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Improvement of the Barkhausen noise method for stress evaluation
96
Journal
IMPROVEMENT
OF THE BARKHAUSEN
R. RAUTIOAHO,
P. KARJALAINEN
of Magnetism
NOISE METHOD
and Magnetic
Materials 73 (1988) 96-102 North-Holland, Amsterdam
FOR STRESS EVALUATION
Materials Engineering Laboraton,
and M. MOILANEN Electronic Measurements Laboratory, University of Oh,
90570 Oh,
Received
1988
2 November
1987; in revised form 12 January
Finland
Envelopes of Barkhausen noise bursts are measured for ferritic-pearlitic, tempered microalloyed, 3.5Ni and 9Ni steels in an unloaded state and under tensile stresses, and the area Ar, peak amplitude Vm and restricted area RAr of these envelopes are determined. RAr is evaluated from the portion of the signature which has simultaneously a high and unique dependence on stress. In the case of ferritic-pearlitic steels, tensile stress increases Barkhausen activity at the leading edge of the envelope which thus constitutes a convenient time range for determining RAr. Ar and Vm values suffer from a non-unique stress dependence in the tempered steel, i.e. the existence of a local maximum, but the stress response of RAr remains unique, with a rather low spatial scatter, at least up to 300 MPa. A directional contribution of the texture to noise values impairs the possibility for evaluating stress magnitudes in the 3.5Ni steel, while the 9Ni steel has a high stress response and pronounced spatial variation in noise. The RAr figures represent an improvement over Vm and Ar values as stress measurements, especially for tempered steels and to a lesser extent ferritic-pearlitic steels, whereas all the parameters mentioned have similar stress responses for Ni steels.
1. Introduction In a previous paper, Rautioaho et al. [l] divided the ferromagnetic structural steels into (1) ferritic-pearlitic, (2) tempered and (3) nickel steels based on the stress response of Barkhausen noise (BN). The ferritic-pearlitic steels exhibit a moderate stress response at low tensile stresses, but this gradually diminishes with increasing tension until Barkhausen noise finally saturates to a level characteristic of the material concerned. The tempered steels have a similar behaviour at low tensile stresses, i.e. BN increases with increasing tension up to a certain maximum, any further increase leading to a gradual decrease in BN. Existence of the maximum was considered in a preceding paper, in which it was associated with a simultaneous increase in coercive force (H,) and stress gradients [2]. A typical feature of the materials in the third 0304-8853/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
group is a low noise level in an unstressed condition and a very high response to tensile stresses. So far this behaviour has been reported to occur in 9Ni steel [3] and has also been observed by the authors in 5.8Ni steel. It can obviously be found in any steel which contains at least 3 ~01% nonmagnetic inclusions distributed at packet and lath boundaries of martensite or bainite. An obvious requirement for the use of the BN method for evaluating residual stresses is a unique dependence between noise level and stress which is strong enough to overcome the generally quite broad spatial scatter in noise values. This is not encountered in materials belonging to the second group in particular. The problem is exaggerated in measurements carried out by recording the rms value or total integrated intensity of noise voltage pulses, as is customarily done. These readings do not utilize all the information contained in BN. B.V.
R. Rautioaho et al. / Barkhawen
The information of a magnetization reversal is included in the envelope of a burst of Barkhausen pulses, also termed here the Barkhausen signature. This envelope depicts the number distribution of pulses along a hysteresis branch. An approach for the utilization of this information has been employed by Barton and Kusenberger [4], Matzkanin and Gardner [5] and Gardner et al. [6], who consider the peak amplitude of the Barkhausen signature to provide a good indication of stress. A measurement of a peak signal voltage nevertheless still omits some information carried by the envelope. The present work is therefore directed towards a more detailed study of the envelope in order to determine whether a restricted range of the signature would offer a more useful dependence of BN on stress and would have a reasonably low spatial variation in the recorded signal. A simplification of this idea has been reported earlier by Tiitto [7] who nevertheless did not pay any special attention to the interrelationship between BN and stress. In the present case the peak amplitude and total area under the signature are also measured and compared with the area of the limited range. The main interest is focused on tempered steels, but an improved ability to evaluate residual stresses is also sought for ferritic-pearlitic and nickel steels as well.
