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Measurement of electromagnetic properties of heat exchanger tubes
NDT&E International 48 (2012) 63–69
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Measurement of electromagnetic properties of heat exchanger tubes Evgueni Todorov n EWI, 1250 Arthur E. Adams Dr., Columbus, OH 43221, USA
a r t i c l e i n f o
abstract
Article history: Received 30 August 2011 Received in revised form 20 February 2012 Accepted 23 February 2012 Available online 5 March 2012
Electromagnetic properties of heat exchanger tubes made of SAF 2205, Type 439 and Sea-Cure stainless steels were determined with magnetic and electrical techniques. The standard magnetic measurements were conducted at direct (DC) and alternate (AC – 200 Hz) current excitation. The magnetic permeability, coercivity, retentivity, and other magnetic parameters were measured in tube axial and circumferential direction, in open and closed magnetic circuits and in wide range of magnetic field strengths from 10 A/m to 82 kA/m. The electrical conductivity was determined with custom-designed fixture in axial direction only at DC and AC (50 – 200 Hz). The unique set of results revealed that a certain anisotropy of properties existed between the two geometric directions. The agreement between the measurements from the different techniques was good where comparison was possible. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Eddy current testing Heat exchanger tubes Electrical conductivity Magnetic permeability Steel electromagnetic properties
1. Introduction
Tube 1. Duplex stainless steel SAF 2205: Actual 19.028-mm outside diameter (OD) by 1.596-mm wall thickness (WT). Nominal 0.75-in. OD by 0.065-in. WT. Tube 2. Type 439 stainless steel: J Actual 15.778-mm OD by 0.829-mm WT. Nominal 0.625-in. OD by 0.035-in. WT. Tube 3. Sea-Cure: J Actual 25.283-mm OD by 0.816-mm WT. Nominal 1-in. OD by 0.035-in. WT. Specimen designation ‘‘Sea-Cure 1 0.035’’. Tube 4. Sea-Cure: J Actual 25.208-mm OD by 0.626-mm WT. Nominal 1-in. OD by 0.028-in. WT. Specimen designation ‘‘Sea-Cure 1 0.028’’. J
Important part of any modern structural integrity program is the use of nondestructive evaluation (NDE) techniques to ensure that fabrication and service-induced flaws and conditions would not affect the safety and service life of critical components such as airframes, jet engines, pipelines, bridges, heat exchangers, rail cars, and others. In addition to NDE personnel experience and training, performance demonstration studies are conducted with large number of representative flaws and flaw conditions to quantify the performance of NDE techniques in terms of probability of detection (POD) and flaw sizing accuracy. The practical trials with specimens are very expensive and time consuming. Computer modeling has been suggested to optimize the NDE technique performance and reduce the number of physical specimens and trials as part of the so-called technical justification [1]. The extensive use of computer modeling approach for NDE is also envisioned for the new generation of nuclear reactors and heat exchangers [2]. Good knowledge of material properties is required, however, to build representative computer models. The use of new ferritic and duplex stainless steel (both magnetic materials) tubing for heat exchangers provided opportunity to consider the computer modeling approach for optimization of inservice NDE eddy current techniques and procedures. The industry was interested in investigating tubes made of three new ferromagnetic materials. Four tubes with length 914.4 mm (3 ft) were submitted for testing as follows:
n
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The material chemical composition (Table 1) was obtained on samples cut from each tube. It complies with the standard specifications provided in Table 1 for reference. As already discussed, knowledge of material properties is needed to build representative computer models. For eddy current technique modeling, reliable and accurate estimates of tube electrical conductivity, magnetic permeability and other magnetic parameters are required. While some general reference data for the tube electrical conductivity was available in the public domain literature [3–5], the magnetic properties were unknown. Then, the first step in implementing the modeling approach was to reliably determine the electrical conductivity and initial magnetic permeability with computer models and other independent techniques. Further, the initial objective was to determine these two electromagnetic parameters with nondestructive model-based eddy current technique where the independent magnetic and electrical conventional destructive techniques would be used to validate the eddy current measurement models. It was later
64
E. Todorov / NDT&E International 48 (2012) 63–69
to amplitudes.
