Nondestructive magnetic measurements in weld and base metals of service exposed CrMo steel

r~UTTERWORTH

NDT&E International, Vol. 28, No. 1, pp. 29--33, 1995

096. 9 94)0000

Copyright © 1995 Elsevier Science Ltd P d n t ~ in Great Britain. All rights reserved 0963-8695/95 $10.00 + 0.00

Nondestructive magnetic measurements in weld and base metals of service exposed C r - M o steel A. Mitra*, Z. J. Chen and D. C. Jiles Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA Received 28 July 1994; revised 25 October 1994

Magnetic hysteresis and micromagnetic Barkhausen parameters were measured in base and weld metals of a service exposed Cr-Mo steel. Although the coercivity was higher in the welds, the difference in hysteresis parameters between welds and base metals was small. Hence, hysteresis parameters were not suitable to distinguish service exposed weld and base metals. However, a significant variation in Barkhausen waveforms was observed between base and weld metals. The difference in Barkhausen activity was most prominent at low magnetizing fields. The results of this study have been correlated with the microstructure of the samples. Keywords: magnetic hysteresis, Barkhausen effect, base and weld metals

The structural integrity of many engineering components is determined by intrinsic properties like stress and microstructure at the surface and/or near subsurface zone t11. The conventional test procedures like X-ray, hardness and metallography which are used to evaluate the quality of the near subsurface state, are expensive, time consuming and sometimes inappropriate in real life situations. So far the nondestructive test procedures used extensively in industry to determine the quality of the component are mainly restricted to eddy current methods and ultrasonic testing t2'31. Many engineering components that are designed for high strength applications are made of steels which are ferromagnetic and it has been found that the magnetic properties are very sensitive to microstructure, corrosion and the stress state of the material t~6]. As changes occur during service exposure of these materials, attempts are now being made to determine their integrity by measuring their magnetic properties [7'8]. There are several different magnetic methods for nondestructive evaluation of materials which have been reviewed recently [9]. In the present work address: Magnetics Group, Laboratory, Jamshedpur 831007, India

* Permanent

National

magnetic hysteresis and micromagnetic Barkhausen measurements have been made to compare the base and weld metals of service aged pipeline steel which was operated at high pressure and temperature. Suitable in situ measuring methods have been developed based on these results.

Material A small section (20 crn long, 2 cm wide and 1.8 cm thick) of a welded 2.25 C r - l M o steel pipe which had been in service for about 210000 h was removed for the present study. The carbon content of the base and weld metals was 0.046% and 0.043% respectively. The pipe had experienced a hoop stress of 32 MPa at 570°C during service exposure.

Experimental Magnetic hysteresis parameters were determined using a computer controlled inspection system, the Magnescope[1% in which a probe was placed on the surface of the material and the hysteresis loop was measured using

Metallurgical

29

A. Mitra et al.

v

40ms

WELD METAL a

Figure 1

WELD METAL

b

FIELD STRENGTH = 8 0 e

FIELD STRENGTH = 30 Oe

Magnetic Barkhausen waveforms of weld and base metals at a magnetizing field amplitude of: (a) 80e; (b) 30 Oe

Table 1. Magnetic hysteresis parameters in base and weld metals

a quasi d.c. magnetic field. Micromagnetic Barkhausen measurements were also taken using a surface Barkhausen probe under the action of a sinusoidal field excitation at a frequency of 8 Hz. The sample was magnetized by a C-core magnet, and a pick-up coil containing a spring loaded ferrite core was placed between the magnetizing pole pieces which were 3 cm apart. The a.c. magnetic field amplitude was measured by placing a Hall probe on the tip of the C-core magnet and passing a known current through the field coil. Barkhausen signals within the frequency range of 10 kHz to 175 kHz were investigated in the present study. The Barkhausen waveforms were monitored by a digital oscilloscope (Lecroy, Model 9314) with a sampling rate of 500 kHz. The rms voltage was measured by a digital multimeter (HP, Model-3457A) having a frequency response of 20 Hz to 1 MHz. The number of events was counted using a universal counter (HP, Model 5316B). The frequency response of the counter was 30 Hz to 100 mHz. The number of events, initially measured over a 2 s gate time, was converted to number of events per magnetizing cycle. A threshold voltage of 350 mV was used to eliminate low level background noise. This threshold voltage was determined by putting the probe on a non-magnetic substance and adjusting the threshold so that no voltage pulses were detected by the counter.

Position

Coercivity (Oe)

Remanence (kG)

Loss (ergs cm -3)

Base metal Weld metal

2.89 3.15

1.87 2.16

12955 13984

Optical micrographs of the sample were also taken to correlate the magnetic properties with the mierostructure.