2. Experimental The basic instrumental arrangement was similar to that used in the preceding papers [l-3]. The magnetizing current fed to the transducer was of a triangular waveform with a frequency of 10 Hz and peak value with 0.5 to 1.0 A. BN was detected by an inductive sensor element, amplified to 40, 50 or 60 dB, rectified and recorded as a function of time in a frequency range from 2 to 100 kHz by a signal analyzer, Data 6000, which displays the envelope of the rectified bursts of Barkhausen pulses and allows free selection of the portion to be analyzed. The envelope was recorded by averaging over 128 to 512 bursts. The total and selected areas of the envelope and its peak amplitude were read, and the Barkhausen signature was plotted by a X-Y recorder, the time origin on the X axis
97
noise method for stress evaluation
coinciding with the minimum value of the external magnetizing field. Two transducers of a commercial measuring unit, Stresscan 100, were used to magnetize the specimens and sense BN pulses. They yielded closely similar Barkhausen signatures, the shape of the first being better suited for stress measurements and the second for evaluations of texture. The samples used for the measurements were 10 mm thick rectangular-shaped specimens which could be strained elastically up to a stress level of 300 MPa by means of a bending device. The materials were: - low-carbon steel in an as-received (hot rolled) state (sample A). _ NbV microalloyed C-Mn steel in an as-received (control rolled) state (sample B). _ a vanadium microalloyed C-Mn steel, tempered for 48 h at 600 o C, l/2 h at 650 o C, or in an as-received (tempered at around 600° C) state (samples Cl, C2, C3). _ a 3.5Ni steel in an as-received condition (quenched from 840” C, tempered at 640 o C) followed by tempering for 1 h at 640 o C (sample D). - a 9Ni steel with a microstructure developed by heat treatment: 1050 o C 3 h/water quench followed by 800 o C 1 h/water quench and tempering for 1 h at 590 o C (sample E). Table 1 presents the chemical compositions of the materials. Samples A and B are ferritic-pearlitic steels, C a tempered steel and D and E Ni steels. E contains 3 ~01% austenite [3] and falls into the material category (3), but the position of D is unclear, a question to be discussed later.
Table 1 Percentage
composition
of the specimens
Sample
C
Mn
Ni
V
Nb
A B Cl, c2, c3 D E
0.08 0.13 0.15 0.09 0.03
0.40 1.27 1.28 0.55 0.67
0.05 0.05 3.5 8.97
0.07 0.07
0.03 0.04
98
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
3. Results 3.1. Stress response
0.6
Envelopes arising from different locations on the sample surfaces and at certain levels of tensile stress were recorded for the materials in order to find their stress sensitive portion and to measure the total area Ar, the restricted, stress sensitive, area RAr and the peak value Vm of these signatures. Some illustrative examples of the curves are presented in figs. 1 to 4, which also show the analysis ranges for the determination of RAr. The ferritic-pearlitic materials A and B have a low spatial variation in noise voltage, typical curves being plotted in fig. la. The increase in tensile stress shifts Barkhausen activity to the leading edge of the signature (fig. lb). A stress sensitive range consists thus of the leading edge of the signature, the spatial variation in which is not essentially larger than that of the trailing edge.
0.4 VOLT) 0.2 0.0
I
1
-o-2O.O15
a
0
0.025
I
I
0.030
0.035
_l
RAr
I 0.015
c
I
0.020
0.020
t. 0.025 0.030 SECONDS
I 0.035
Fig. 2. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100, 200 and 300 MPa (b) for the C-Mn steel tempered for 0.5 h at 650 o C.
2--
“CLTsl
O-
I
1
0.01
0.02
0.01
0.02
0.03
1
0.04
0.03 0.04 SECONDS Fig. 1. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100, 200 and 300 MPa (b) for the ferritic-pearlitic low carbon steel.
The analysis range from 10 to 22 ms was selected for steels A and B for determining RAr. Typical signatures for the tempered steel C2 are depicted in fig. 2. Fig. 2a shows that there the leading edge experiences much more pronounced spatial scatter than the trailing edge. The tensile stress of 100 MPa increases Barkhausen activity especially at the beginning of the curve, whereas higher stresses shift the signature to the right on the time axis. Simultaneously the total area Ar and peak value Vm decrease. A similar stress induced shift is also observed in samples Cl and C3. Accordingly, a convenient analysis range for C2 is from 28 to 40 ms, for instance (fig. 2b), while ranges from 20 to 32 ms and from 26 to 41 ms were selected for Cl and C3 respectively. These time intervals amount to 20 to 30% of the rising time of the driving field. The 3SNi steel exhibits the highest stress response at around the peak position of the signatures, while the spatial variation in Barkhausen
99
R. Rautioaho et al. / Barkhauren noise method for stress evaluation
t
a
0.02
0.03
O.Oh
b
‘0 2
I
I
0.01
.