Table 1 Chemical composition (%). Element
Carbon (C) Manganese (Mn) Phosphorus (P) Sulfur (S) Slicon (Si) Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Nitrogen (N) Aluminum (Al) Titanium (Ti) Iron (Fe) a
SAF 2205 (UNS Type 439 (UNS S31803/ S43035) S32205a)
Sea-Cure Sea-Cure 1 0.035 (UNS 1 0.028 S44660) (UNS S44660)
0.018 1.38
0.022 0.285
0.028 0.302
0.031 0.405
0.024
0.020
0.026
0.025
o 0.001 0.37 22.4
0.004 0.367 17.2
0.006 0.404 26.3
0.004 0.424 25.8
5.61 3.18
0.157 –
1.94 3.71
1.87 3.74
0.18 –
0.0084 0.014
0.018 –
0.015 –
– Balance
0.333 Balance
0.096 Balance
0.168 Balance
realized that with little additional efforts, the independent magnetic measurements would provide significantly more data especially at stronger magnetic fields and along different directions of the tube geometry which would be of value to the future inspection and modeling efforts. Thus, the results from the comprehensive tube characterization with magnetic and electrical independent techniques is presented and discussed in this paper.
2. Methods and theory 2.1. Magnetic measurements
ð1Þ
where B – magnetic flux density or magnetic induction in Tesla (T), m0 ¼ 4p107 – magnetic permeability (constant) of free space or air in Henry/meter (H/m), H – magnetic filed strength or intensity in ampere/meter (A/m), M – material magnetization in A/m, I – magnetic polarization in T, and mr – relative magnetic permeability (dimensionless). The relative magnetic permeability depends on the magnetic field strength H. There are many specific types of the relative magnetic permeability used to describe material magnetic property behavior for various practical purposes. The amplitude or peak relative magnetic permeability is frequently quoted and shown in Eq. (2). The amplitude or peak values of B and H are used to calculate it. B
m0 H
DB
ð3Þ
m0 DH
B H þ 4pM
ð4Þ
where B is expressed in Gauss (G), H – in Oersted (Oe), M¼ I – in electromagnetic unit/cubic centimeter (emu/cm3), and m – magnetic permeability in G/Oe. The magnetic permeability expressed in cgs–emu units in Eq. (4) is equal in value to the relative magnetic permeability equations (2) or (3) as applicable. Except for the original vendor reports, all magnetic measurement data was converted, processed and reported in SI units. Equations to convert cgs–emu into SI units can be found elsewhere in the open literature. Sometime, magnetic measurements are performed in open magnetic circuit where the magnetic field strength in the sample Hin is not equal to the external magnetic field Hext. It is then said that the external magnetic field in the sample is reduced by a demagnetizing field Hdm created by the magnetic poles at the sample ends [Eq. (5)]. Hin ¼ Hext Hdm
Two major sets of magnetic measurements in closed and open magnetic circuit were conducted to determine the magnetic permeability, residual magnetization, coercivity, retentivity, and others. The governing equation (in International System of Units – SI) expressing the magnetization of a magnetic material when placed in external magnetic filed is shown below:
mrampl ¼
dB
m0 dH
The differential relative magnetic permeability is used to describe material behavior when relatively weak alternating magnetic field is superimposed on strong permanent biasing magnetizing field. The differential relative magnetic permeability is also used to monitor the degree of saturation in strong fields where it approaches 1. The amplitude and differential relative magnetic permeabilities are identical in weak field when the material is magnetized in the linear portion of the initial magnetization curve. Some of the vendor magnetic measurements in this study were reported in centimeter–gram–second (cgs–emu) system of units. Eq. (4) is used to calculate the material magnetic permeability in cgs–emu system:
m¼
Material standard specification for reference.
