuring the residual stress values for these samples. In addition, residual stress depth profiles were carried out to some of the induction hardened samples in set 1. The residual stress depth distributions were compressive residual stress on the
Hardness [HV1]
(CHD) is determined to be the distance where hardness drops to 550 HV (50 HRC) measured with HV0.1, HV1 or HV5 [25]. An example of hardness profiles for 18CrNiMo7–6 carburizing casehardened rod specimens of set 5 is shown in Fig. 3.
HH09
HH20
750
HH11
HH25
700
HH14
HH30 HH17
650 600 550 500 450 400 0
0.5
1 1.5 2 2.5 3 3.5 Distance from the surface [mm]
4
4.5
Fig. 3. Hardness depth profiles for set 5 of carburizing case-hardened samples.
surface layer, decreasing and changing into tensile residual stress in the depth equal to the hardened layer thickness. 3.2. Results of Barkhausen noise signal analysis A new approach for the BN case-depth studies was performed with measuring MVS from hardened samples. The calculated slope of the MVS curve changed as the magnetizing voltage was increased referring to increase in the value of the applied magnetic field. The slopes were calculated for each voltage sweep
S. Santa-aho et al. / NDT&E International 52 (2012) 95–102
99
Table 3 The measured RMS and X-ray diffraction residual stress values for different sample sets. Set 1
Set 2
Set 3
Sample
Rht [mm]
RMS [MP]
RS [MPa]
Sample
Rht [mm]
RMS [MP]
RS [MPa]
Sample
Rht [mm]
RMS [MP]
RS [MPa]
S52 S48 S51 CD37 CD36 CD39 S55 CD38 S57 S56
0.65 0.75 0.85 1.12 1.15 1.25 1.55 1.75 1.8 2.2
113 118 126 53 52 54 111 61 113 111
320 370 350 440 390 400 330 390 380 390
TG1 TG2 TG3 TG4 TG5
0.825 0.875 1.1 1.45 1.85
32 31 41 43 45
520 540 480 500 510
B8 B9 B7 B6 B5 B4 B3 B2 B1
0.65 0.7 0.75 0.9 0.95 1.05 1.15 1.3 1.45
49 50 57 53 68 66 62 76 69
200 200 230 240 230 260 250 240 300
89
376
38
510
61
239
Average Set 4
Set 5
Set 6
Sample
Rht [mm]
RMS [MP]
RS [MPa]
Sample
CHD [mm]
RMS [MP]
RS [MPa]
Sample
Rht [mm]
RMS [MP]
S20 IND5 IND10 S23 S25 IND9 IND8 S27 IND11
0.35 1 1 1.05 1.25 1.25 1.3 1.3 1.9
98 35 60 105 101 63 56 104 62
110 380 430 140 260 400 450 320 400
HH09 HH11 HH14 HH17 HH20 HH25 HH30
1.2 1.5 1.8 2 2.5 2.9 3.5
55 145 132 124 155 67 78
90 þ 310 þ 190 þ 130 þ 100 190 130
#504 #509 #478 #522
2.8 2.9 3 4
54 79 56 70
76
321
Average
Fig. 4. The slope calculated from MVS as a function of voltage.
by fitting a first order polynomial to 25 consecutive data points. An example of the MVS slope changes in one sweep is shown in Fig. 4. The maximum values of the MVS slopes were recorded and used in later analyses. The MVS curve created this way can thus be correlated to the hysteresis curve and the slope value calculated from the MVS curve to the permeability or susceptibility value calculated from the hysteresis loop. Both of these values, permeability and susceptibility, characterize the material magnetic behaviour when the ferromagnetic specimen is exposed to an applied magnetic field. According to the literature, the reciprocal initial susceptibility has a linear behaviour with respect to the applied tensile stress [21]. The permeability calculated from the slope is found to be influenced by the magnetic domain coupling and also to the domain density [19]. The magnetizing frequency value determines partly the depth where the measurement information comes from [12]. Thus the MVS performed with different frequencies can give information from different depths. The use
108
65
of two different magnetizing frequencies in the MVS gives information about the compositional gradient layer influenced by the microstructural features and also about residual stresses. The MVS behaviour showed a systematic change with respect to surface hardness values. This was shown in Fig. 2b. Also the surface residual stresses change the MVS appearance as shown in Fig. 2a when all other characteristics of the samples are similar. Thus, it is reasonable to assume that the calculated slope from the MVS curve can characterize the material properties. Some papers [6,7] indicate that the maximum permeability decreases as the case-depth increases. Only the carburizing case-hardened samples in our studies, having tensile residual stresses on the surface, showed this behaviour. However, the studies [6,7,17] did not give the information about the residual stress for their samples and the studied sample sets were also quite small. Fig. 5 presents the relationship between the MVS slopes obtained from the 60 Hz MVS and case-depths. Correlations were linear within all sample sets. However, the residual stress state affected the direction of the correlation. With compressive residual stresses the correlation was positive and with tensile residual stresses the correlation was negative. This behaviour can be seen clearly in set 5 including carburizing case-hardened samples with either tension or compression residual stress on the surface. The MVS slope values for different surface residual stress values were noticed to change. The magnetic hysteresis behaviour is known to vary because the magnetostriction coefficients change according to the stresses from magnetic field exposure of ferromagnetic material. The tensile stress causes the magnetostrictive coefficient to increase more slowly in the magnetization [27,28]. Thus the different calculated slope values for samples with compressive or tensile stresses were noticed to change. However, no conclusions can be drawn from the MVS slope values at one magnetizing frequency value comparing to case-depths. Thus, the information in the MVS slope was refined to take into account the effect of residual stresses and to get more detailed information about the case-depths. The slopes at different frequencies were subtracted