curves is observed in C2 and a faint maximum in the Ar vs. stress curves in Cl and C3, whereas neither the existence of any maximum nor any saturation of stress response can be seen in the RAr vs. stress curves, the figures at a stress level of 300 MPa being about 50% higher than those for the unloaded samples. The Vm vs. stress curves approach saturation at around 300 MPa in the latter specimens. Vm also has a higher stress response than Ar in Cl, C2 and C3. In the Ni steels (fig. 7) Vm has an almost equal stress response as Ar, and RAr offers no essential improvement. Consequently, the tensile stress of 300 MPa increases any one of these parameters by about 60% compared with the figures in an unloaded condition in the case of the 3SNi steel and 800% in the case of the 9Ni steel. 3.2. Spatial variation and texture
RAr 0.01
I 0.02 SECONDS
I 0.03
I 0.04
The studied
spatial variation in the parameters was more closely for samples Cl, C2, C3 and
Fig. 3. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100, 200 and 300 MPa (b) for the 3.5Ni steel.
activity is also somewhat increased in this region (fig. 3). Nevertheless, the range from 13 to 21 ms was considered reasonable in this material. On the other hand, the 9Ni steel shows a pronounced spatial variation in the signature everywhere along the time axis. Moreover, tensile stress only slightly concentrates Barkhausen activity in the trailing edge (fig. 4). Consequently, selection of any special analysis range seems meaningless, although a range from 27 to 46 ms was adopted here. The stress response curves for Ar, RAr and Vm are presented in figs. 5 to 7. For the ferritic-pearlitic steels A and B all parameters follow the same trend with increasing tension, the stress response extending up to a stress level of 200 MPa. RAr attains an about 50% (A) or 150% (B) higher stress response than Ar, the maximum RAr values exceeding those measured in an unstressed state by 45% (A) and 160% (B), and Vm closely following RAr (A) or Ar (B). Among the tempered steels, the existence of a clear maximum in the Ar or Vm vs. tensile stress
0.6 VOLTS 0.2
-0.2/l 0.02
0.03
0.04
0.05
0.04
0.05
2
1 "OLTm
0 RAr -1 0.02
0.03
I
,
SECONDS
Fig. 4. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100,200 and 300 MPa (b) for the 9Ni steel.
100
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
a
a
_._ ,m’
-
/
l-O
/j
150
100
0
. I b
/
0 Tensile
o-
-
I
I
I
100
200
300
stress
s.1
RAr
-0
RAr
o/y’o
200
3
-m Vm
-
l
Ar
I b
Vm
’
‘\
(MPa)
Fig. 5. Stress response curves for the ferritic-pearlitic
steels.
D. Because the stress response curves of Ar and RAr are quite similar in steel D, only Vm and Ar were recorded for this sample. Since we used only one sample in each material condition and tested an area of about 50 cm2, we may not be getting much more than an intimation of the true spatial scatter in the parameters concerned, but the results, presented in table 2, can still be utilized for a rough comparison between the parameters. Table 2 indicates that the relative standard deviation, 100 * standard deviation/parameter, is generally largest for the quantity Ar and smallest for RAr, the latter figures being 50 to 70% of the former. Except for sample C3, the relative standard deviation of Vm is smaller than that of Ar. The figures in table 2 are computed of the basis of 10 to 20 measurements. The directional anisotropy in noise parameters, i.e. the magnetic (and mechanical) texture, was found to be pronounced only in sample D. The figures denoted by (L) in table 2 describe the spatial scatter in Ar and Vm and those denoted by (L + T) the combined contribution of spatial vari-
”
0
100 Tensile
stress
200 300 IMPa)
Fig. 6. Stress response curves for the tempered steels.
Table 2 Values for the noise parameters Ar, Vm and RAr together with their relative standard deviations for the tempered and 3.5Ni steels Sample
Ar
lOOu,/ Ar
27.0 30.0 22.2 12.7 14.3
looO”/ Vm
10.4 16.0 26.6 9.4 16.8
1473 2203 1220 705 761
RAr
Lao”/ RAr
(mVs)
(mV)
(mVs) Cl c2 c3 D(L) D(L+T)
Vm
8.1 9.3 28.4 7.3 12.2
12.1 12.0 12.0
6.6 8.3 20.0
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
0’
0’
/
about double that in row (L). The effect of texture on the Barkhausen signature can be observed in fig. 8, where it is seen clearly to impair the capability for assessing the magnitude of the residual stresses in material D (compare figs. 3b and 8).
RAr
--o
101
4. Discussion a 100
200
300
0 Tensile
stress INPa)
Fig. 7. Stress response curves for the nickel steels
ation and texture. L and T refer here to measurements carried out with magnetization directed longitudinally (L) or transversely (T) to the long dimension of the rectangular-shaped specimens. The relative standard deviation in row (L + T) is
t
1
0.01
I
0.02
0.03
0.04
SECONDS
Fig. 8. Barkhausen signatures for the 3.5Ni steel illustrating the contribution of texture, with magnetization directed parallel to the long dimension of the rectangular-shaped specimen (L) or transverse to it (T).