B ¼ m0 H þ m0 M ¼ m0 H þI ¼ m0 mr H,
mrdif f ¼
ð2Þ
Another type of relative magnetic permeability is the so called differential relative magnetic permeability shown in Eq. (3). It is expressed as a first derivative of B–H curves (initial and hysteresis loops) or ratio of small increments of B and H as opposed
ð5Þ
The external magnetic field strength can only be measured. For cases where the demagnetization is strong (e.g., open magnetic circuit), a relative magnetic permeability of the sample shape is defined as shown in Eq. (6) which is smaller than the material (or intrinsic) relative magnetic permeability:
mrshape ¼
B
m0 Hext
o
B
m0 Hin
¼
B
m0 ðHext NMÞ
¼ mrmaterial
ð6Þ
The demagnetizing factor N (Eq. (6)) is dimensionless and constant in the linear portion of the initial magnetization curve. It depends on the magnetic field strength if magnetization is conducted in the non-linear portion of the initial magnetization curve. The problem with the open circuit magnetic measurements is that the demagnetizing factor N is rarely known or it is difficult to estimate and measurement of material magnetic permeability is impossible. Magnetic measurements in closed magnetic circuits (N ¼0) or with significantly reduced demagnetization (localized magnetization) are used to obtain the material properties. 2.1.1. Vibrating sample magnetometer (VSM) method. Open magnetic circuit A magnetic material specimen is placed in a constant magnetic field. The specimen is magnetized and magnetic poles are formed. The specimen is then vibrated causing change in the magnetic field surrounding the specimen. The changes of the magnetic field surrounding the specimen are sensed with coils in all three directions in space. The signals in the sensor coils will depend on the magnetic properties of the material and the specimen shape. Thus, the magnetic properties of the specimen (and material) can
E. Todorov / NDT&E International 48 (2012) 63–69
be estimated in different direction of magnetization. The VSM measurements are conducted in open magnetic circuit. Four half-ring-shaped specimens with height approximately 25.4 mm were cut from each tube for this test. The vendor cut a rectangular shape specimen 4.75 4.75 mm from each halfring to conduct the test. The measurements were acquired in two different directions – axially (A) along the tube length and circumferentially (C) along the tube circumference. The vendor used an ADE/DMS Model 880 Vibrating Sample Magnetometer manufactured by MicroSense, LLC. 2.1.2. Standard DC and AC methods. closed magnetic circuit The DC and the AC measurements in closed magnetic circuit are the standard [6,7] methods for determining the magnetic properties of the materials. Eight specimens – four short tubes/bars (75-mm length) and four rings (6-mm height) were cut from each of the tubes to conduct the A- and C-direction measurements. An electrical discharge machining (EDM) cutting process was used to provide very tight tolerance in the range of 70.05 mm for the length and parallel end-surfaces. The EDM cutting was conducted in such a way that no effect on magnetic measurements was anticipated due to the machining. The short tubes and rings dimensions were measured with precision mechanical gauges before the magnetic measurements were initiated. Each ring sample has a primary (H) and secondary (B) coil wound with 24AWG and 30AWG magnet wire, respectively. A secondary (B) coil only was wound on each short tube with 30AWG magnet wire over insulating tape with thickness approximately 0.1 mm. The short tube was then placed in a KJS Associates Model YOKE-100 electromagnet and clamped into a closed magnetic test circuit. A calibrated Hall probe was placed at the surface of the coil to measure the applied field strength H. The search coil winding was connected to the system fluxmeter to determine the flux density in the sample. All measurements were made with a KJS Associates, Inc. Model SMT-700 computer-automated soft magnetic hysteresigraph system. Two measurement sets – DC and AC excitation were acquired with the rings samples, while only DC measurements were acquired with the short tube specimens.
65
the two lengths was calculated [Eq. (10)].