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and divided with each other in the analysis. Also the reciprocal of the maximum slopes was calculated. To analyse the sweeps, correlation coefficients between the case-depths and the refined slope information were calculated. Correlation is a statistical measure describing the relationship of two variables. Correlation coefficient varies between 1 and 1 so that 1 refers to a perfect correlation and 1 to a perfect negative correlation [26]. Also the reciprocal of the maximum slopes was calculated. Table 4 shows these correlation coefficients between the sample sets and the case-depths. The samples were divided into two groups according to the residual stress being tension or compression. Maximum correlation for samples with compressive residual stress on the surface, 0.86, was measured with division of MVS slopes for frequencies 20 Hz and 125 Hz. This frequency range was the widest frequency range studied. Slope division also produced higher correlations in all studied frequency ranges than subtracted sweeps. Higher correlations were observed if they were calculated only for one set at the time. For example to division 20 Hz/125 Hz for set 3, the correlation value was 0.98. The differences between the sample sets may be due to the different manufacturing equipment and process parameters. Within each sample set, the manufacturing process was similar. The maximum correlation for carburizing case-hardened samples with tensile residual stress on the surface was noticed to be
60
Set 1 Set 2 Set 3 Set 4 Set 5, + RS Set 5, -RS Set 6
Slope (60 Hz)
50 40 30 20 10 0
0
0.5
1
1.5 2 2.5 3 Hardening depth [mm]
3.5
4
4.5
Fig. 5. The hardening depth for different sample series as a function of the calculated slope for 60 Hz voltage sweeps. RS denotes compressive residual stresses and þ RS tensile residual stresses.
higher, 0.97, than to the induction and carburized samples having compressive residual stresses on the surface, 0.86. This was calculated for carburized samples with division of sweep slopes for frequencies 30 Hz and 20 Hz. The induction hardened teeth series had maximum correlation, 0.81, in slope division between 20 Hz and 125 Hz and also the same correlation in subtraction of 30 Hz slope and 125 Hz slope. The decreased correlation may be due to the differences in the samples, the materials and sample sizes, referring to tooth module, within set 6 varied. The differences in the best frequency ranges for induction and tensile carburizing samples indicates that the change in the magnetic properties occurs more gradually over a long distance in the induction hardened samples and thus needs larger frequency range to be taken into account. Zhang et al. [6] noticed also different behaviour of saturation magnetization as a function of case-depth for induction and carburizing hardened samples. They noticed that carburized samples produced larger changes in the ratio between the saturation magnetization and the case-depth values. An example of the dependency between the reciprocal of MVS subtraction and the case-depths is given in Fig. 6. The calculated values give linear relationship in shallower hardening depths up to 2.5 mm. They also distinguish the samples according to their surface residual stress values being tension or compression into two separate groups that both have linear relationship as a function of hardened layer thickness up to 2.5 mm. The best calculated correlation for division of two MVS slopes is given in Fig. 7 for magnetizing frequencies 20 Hz and 125 Hz. The slope ratio is correlated to the hardening depth in all sample sets, including both the tensile and compressive residual stress carburizing case-hardened samples. The calculated inclinations between slope ratio of 20 Hz and 125 Hz and case-depths for different sets are presented in Table 5. For sample sets 3 and 5 with lower compressive residual stress values the inclinations are larger. However, for the carburizing case-hardened sample set with tensile residual stress on the surface the inclination is smaller than for the carburized samples with compressive residual stress. Turned samples were separated from sets 1 and 4 and produced a group with lowest inclination. The sample sets with residual stress values from 300 MPa to 400 MPa had inclinations between these two groups. The inclination seems to be affected highly by the residual stress of the surface (Table 5). Table 5 shows that when the inclinations are compared to the surface residual stresses, generally decrease is observed with increasing compressive residual stress. However, set 3 deviates from this observation. The different manufacturing equipment may explain this divergent behaviour for set 3.