The leading edges of the Barkhausen signatures are exaggerated by increasing tensile stress in the case of the ferritic-pearlitic steels A and B. This phenomenon is caused by the stress alignment effect [2], which in this case extends to a stress level of 200 MPa. A convenient analysis range thus consists of the leading edge of the signature induced by a low driving field. The same increase in stress response can be achieved, however, by using such a low magnetizing current (and field) that the noise pulses corresponding to the leading portion are the only ones generated in the material. A sophisticated signal analyzing system hence gives no essential advantage over customary measuring devices in the case of ferritic-pearlitic steels. Tensile stresses above 200 MPa cannot be evaluated in terms of magnitude in steels like A and B. Due to the unique behaviour of the stress response curves, the figures for RAr represent an indisputable improvement over the Ar values in the tempered steels of type C after selection of the proper time range. The increase in the leading edges of the signatures at a stress level of 100 MPa arises from the stress alignment effect, while the shift to the right on the time axis with increasing tension above 100 MPa is connected with the increase in coercive force and stress gradients, as suggested in the preceding paper [2]. This behaviour is most pronounced in sample C2, and weakens in C3 and Cl, respectively. The 3.5Ni steel behaves in a similar fashion to the above tempered steels. This is not so surprising, because it is in fact a tempered steel which only contains a tiny amount of retained austenite, as revealed by X-ray diffraction. Nevertheless, the parameters do not exhibit a maximum in their noise vs. stress curves, and the stress response curves for Ar, RAr and Vm resemble one another.
102
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
The contribution of texture has its origin in the microstructure, a subject not considered here. The high stress response in the 9Ni steel is caused by the retained austenite present in this steel, as considered in the preceding paper [3]. The spatial scatter in BN values is pronounced in an unstressed state (fig. 7a), but at least relative noise variations seem to diminish with increasing tension. Due to a strong interaction between nonmagnetic austenite paricles and the movement of Bloch walls [3], one obvious reason for this scatter is a non-uniform distribution of these particles. The measurement of RAr represents an improvement on customarily employed methods, especially in tempered steels where the stress sensitivity becomes unique and the spatial scatter diminishes. It is also as good as either Ar or Vm for the other types of material. Compared with Ar (or the rms value), Vm has a higher or similar stress response and a lower spatial variation, at least in the tempered steels. It should hence be preferred to Ar. It can be recommended, however, that all the parameters should be measured and their combined information utilized for evaluating residual stresses by the BN method in order to obtain the most reliable estimates for stress magnitudes.
5. smlunaly The following observations were made: 1. In the case of ferritic-pearlitic steels the stress sensitivity of the BN method can be improved either by restricting the time range of the recorded pulses to the leading edge of the noise voltage vs. time curve or by using such a small magnetizing field (current) that only a fraction of the total BN activity is generated in the sample. The magnitude of stresses extending at least up to a half of the
yield strength can be evaluated by these procedures. 2. In tempered steels a unique stress response, and simultaneously a reduced spatial variation in the measured signal, is obtained by recording pulses from a time interval amounting to 20 to 30% of the rising time of the field, with the leading boundary approximately, conciding with the peak amplitude. 3. The stress response is not essentially improved or the spatial variation reduced by restricting the analysis range for the Ni steels. A directional contribution to noise values impairs the capability for determining the magnitude of residual stresses in the case of the 3.5Ni steel. The 9Ni steel has a high response and pronounced spatial scatter in noise due to the retained austenite present in this steel.
Acknowledgement Dr. T. Ogawa of the Nippon Steel Corporation, Japan, is thanked for supplying the 3SNi steel.
References 111R. Rautioaho, P. Karjalainen and M. Moilanen, J. Magn. Magn. Mat. 61 (1986) 183.
PI R. Rautioaho, P. Kaxjalainen and M. Moilanen, J. Magn.
Magn. Mat. 68 (1987) 314. 131 R. Rautiosho, P. Karjalainen and H. Moilanen, J. Magn. Magn. Mat. 68 (1987) 221. t41 C.G. Gardner, G.A. Mazkanin and D.L. Davidson, Intern. J. Nondest. Test. 3 (1971) 131. 151 J.R. Barton and F.N. Kusenberger, Paper 74GT-51, ASME Gas Turbine Conf., Zurich, Switzerland (1974). Proc. of the [61 G.A. Matzkanin and C.G. Gardner, ARPA/AFML Review of Quantitative NDE, AFML-TR75-212 (1976) 791. 171 S. Tiitto, Acta Polytechn. Scandinavica 119 (1977) p. 18.