sd ¼
2ðL1 L2 Þ
ð10Þ
pðR1 R2 ÞWTðOD þ IDÞ
The purpose of this third conductivity value was to identify whether the resistance measurements had a systematic error. If a systematic error existed, it would cause a significant difference between the sd and the other two conductivities. Eqs. (7) through (10) are based on the assumptions that the current density along the tube tested length and through the tube cross section is uniform (no ‘‘skin effect’’), the conductor cross-section is uniform along the conductor length and the tube resistivity or conduc tivity is uniform along the conductor length and through the cross-section. Based on literature references [3–5], the resistance of the tubes was expected to be in the range from 5 to 15 mO. For comparison, the resistance of a center contact in a typical widely popular connector pair (plug-to-jack) as BNC is approximately 1.5 mO [8]. These are extremely low values that pose unique challenges to the measurement technique, circuit and instrumentation. Initially, a 4-terminal pair (4 TP) AC configuration with precision impedance analyzer was selected to eliminate the effects of the contact resistance and thermal electromotive forces. Unfortunately, an off-the-shelf fixture to implement this measurement technique was not commercially available. A special fixture was designed and built shown in Fig. 1. The AC measurements, however, were slow and very demanding in terms of calibration and implementation. Conventional DC measurements were later considered and conducted using the same fixture and a classical 4-terminal (4 T) Kelvin circuit. The DC measurements were fast and very consistent. All of the contact DC and AC electrical conductivity measurements were conducted in A-direction only. A Micro-Ohmmeter Model 5600 (Chauvin Arnoux, Inc.) was used for the DC while an Impedance Analyzer HP 4294 A (Agilent) was used for the AC measurements as shown in Fig. 1.
3. Results and discussion 3.1. Magnetic measurements in open and closed magnetic circuit
2.2. Electrical measurements: four-terminal circuit The material resistivity or conductivity is another parameter that affects the nondestructive testing of the materials with the electromagnetic or eddy current techniques. The main formula that links a material electrical conductivity or resistivity with the conductor length and cross section is shown in Eq. (7):
s¼
1
r
¼
L , RS
ð7Þ
where s – electrical conductivity in Siemens/meter (S/m), r – electrical resistivity in Ohm meter (Om), L – conductor length in meter (m), and S – conductor cross sectional area in m2. Further, two lengths of each tube were measured – L1 and L2. The electrical conductivities s1 and s2 for each of the lengths were calculated using formulas (8) and (9):
s1 ¼ s2 ¼
2L1
,
ð8Þ
2L2 , pR2 WTðOD þ IDÞ
ð9Þ
pR1 WTðOD þ IDÞ
Two sets of data were acquired with the VSM in open magnetic circuit for each tube and direction – major hysteresis loops and initial magnetization curves (16 raw data files). As previously discussed, the data represented the magnetic properties of the specimen shape for field strengths lower than the saturation levels. The material magnetic properties were not possible to determine due to the strong demagnetizing effect. The vendor conducted a second set of measurements (additional 16 raw data files) in an attempt to estimate the demagnetization factor N and correct the magnetic field strength as shown in Eq. (6). The raw data files from the second set were processed in-house to determine the initial and the maximum relative magnetic permeability. The data was approximated (smoothed) using a second and
Micro-Ohmmeter
Impedance Analyzer
Fixture
where R1 and R2 – measured resistances in O, WT – tube wall thickness in m, OD – outer diameter in m, and ID – inner diameter in m. In addition, the conductivity sd for the difference between
Fig. 1. Equipment arrangement for DC and AC resistance measurements.
66
E. Todorov / NDT&E International 48 (2012) 63–69
a third order polynomials followed by analytical differentiation to determine the differential relative magnetic permeability at different field strengths. The estimates were inconsistent, scattered, and inaccurate compared to the more accurate closed magnetic circuit measurements. A discussion with the VSM manufacturer confirmed that the procedure used by the VSM vendor to estimate the demagnetization factor was not suitable for this type of specimens. In summary, the VSM data in weak and medium fields was not adequate to determine the material magnetic properties due to the strong demagnetization in open magnetic circuit. However, the VSM data (first set) acquired at very strong saturating field (no demagnetization) was actually comparing very well to the closed circuit measurements discussed later. A total of 36 data sets were acquired in closed magnetic circuit with DC and AC excitation in both directions of magnetization. The measurements were first conducted to cover the entire range from the weak to the maximum field strength of 75 kA/m (tubes) and 35 kA/m (rings). The magnetic properties of the materials in medium and strong fields were well defined. The magnetic properties in weak fields, however, were difficult to obtain due to the lack