Table 4 Calculated correlations for different frequency sweeps for induction and carburizing case-hardened samples with compression residual stress on the surface and calculated correlations for case-hardened sample with tensile residual stress. Highest absolute value of correlations shown in bold.
20 Hz, 30 Hz 20 Hz, 60 Hz 20 Hz, 125 Hz 30 Hz, 20 Hz 30 Hz, 60 Hz 30 Hz, 125 Hz 60 Hz, 20 Hz 60 Hz, 30 Hz 60 Hz, 125 Hz 125 Hz, 20 Hz 125 Hz, 30 Hz 125 Hz, 60 Hz
Maximum slope for sweep subtraction Correlation 1 ( RS)
Maximum slope for sweep division Correlation 2 ( RS)
Maximum slope for 1/ Sweep slope sweep subtraction subtraction Correlation 3 ( RS) Correlation 1 (þRS)
Maximum slope for sweep division Correlation 2 (þRS)
1/ Sweep slope subtraction Correlation 3 (þRS)
0.10 0.28 0.50 0.10 0.38 0.56 0.28 0.38 0.54 0.50 0.56 0.54
0.66 0.82 0.86 0.59 0.79 0.79 0.69 0.71 0.66 0.76 0.69 0.60
0.07 0.51 0.74 0.07 0.70 0.18 0.51 0.70 0.10 0.74 0.18 0.10
0.96 0.92 0.88 0.97 0.86 0.84 0.94 0.87 0.66 0.86 0.81 0.62
0.87 0.83 0.80 0.87 0.80 0.33 0.83 0.80 0.57 0.80 0.33 0.57
0.92 0.87 0.86 0.92 0.84 0.82 0.87 0.84 0.53 0.86 0.82 0.53
S. Santa-aho et al. / NDT&E International 52 (2012) 95–102
Sample set 6 was left out from the examination in Fig. 7 because the RS data was not available. In general, the deviation in all the results may be due to the different manufacturing equipments because all of the samples were not manufactured with same induction hardening machine. Also the changes in the process parameters can have influence on the samples. The hardened layer thickness value range was the narrowest for sample set 3. There were totally 9 samples within a hardened layer thickness range of 0.8 mm. Thus, in Table 5 the calculated inclination for this sample set can be misleading because of the smaller range of hardened layer thicknesses compared to other sample sets. Fig. 8 presents the regression line together with the calculated upper and lower confidence limits for the same data set as shown in
1/slope (125 Hz)-slope (20 Hz)
0.7
Set 5, + RS Set 2,3,5, -RS
0.6
0.4 0.3 0.2 0.1
0
0.5
1
1.5 2 2.5 Hardening depth [mm]
3
3.5
4
Fig. 6. The hardening depth for different sample series as a function of the calculated slope for reciprocal of subtraction of 20 Hz and 125 Hz slopes. RS denotes compressive residual stresses and þRS tensile residual stresses.
1
Slope 20 Hz / slope 125 Hz
0.9 0.8 0.7 0.6 0.5 0.4
Set 1 Set 2 Set 3 Set 4 Set 5, + RS Set 5, -RS
0.3 0.2 0.1 0
Fig. 7. The confidence interval can be used to indicate the reliability of the regression line. A linear regression analysis was used for the MVS slope division data by utilizing the least squares method calculated for the data sets 1–5. The confidence level of 95% was used. Thus, the confidence limits show the range where the estimated values are located with a probability of 95%. We can estimate that with certain measured slope division values the hardened layer thickness variations are within a range of certain limits with 95% probability. For example with MVS slope division value of 0.3 the hardened thickness layer is within a range of 0.65–0.95 mm. The calculated inclinations from the MVS slope division for different sample sets seemed to be affected highly by the residual stress of the surface. The higher the compressive stress the smaller the changes are in the MVS slope ratio. This is probably due to the decreased Barkhausen noise activity with higher compressive stresses [29].
4. Conclusions
0.5
0
101
0
0.5
1
1.5 2 2.5 Hardening depth [mm]
3
3.5
4
Fig. 7. The division of 20 Hz MVS slope with 125 Hz MVS slope for sample sets 1–5. RS denotes compressive residual stresses and þ RS tensile residual stresses.