Resu Its The measured magnetic hysteresis parameters for both base and weld metals are shown in Table I. It shows that coereivity, remanence and hysteresis loss were lower in the base metal than the weld portion of the pipe, but this difference was not significant enough to allow their properties to be distinguished in a real life situation. The Barkhausen waveforms at magnetizing field amplitudes of 8 0 e (640 A m - i) and 30 Oe (2.4 kA m - 1) for base and weld metals are shown in Figure la and lb. The difference

30

Nondestructive magnetic measurements 1.40 1.20

t ~ rit~mber of events detected in each magnetizing cycle (No) when the threshold voltage is 350 inV. In base metal No increased sharply with field and reached a maximum value at a magnetizing field of amplitude 15 Oe (1200 A m - 1) and then slowly decreased with the increase of field. In the case of weld metal No also increased with field at the beginning, but the rate of increase was slow compared with base metal. Typical Barkhausen parameters at field amplitudes of 8 0 e (640 A m - 1) and 30 Oe (1200 A m - 1) are shown in Table 2.

L

--

/

4$

1.00 :> @

0.80

=?

0.60

tO

0.40

•It

]$

The optical micrographs of the base and weld metals and their interface are shown in Figure 5a, b, c respectively. The grain size in the base metal was found to be larger than the weld. At the grain boundaries of the interface, cavity-like defects were observed.

0.20 0

0

I

I

I

l

I

I

I

I

10

20

30

40

50

60

70

80

90

Magnetizing Field (Oe)

Discussion

Figure 2 Dependence of RMS voltage of Barkhausen activity against magnetizing field amplitude in base and weld metals

Among the hysteresis parameters it was found that coercivity and permeability, which are inversely related, are very structure sensitive. Thompson et al. Elll found that the coercivity of the welds in high strength steel in its virgin state was almost double that of the base metal, but after renormalization at 800°C the difference in coercivity was minimized. In the present case no significant change in coercivity was observed, although it was higher in welds than the base metal. During the long service exposure of 210000 h at high temperature and pressure, grain growth took place both in base and weld metals. Coercivity decreased with the grain growth and reached a limiting value at higher grain size[12]. As a result no significant difference in coercivity in weld and base metals was observed. Hence, coercivity does not appear to be a suitable parameter for comparison of base and weld properties of service exposed material.

10000 8000 (D

O

>=

60004000"

UJ

20000 100

260 3(~0 400 Threshold Voltage (mV)

550

Figure 3 Plot of the number of detected events per magnetizing cycle against the threshold voltage for background noise ( A ) , base metals ( I ) and weld metals ('k) at a magnetizing field amplitude of 15 Oe

2500

between the Barkhausen wavdorms in the base and weld metals was greater at low field (Figure la). To find the field dependence of the Barkhausen signal, different Barkhausen parameters were measured at different magnetizing field amplitudes.

!

Figure 2 represents the rms voltage (Vrm,) at different magnetizing field amplitudes for base and weld metals. Vrm, increases rapidly with field at the beginning and tends to saturate at high fields. The rate of increase of Vrm~ with field in the base metal was higher and Vrm~ reached its saturation value at lower fields in the base metal compared with the weld. The saturation value of Vr~, in base and weld metal is almost identical.

2000

1500

1000

50O

0

Figure 3 shows the effect of threshold voltage on the number of Barkhausen events detected in base and weld metals. The background noise at a magnetizing field amplitude of 15 Oe (1200 A m -1) is also shown in the same plot. Figure 4 represents the field dependence of

0

10

20

30

40

50

60

70

80

90

Magnetizing Field (Oe) Figure 4 Variation of the number of detected Barkhausen events with magnetizing field amplitude in weld and base metals at a threshold voltage of 350 mV

31

A. Mitra et aL

However, higher values of coercivity were observed in weld metal compared with the base metal due to smaller grain size and the formation of a greater number of defects at the grain boundaries (cavity-like). Lower coercivity implies a lower number density and strength of pinning centres for the domain wall movement. Thus the domain wall can move a longer distance in the base metal, resulting in larger Barkhausen jump amplitudes compared with those in the weld. Barkhausen waveforms (Figure 1) of the weld and base metals, in particular at a low magnetizing field, indicate that this micromagnetic technique can distinguish between weld and base metals of service exposed materials. The observed field dependence of the Barkhausen signal can be explained by using the model of Jiles et al. t13J. According to the model the Barkhausen activity (Mj,) can be expressed as:

(1)

=

where ? is a dimensionless proportionality constant, Zir, is the differential irreversible susceptibility and H is the time derivative of the applied magnetic field. The differential susceptibility of a material increases with the field until the magnetization reaches the point of inflection of the normal magnetization curve. Accordingly Barkhausen activity increases rapidly during the initial part of the magnetization curve. When the field is increased further, differential susceptibility decreases which indicates no further domain orientation takes place with the increase of field amplitude. As a result the Barkhausen voltage reached saturation at a high field amplitude. Lower coercivity in the base metal indicates that less field is necessary to orient all the domains along the field direction. Hence the Barkhausen voltage in the base metal reached its saturation value at a lower field amplitude compared with the weld metal. Microstructural evidence showed smaller grain size and more intergranular defects in weld metals which gave rise to greater number density and strength of pinning centres in weld metal. This restricted the domain wall movements and consequently more Barkhausen events of smaller amplitude are expected to occur in weld metal. But the observed lower number of detected Barkhausen events in weld metals compared to base metals (Figure 4) might be due to the elimination of those events whose amplitude fell below the detection threshold voltage (350 mV). The threshold voltage dependence of the number of events showed a higher No in weld metal when the threshold voltage was low (Figure 3).