of resolution (small number of data points). Subsequent data sets were acquired at reduced maximum field strengths to improve the resolution at weak field strengths. The AC measurements were acquired at two excitation frequencies – 200 Hz and 15 kHz. The relative magnetic permeability measurements versus H at 15 kHz (not discussed in this paper) were unreliable due to the unaccounted and strong effect of the induced eddy currents in the specimens. A similar procedure as discussed above was used to smooth the experimental measurements and calculate the differential relative magnetic permeability at different field strengths. The amplitude or peak [Eq. (2)] mr,in and mr,max measurements extracted from different data sets are summarized in Table 2. The permeability values are reported along with the H field at which the permeability was calculated. Regarding the DC measurements, the mr,max for SAF 2205 tube is smaller compared to the other three tubes. The mr,max is similar for 439 and Sea-Cure tubes. Except for the steel SAF 2205, significant anisotropy in mr,max is observed between the A- and C-directions. The mr,max is almost two times larger in C-direction compared to the A-direction. Except for type 439, the mr,in is less than 100. Such high values of the mr,in for Type 439 are questionable based on previous experience and confirmed by other experimental results discussed further in this study. The AC measurements in C-direction are also presented in Table 2. The mr,in values for the Type 439 and Sea-Cure 1 0.035 are similar while the mr,in for Sea-Cure 1 0.028 is 10% larger. Although having different wall thickness, this difference in the mr,in
was not expected for the same material at the same C-direction. The mr,in although acquired in stronger fields (vendor equipment limitation) is less than 100 (except for Sea-Cure 1 0.028) compared to the DC measurements in Table 2. The mr,max values for the Type 439 and Sea-Cure tubes are very close (less than 4% difference) to those measured at the DC. The mr,in AC value for SAF 2205 is higher than the one at DC, however, the mr,max value is very close (less than 2% difference) to the one at DC. In summary, the AC measurements compare well to the DC measurements shown in Table 2 at stronger fields. The differential mr,in and mr,max calculated in-house along the initial magnetization curve, across various techniques and field strengths is presented in Table 3 [9]. The differential mr,in DC values are more consistent especially for the type 439 compared to Table 2. The differential mr,in values for Type 439 and Sea-Cure 1 0.035 are closer than those between Sea-Cure 1 0.035 and Sea-Cure 1 0.028. Except for SAF 2205, an anisotropy (10% to 20%) is observed for Type 439 and Sea-Cure tubes in weak fields (Table 3). The anisotropy trend in weak fields is opposite (A-direction has higher permeability than C-direction) to the trend in stronger fields where the mr,max was calculated. Similar to the trend in Table 2, significant anisotropy in differential mr,max is observed (Table 3) between the A- and C-directions for type 439 and Sea-Cure with the C-direction having higher permeability than the A-direction. For steel SAF2205, the trend in the differential mr,max was opposite (A-direction has higher permeability than C-direction) and less pronounced ( 11% difference) compared to other two materials. The differential mr,max values were very close between the AC and DC for the ring specimens (C-direction). For C-direction, the AC differential mr,in were larger (except Sea-Cure 1 0.035) than the DC differential mr,in values. The increase of magnetic permeability at AC was actually expected because it was calculated at stronger fields (60 to 75 A/m) determined by equipment limitations. The data provided in ‘‘DC mr at AC field’’ column in Table 3 support this expectation for SAF 2205 (ideal case). The other three values in the same column confirm the expected trend, although, a bit larger than the measured AC values in the ‘‘AC mr,in‘‘ column. The AC and DC differential mr,in, and the DC differential mr,in calculated at the AC field strengths (‘‘DC mr at AC field’’) had somewhat more variability than the differential mr,max for the AC and DC. It is not clear whether the increased variability was simply due to the different measurement techniques as noted in the standards [6] or optimized measurement procedure (outside of study objectives) for weak fields would provide more consistent results. The parameters in open and closed magnetic circuit obtained at strong saturating fields and along major hysteresis loops are summarized in Table 4 for all four tubes and both A- and
Table 2 Amplitude/peak magnetic measurements in closed magnetic circuit. Tube material
Direction
DC
AC
lr,in
lr,max
lr,in
lr,max
A C
53.1940a 19940
103.591118.5 879392.94 8691293
na 34.39105
na 84.691376
Type 439
A C
242.8940 737940
321.991167.1 3089390.7 6149627 6269609
na 90.9968.8
na 604.99635
Sea-Cure 1 0.035
A C
74.2940 na
317.19785.8 3119393.19 5059528 5249512
na 91971.9
na 524.69526
Sea-Cure 1 0.028
A C
56940 72940
321.69677.6 3409394.08 5679414 5559432
na 101.4964.3
na 542.99457
SAF 2205
a
Magnetic field strength (A/m) at which permeability was measured.