One important research area of the Barkhausen noise development is its application to case-depth measurements of hardened components. Since the ultrasound based measurement is not applicable to the carburized samples, the applicability of some other measurement technique such as magnetic Barkhausen noise measurement is worthwhile to be studied. The different hardening processes and process parameters create differences in hardened layer thickness, but produce also a compositional and microstructural gradient with different magnetic properties affected by residual stresses, too. The surface hardness variations can be noticed as altered RMS values of the samples. In this study, the residual stress state of the induction hardened samples was quite uniform so the RMS values are mostly influenced by the hardness. However, some of the studied carburizing case-hardened samples had tensile residual stresses on the surface that was produced in the hardening process or due to some residual oxide layer left on the sample surface. The previous BN studies concerning the case-depth measurements have concluded that the case-depth can be reliably estimated down to 1 mm. Here it was noticed, that by studying the magnetizing voltage sweeps it is possible to evaluate the compositional gradients referring to the case-depth values even deeper. It was also concluded that any MVS slope with certain frequency does not explain the changes in the case-depth values on their own. Thus, the ratio of the maximum MVS slopes at different frequencies needed to be calculated. When comparing the ratios within one sample set, the correlation was close to 1 indicating a linear behaviour. Even taking into account many different sample sets the MVS slope ratios led to high correlations, 0.86, when comparing these ratios to the case-depth values. Also the estimation of the hardened layer thickness variations with certain measured slope division values could be performed with calculating the regression line together with the confidence limits. The current needs for the non-destructive case-depth determination can consist of layer thickness estimation e.g. for regrinding of hardened component. The studied sweep method can be applicable to carburized samples also in such case that a calibration series of that particular material is manufactured.
Table 5 Calculated inclinations from Fig. 7 for MVS slope 20 Hz/125 Hz with different sample sets.
Inclination 20 Hz/125 Hz Residual stress average [MPa]
Set 1, ground
Set 1, turned
Set 2
Set 3
Set 4, ground
Set 4, turned
Set 5, RS
Set 5, þ Rs
0.15 410
0.06 360
0.2 510
0.38 240
0.13 410
0.05 210
0.21 140
0.1 þ180
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1 0.9
Slope 20 Hz / slope 125 Hz
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Set 1 0
Regression line
0.25 0.5 0.75
1
Lower 95 % conf. limit
Upper 95 % conf. limit
1.25 1.5 1.75 2 2.25 2.5 2.75 Hardening depth [mm]
3
3.25 3.5 3.75
Fig. 8. The 95% confidence limits calculated for division of 20 Hz and 125 Hz MVS slope for sample sets.
With such samples, a regression line with confidence limits can be performed to be used in the non-destructive testing of hardened samples. For now, the overall regression line gives information about the limits between what the layer thickness lies. The challenge of the utilization of this method is still to increase the accuracy and confidence of the method. For this reason more actual hardened components need to be tested verifying the applicability of the method.
Acknowledgement Support from TEKES for the NOVEBARK Research Project, Foundation of Walter Ahlstrom and Graduate School of TUT is gratefully acknowledged. The authors would also like to thank personnel of Stresstech group, Moventas Wind and Santasalo (Finland), Takoma ¨ Sweden), Katsa Oy (Finland) for Gears (Finland), Bodycote (Malmo, their technical support. Research assistants at the Laboratory of Materials Characterization (Tampere University of Technology) are thanked for sample preparation. References [1] Dubois M, Fiset M. Evaluation of case depth on steels by Barkhausen noise measurements. Mater Sci Technol 1995;11(3):264–7. [2] Bach G, Goebbels K, Theiner WA. Characterization of hardening depth by Barkhausen noise measurements. Mater Eval 1988;46:1576–80. [3] Vaidyanathan A, Moorthy V, Jayakumar T, Raj B. Evaluation of induction hardened case depth through microstructural characterization using magnetic Barkhausen emission technique. Mater Sci Technol 2000;16:202–8. [4] Moorthy V, Shaw BA, Hopkins P. Surface and subsurface stress evolution in case-carburised steel using high and low frequency magnetic Barkhausen emission measurements. J Magn Magn Mater 2006;299(2):363–75. [5] Moorthy V, Shaw BA, Brimble K. Testing of case depth in case carburized steels using magnetic Barkhausen emission technique. Mater Eval 2004;62(5):523–7. [6] Zhang C, Bowler N, Lo C. Magnetic characterization of surface-hardened steel. J Magn Magn Mater 2009;321:3878–87. [7] Lo CCH, Kinser ER, Melikhov Y, Jiles DC. Magnetic non-destructive characterization of case depth in surface-hardened steel components. In: Thompson DO, Chimenti DE, editors. Review of progress in quantitative non-destructive evaluation 25B. AIP Conference Proceedings 2006;820:1253–60. [8] Internet: /http://www.qnetworld.com/case_testing/p3121.htmlS, 12.12.2011.
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