Journal
IMPROVEMENT
OF THE BARKHAUSEN
R. RAUTIOAHO,
P. KARJALAINEN
of Magnetism
NOISE METHOD
and Magnetic
Materials 73 (1988) 96-102 North-Holland, Amsterdam
FOR STRESS EVALUATION
Materials Engineering Laboraton,
and M. MOILANEN Electronic Measurements Laboratory, University of Oh,
90570 Oh,
Received
1988
2 November
1987; in revised form 12 January
Finland
Envelopes of Barkhausen noise bursts are measured for ferritic-pearlitic, tempered microalloyed, 3.5Ni and 9Ni steels in an unloaded state and under tensile stresses, and the area Ar, peak amplitude Vm and restricted area RAr of these envelopes are determined. RAr is evaluated from the portion of the signature which has simultaneously a high and unique dependence on stress. In the case of ferritic-pearlitic steels, tensile stress increases Barkhausen activity at the leading edge of the envelope which thus constitutes a convenient time range for determining RAr. Ar and Vm values suffer from a non-unique stress dependence in the tempered steel, i.e. the existence of a local maximum, but the stress response of RAr remains unique, with a rather low spatial scatter, at least up to 300 MPa. A directional contribution of the texture to noise values impairs the possibility for evaluating stress magnitudes in the 3.5Ni steel, while the 9Ni steel has a high stress response and pronounced spatial variation in noise. The RAr figures represent an improvement over Vm and Ar values as stress measurements, especially for tempered steels and to a lesser extent ferritic-pearlitic steels, whereas all the parameters mentioned have similar stress responses for Ni steels.
1. Introduction In a previous paper, Rautioaho et al. [l] divided the ferromagnetic structural steels into (1) ferritic-pearlitic, (2) tempered and (3) nickel steels based on the stress response of Barkhausen noise (BN). The ferritic-pearlitic steels exhibit a moderate stress response at low tensile stresses, but this gradually diminishes with increasing tension until Barkhausen noise finally saturates to a level characteristic of the material concerned. The tempered steels have a similar behaviour at low tensile stresses, i.e. BN increases with increasing tension up to a certain maximum, any further increase leading to a gradual decrease in BN. Existence of the maximum was considered in a preceding paper, in which it was associated with a simultaneous increase in coercive force (H,) and stress gradients [2]. A typical feature of the materials in the third 0304-8853/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
group is a low noise level in an unstressed condition and a very high response to tensile stresses. So far this behaviour has been reported to occur in 9Ni steel [3] and has also been observed by the authors in 5.8Ni steel. It can obviously be found in any steel which contains at least 3 ~01% nonmagnetic inclusions distributed at packet and lath boundaries of martensite or bainite. An obvious requirement for the use of the BN method for evaluating residual stresses is a unique dependence between noise level and stress which is strong enough to overcome the generally quite broad spatial scatter in noise values. This is not encountered in materials belonging to the second group in particular. The problem is exaggerated in measurements carried out by recording the rms value or total integrated intensity of noise voltage pulses, as is customarily done. These readings do not utilize all the information contained in BN. B.V.
R. Rautioaho et al. / Barkhawen
The information of a magnetization reversal is included in the envelope of a burst of Barkhausen pulses, also termed here the Barkhausen signature. This envelope depicts the number distribution of pulses along a hysteresis branch. An approach for the utilization of this information has been employed by Barton and Kusenberger [4], Matzkanin and Gardner [5] and Gardner et al. [6], who consider the peak amplitude of the Barkhausen signature to provide a good indication of stress. A measurement of a peak signal voltage nevertheless still omits some information carried by the envelope. The present work is therefore directed towards a more detailed study of the envelope in order to determine whether a restricted range of the signature would offer a more useful dependence of BN on stress and would have a reasonably low spatial variation in the recorded signal. A simplification of this idea has been reported earlier by Tiitto [7] who nevertheless did not pay any special attention to the interrelationship between BN and stress. In the present case the peak amplitude and total area under the signature are also measured and compared with the area of the limited range. The main interest is focused on tempered steels, but an improved ability to evaluate residual stresses is also sought for ferritic-pearlitic and nickel steels as well.