Figure 5 Micrograph of (a) the base metal; (b) the weld metal; (c) the interface of base and weld metals

Table 2. M a g n e t i c Barkhausen parameters in base and weld metals

Position

RMS voltage (V) at 80e 30 Oe

Base metal Weld metal

0.40 0.16

1.50 0.64

Peak to peak voltage (V) at 80e 30 Oe 3.51 1.50

32

13.33 8.64

Events/cycle at 80e 30 Oe 1849 869

1851 1843

Nondestructive magnetic measurements the sample. Financial support for AM from USAID and DST, Govt. of India and for ZJC from NRC and EPRI is acknowledged.

As the field amplitude increased, the magnetization sweep rate also increased to keep the frequency constant. So the time interval between the subsequent pulses decreased to accommodate all the Barkhausen events. Hence overlapping of pulses occurs t14]. As a result the number of detected Barkhausen events, No, decreased with field amplitude.

References 1 Field, M. and K ~ , J.F. 'Review of surfa~ integrity ofmachin~ components' Ann CIRP 20 (1971) pp 153-163 2 Simmer, R.D. 'Eddy current testing, today and tomorrow' Mater Eva152 (1994) pp 28-32 3 Good, M.S. and Rose, J . L 'Measurement of thin ease depth in hardened steel by ultrasonic pulse-echo angulation techniques' in Nondestructive Method for Material Property Determination Plenum, New York (1983) pp 192-203 4 Titto, S. 'On the influence of microstrueture on magnetic transitions in steel' Acta Polytech Scandinavica: Appl Phys Ser No. 119 (1977) p 1 5 Chattoraj, I., B I m ~ m i s h ~ A.K. and Mitr~ A. 'The effect of corrosion on the magnetic properties of rapidly solidified Fe-Si-B fiber' Scripta Met Mat 26 (1992) pp 1013-1018 6 I.~ngman, R. 'Measurement of the mechanical stress in mild steel by means of rotating magnetic field strength" NDTlntern 14 (1981) pp 255-262 7 Chen, Zd., Strom, A. and Jiles, D.C. 'Micromagnetic surface measurement for evaluation of surface modification due to cyclic loading" IEEE MAG-29 (1993) pp 3031-3033 8 Tbeiner, W.A., Altl~ter, L and Kern, R. 'Determination of subsurface microstrueture state by micromagnetic NDT' in Nondestructive Characterization of Materials 11, Eds C.O. Rudd, R.E. Green and J.F. Bussiere, Plenum, New York (1986) pp 233-240 9 Jiles, D.C. 'Review of magnetic methods for NDE' ND T Intern 21 (1988) pp 311-319 and 23 (1990) pp 83-92 10 Jiles, D.C., Harihanm, S. and Devine, M.K. 'Magnescope: A portable magnetic inspection system for evaluation of steel structure and components' IEEE MAG-26 (1990) pp 2577-2579 11 Thompson, S.M., Allen, P J . and Tanner, KK. 'Magnetic properties of welds in high strength peaditic steel' IEEE MAG-26 (1990) pp 1084-1086 12 Jiles, D.C. 'Influence of size and morphology ofeutectoid carbides on the magnetic properties of carbon steels' J Appl Phys 63 (1988) pp 2980-2982 13 Jiles, D.C., Sil~mi, LB. and Williams, G. 'Modeling of micromagnetic Barkhausen activity using stochastic process extension to the theory of hysteresis' J Appl Phys 73 (1993) pp 5830-5832 14 Bittel, H. 'Noise of ferromagnetic materials' IEEE MAG-5 (1969) pp 359-365

Conclusion Magnetic properties of weld and base metal of service exposed Cr-Mo steel were measured by a nondestructive technique. No significant differences in magnetic hysteresis parameters were observed as a result of service aging, although they were higher in weld metal compared with base metal. However, wide variation in the Barkhausen waveforms, in particular at lower magnetizing field amplitudes, was found between weld and base metals. This indicated that the micromagnetie Barkhausen emission technique is useful for distinguishing the properties of weld and base metals of service exposed materials. The microstructures of the materials revealed that weld metal had lower grain size and more defect centres compared with base metal. Hence domain wall motion was more restricted in the welds and more magnetic field was necessary to orient all the domains along the field direction. Thus saturation of the Barkhausen activity occurred at a higher field amplitude in welds compared with base metals. The number of Barkhausen events detected decreased at higher field amplitudes in both base and weld metals due to the overlapping of the pulses.

Acknowledgement The authors wish to thank Dr Carl D. Lundin and Dr K. K. Khan of the University of Tennessee for providing

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