2. Experimental The basic instrumental arrangement was similar to that used in the preceding papers [l-3]. The magnetizing current fed to the transducer was of a triangular waveform with a frequency of 10 Hz and peak value with 0.5 to 1.0 A. BN was detected by an inductive sensor element, amplified to 40, 50 or 60 dB, rectified and recorded as a function of time in a frequency range from 2 to 100 kHz by a signal analyzer, Data 6000, which displays the envelope of the rectified bursts of Barkhausen pulses and allows free selection of the portion to be analyzed. The envelope was recorded by averaging over 128 to 512 bursts. The total and selected areas of the envelope and its peak amplitude were read, and the Barkhausen signature was plotted by a X-Y recorder, the time origin on the X axis
97
noise method for stress evaluation
coinciding with the minimum value of the external magnetizing field. Two transducers of a commercial measuring unit, Stresscan 100, were used to magnetize the specimens and sense BN pulses. They yielded closely similar Barkhausen signatures, the shape of the first being better suited for stress measurements and the second for evaluations of texture. The samples used for the measurements were 10 mm thick rectangular-shaped specimens which could be strained elastically up to a stress level of 300 MPa by means of a bending device. The materials were: - low-carbon steel in an as-received (hot rolled) state (sample A). _ NbV microalloyed C-Mn steel in an as-received (control rolled) state (sample B). _ a vanadium microalloyed C-Mn steel, tempered for 48 h at 600 o C, l/2 h at 650 o C, or in an as-received (tempered at around 600° C) state (samples Cl, C2, C3). _ a 3.5Ni steel in an as-received condition (quenched from 840” C, tempered at 640 o C) followed by tempering for 1 h at 640 o C (sample D). - a 9Ni steel with a microstructure developed by heat treatment: 1050 o C 3 h/water quench followed by 800 o C 1 h/water quench and tempering for 1 h at 590 o C (sample E). Table 1 presents the chemical compositions of the materials. Samples A and B are ferritic-pearlitic steels, C a tempered steel and D and E Ni steels. E contains 3 ~01% austenite [3] and falls into the material category (3), but the position of D is unclear, a question to be discussed later.
Table 1 Percentage
composition
of the specimens
Sample
C
Mn
Ni
V
Nb
A B Cl, c2, c3 D E
0.08 0.13 0.15 0.09 0.03
0.40 1.27 1.28 0.55 0.67
0.05 0.05 3.5 8.97
0.07 0.07
0.03 0.04
98
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
3. Results 3.1. Stress response
0.6
Envelopes arising from different locations on the sample surfaces and at certain levels of tensile stress were recorded for the materials in order to find their stress sensitive portion and to measure the total area Ar, the restricted, stress sensitive, area RAr and the peak value Vm of these signatures. Some illustrative examples of the curves are presented in figs. 1 to 4, which also show the analysis ranges for the determination of RAr. The ferritic-pearlitic materials A and B have a low spatial variation in noise voltage, typical curves being plotted in fig. la. The increase in tensile stress shifts Barkhausen activity to the leading edge of the signature (fig. lb). A stress sensitive range consists thus of the leading edge of the signature, the spatial variation in which is not essentially larger than that of the trailing edge.
0.4 VOLT) 0.2 0.0
I
1
-o-2O.O15
a
0
0.025
I
I
0.030
0.035
_l
RAr
I 0.015
c
I
0.020
0.020
t. 0.025 0.030 SECONDS
I 0.035
Fig. 2. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100, 200 and 300 MPa (b) for the C-Mn steel tempered for 0.5 h at 650 o C.
2--
“CLTsl
O-
I
1
0.01
0.02
0.01
0.02
0.03
1
0.04
0.03 0.04 SECONDS Fig. 1. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100, 200 and 300 MPa (b) for the ferritic-pearlitic low carbon steel.
The analysis range from 10 to 22 ms was selected for steels A and B for determining RAr. Typical signatures for the tempered steel C2 are depicted in fig. 2. Fig. 2a shows that there the leading edge experiences much more pronounced spatial scatter than the trailing edge. The tensile stress of 100 MPa increases Barkhausen activity especially at the beginning of the curve, whereas higher stresses shift the signature to the right on the time axis. Simultaneously the total area Ar and peak value Vm decrease. A similar stress induced shift is also observed in samples Cl and C3. Accordingly, a convenient analysis range for C2 is from 28 to 40 ms, for instance (fig. 2b), while ranges from 20 to 32 ms and from 26 to 41 ms were selected for Cl and C3 respectively. These time intervals amount to 20 to 30% of the rising time of the driving field. The 3SNi steel exhibits the highest stress response at around the peak position of the signatures, while the spatial variation in Barkhausen
99
R. Rautioaho et al. / Barkhauren noise method for stress evaluation
t
a
0.02
0.03
O.Oh
b
‘0 2
I
I
0.01
.
curves is observed in C2 and a faint maximum in the Ar vs. stress curves in Cl and C3, whereas neither the existence of any maximum nor any saturation of stress response can be seen in the RAr vs. stress curves, the figures at a stress level of 300 MPa being about 50% higher than those for the unloaded samples. The Vm vs. stress curves approach saturation at around 300 MPa in the latter specimens. Vm also has a higher stress response than Ar in Cl, C2 and C3. In the Ni steels (fig. 7) Vm has an almost equal stress response as Ar, and RAr offers no essential improvement. Consequently, the tensile stress of 300 MPa increases any one of these parameters by about 60% compared with the figures in an unloaded condition in the case of the 3SNi steel and 800% in the case of the 9Ni steel. 3.2. Spatial variation and texture
RAr 0.01
I 0.02 SECONDS
I 0.03
I 0.04
The studied
spatial variation in the parameters was more closely for samples Cl, C2, C3 and
Fig. 3. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100, 200 and 300 MPa (b) for the 3.5Ni steel.
activity is also somewhat increased in this region (fig. 3). Nevertheless, the range from 13 to 21 ms was considered reasonable in this material. On the other hand, the 9Ni steel shows a pronounced spatial variation in the signature everywhere along the time axis. Moreover, tensile stress only slightly concentrates Barkhausen activity in the trailing edge (fig. 4). Consequently, selection of any special analysis range seems meaningless, although a range from 27 to 46 ms was adopted here. The stress response curves for Ar, RAr and Vm are presented in figs. 5 to 7. For the ferritic-pearlitic steels A and B all parameters follow the same trend with increasing tension, the stress response extending up to a stress level of 200 MPa. RAr attains an about 50% (A) or 150% (B) higher stress response than Ar, the maximum RAr values exceeding those measured in an unstressed state by 45% (A) and 160% (B), and Vm closely following RAr (A) or Ar (B). Among the tempered steels, the existence of a clear maximum in the Ar or Vm vs. tensile stress
0.6 VOLTS 0.2
-0.2/l 0.02
0.03
0.04
0.05
0.04
0.05
2
1 "OLTm
0 RAr -1 0.02
0.03
I
,
SECONDS
Fig. 4. Barkhausen signatures illustrating spatial scatter (a) and contributions from stress levels of 0, 100,200 and 300 MPa (b) for the 9Ni steel.
100
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
a
a
_._ ,m’
-
/
l-O
/j
150
100
0
. I b
/
0 Tensile
o-
-
I
I
I
100
200
300
stress
s.1
RAr
-0
RAr
o/y’o
200
3
-m Vm
-
l
Ar
I b
Vm
’
‘\
(MPa)
Fig. 5. Stress response curves for the ferritic-pearlitic
steels.
D. Because the stress response curves of Ar and RAr are quite similar in steel D, only Vm and Ar were recorded for this sample. Since we used only one sample in each material condition and tested an area of about 50 cm2, we may not be getting much more than an intimation of the true spatial scatter in the parameters concerned, but the results, presented in table 2, can still be utilized for a rough comparison between the parameters. Table 2 indicates that the relative standard deviation, 100 * standard deviation/parameter, is generally largest for the quantity Ar and smallest for RAr, the latter figures being 50 to 70% of the former. Except for sample C3, the relative standard deviation of Vm is smaller than that of Ar. The figures in table 2 are computed of the basis of 10 to 20 measurements. The directional anisotropy in noise parameters, i.e. the magnetic (and mechanical) texture, was found to be pronounced only in sample D. The figures denoted by (L) in table 2 describe the spatial scatter in Ar and Vm and those denoted by (L + T) the combined contribution of spatial vari-
”
0
100 Tensile
stress
200 300 IMPa)
Fig. 6. Stress response curves for the tempered steels.
Table 2 Values for the noise parameters Ar, Vm and RAr together with their relative standard deviations for the tempered and 3.5Ni steels Sample
Ar
lOOu,/ Ar
27.0 30.0 22.2 12.7 14.3
looO”/ Vm
10.4 16.0 26.6 9.4 16.8
1473 2203 1220 705 761
RAr
Lao”/ RAr
(mVs)
(mV)
(mVs) Cl c2 c3 D(L) D(L+T)
Vm
8.1 9.3 28.4 7.3 12.2
12.1 12.0 12.0
6.6 8.3 20.0
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
0’
0’
/
about double that in row (L). The effect of texture on the Barkhausen signature can be observed in fig. 8, where it is seen clearly to impair the capability for assessing the magnitude of the residual stresses in material D (compare figs. 3b and 8).
RAr
--o
101
4. Discussion a 100
200
300
0 Tensile
stress INPa)
Fig. 7. Stress response curves for the nickel steels
ation and texture. L and T refer here to measurements carried out with magnetization directed longitudinally (L) or transversely (T) to the long dimension of the rectangular-shaped specimens. The relative standard deviation in row (L + T) is
t
1
0.01
I
0.02
0.03
0.04
SECONDS
Fig. 8. Barkhausen signatures for the 3.5Ni steel illustrating the contribution of texture, with magnetization directed parallel to the long dimension of the rectangular-shaped specimen (L) or transverse to it (T).
The leading edges of the Barkhausen signatures are exaggerated by increasing tensile stress in the case of the ferritic-pearlitic steels A and B. This phenomenon is caused by the stress alignment effect [2], which in this case extends to a stress level of 200 MPa. A convenient analysis range thus consists of the leading edge of the signature induced by a low driving field. The same increase in stress response can be achieved, however, by using such a low magnetizing current (and field) that the noise pulses corresponding to the leading portion are the only ones generated in the material. A sophisticated signal analyzing system hence gives no essential advantage over customary measuring devices in the case of ferritic-pearlitic steels. Tensile stresses above 200 MPa cannot be evaluated in terms of magnitude in steels like A and B. Due to the unique behaviour of the stress response curves, the figures for RAr represent an indisputable improvement over the Ar values in the tempered steels of type C after selection of the proper time range. The increase in the leading edges of the signatures at a stress level of 100 MPa arises from the stress alignment effect, while the shift to the right on the time axis with increasing tension above 100 MPa is connected with the increase in coercive force and stress gradients, as suggested in the preceding paper [2]. This behaviour is most pronounced in sample C2, and weakens in C3 and Cl, respectively. The 3.5Ni steel behaves in a similar fashion to the above tempered steels. This is not so surprising, because it is in fact a tempered steel which only contains a tiny amount of retained austenite, as revealed by X-ray diffraction. Nevertheless, the parameters do not exhibit a maximum in their noise vs. stress curves, and the stress response curves for Ar, RAr and Vm resemble one another.
102
R. Rautioaho et al. / Barkhausen noise method for stress evaluation
The contribution of texture has its origin in the microstructure, a subject not considered here. The high stress response in the 9Ni steel is caused by the retained austenite present in this steel, as considered in the preceding paper [3]. The spatial scatter in BN values is pronounced in an unstressed state (fig. 7a), but at least relative noise variations seem to diminish with increasing tension. Due to a strong interaction between nonmagnetic austenite paricles and the movement of Bloch walls [3], one obvious reason for this scatter is a non-uniform distribution of these particles. The measurement of RAr represents an improvement on customarily employed methods, especially in tempered steels where the stress sensitivity becomes unique and the spatial scatter diminishes. It is also as good as either Ar or Vm for the other types of material. Compared with Ar (or the rms value), Vm has a higher or similar stress response and a lower spatial variation, at least in the tempered steels. It should hence be preferred to Ar. It can be recommended, however, that all the parameters should be measured and their combined information utilized for evaluating residual stresses by the BN method in order to obtain the most reliable estimates for stress magnitudes.
5. smlunaly The following observations were made: 1. In the case of ferritic-pearlitic steels the stress sensitivity of the BN method can be improved either by restricting the time range of the recorded pulses to the leading edge of the noise voltage vs. time curve or by using such a small magnetizing field (current) that only a fraction of the total BN activity is generated in the sample. The magnitude of stresses extending at least up to a half of the
yield strength can be evaluated by these procedures. 2. In tempered steels a unique stress response, and simultaneously a reduced spatial variation in the measured signal, is obtained by recording pulses from a time interval amounting to 20 to 30% of the rising time of the field, with the leading boundary approximately, conciding with the peak amplitude. 3. The stress response is not essentially improved or the spatial variation reduced by restricting the analysis range for the Ni steels. A directional contribution to noise values impairs the capability for determining the magnitude of residual stresses in the case of the 3.5Ni steel. The 9Ni steel has a high response and pronounced spatial scatter in noise due to the retained austenite present in this steel.
Acknowledgement Dr. T. Ogawa of the Nippon Steel Corporation, Japan, is thanked for supplying the 3SNi steel.
References 111R. Rautioaho, P. Karjalainen and M. Moilanen, J. Magn. Magn. Mat. 61 (1986) 183.
PI R. Rautioaho, P. Kaxjalainen and M. Moilanen, J. Magn.
Magn. Mat. 68 (1987) 314. 131 R. Rautiosho, P. Karjalainen and H. Moilanen, J. Magn. Magn. Mat. 68 (1987) 221. t41 C.G. Gardner, G.A. Mazkanin and D.L. Davidson, Intern. J. Nondest. Test. 3 (1971) 131. 151 J.R. Barton and F.N. Kusenberger, Paper 74GT-51, ASME Gas Turbine Conf., Zurich, Switzerland (1974). Proc. of the [61 G.A. Matzkanin and C.G. Gardner, ARPA/AFML Review of Quantitative NDE, AFML-TR75-212 (1976) 791. 171 S. Tiitto, Acta Polytechn. Scandinavica 119 (1977) p. 18.