E. Todorov / NDT&E International 48 (2012) 63–69
67
Table 3 DC and AC differential relative magnetic permeability in closed magnetic circuit (in-house calculated based on raw B–H data). Tube material
Direction
DC lr,in
AC lr,in
DC lr at AC field
DC lr,max
AC lr,max
SAF 2205
A C
33.690.5a 33.690.5
na 36.8975
na 36.5975
139.69750 125.29890
na 123.99913
Type 439
A C
95.3910 86.495
na 92.5960
na 97.7960
577.59361 1063.29374
na 1048.49388
Sea-Cure 1 0.035
A C
99.590.5 79.191.5
na 66.9970
na 119.6970
582.39373 934.69285
na 917.19296
Sea-Cure 1 0.028
A C
107.290.5 96.292
na 123.8970
na 160970
563.69352 917.19240
na 923.89254
a
Magnetic field strength (A/m) at which permeability was calculated.
Table 4 VSM and DC closed circuit comparison in strong fields and major hysteresis loops. Tube material
Direction
VSM (in-house calculated)
Axial (A)/ Circ. (C)
Hc (A/m)
Br (mT)
Hs (kA/m)
Ms (kA/m)
Bs (T)
Hc (A/m)
Br (mT)
Hmax (kA/m)
Mmax (kA/m)
Bmax (T)
SAF 2205
A C
868.8 1143
7.118 9.111
477.0 712.9
437.291.01a 430.991.01
1.149 1.437
787.6 916.4
178 165
81.06 42.59
439.091.20 393.892.72
0.654 0.548
Type 439
A C
938.7 866.5
10.12 9.142
780.2 744.8
129291.01 132091.01
2.604 2.595
558.6 391.0
378 581
82.13 42.68
130191.05 128592.21
1.738 1.670
Sea-Cure 1 0.035
A C
923.5 819.4
10.59 9.271
673.2 652.1
100291.01 100591.01
2.106 2.082
497.5 340.0
311 438
82.01 39.70
980.791.05 981.391.94
1.335 1.294
Sea-Cure 1 0.028
A C
683.1 575.2
9.029 7.746
738.3 735.1
103091.01 101791.01
2.223 2.201
440.9 293.1
273 394
82.67 43.00
954.991.05 958.391.90
1.304 1.262
a
DC in closed circuit (vendor reported)
Relative differential magnetic permeability where saturation (Ms) or maximum (Mmax) material magnetization was achieved.
C-directions. The degree of saturation was determined by calculating the differential mr. The coercivity (Hc) and retentivity (Br) were calculated from the major hysteresis loops at saturation (VSM) and maximum (closed circuit) magnetic fields. The magnetic saturation parameters Hs, Bs, and Ms were determined at a differential mr threshold value of 1.01. The closed circuit magnetization (Mmax) measurements in strong fields are very close to (differential mr from 1.05 to 2.72) but still smaller than the saturation magnetization (Ms) achieved with the VSM technique (differential mr of 1.01). Much stronger field intensities (up to 30 times) were required, however, to achieve saturation in the open VSM circuit. The difference between the two techniques in Br is large (up to 64 times) as shown in Table 4. The difference in the coercivity between the open and closed circuit measurements is less dramatic – up to 2.5 times. The material retentivity and coercivity should be measured in closed magnetic circuit if the demagnetization effect in open circuit cannot be eliminated. 3.2. Electrical conductivity measurements with four-terminal circuit The resistance and the electrical conductivity measurements at DC and AC for both lengths and length difference are presented in Table 5 along with the conductivity reference data found in the open literature and sources. The DC resistance uncertainty range is also included in the table. The AC measurements were conducted at four frequencies – 50, 100, 150, and 200 Hz. The AC average resistance and electrical conductivity were calculated as well. The higher frequencies might have provided more accurate AC results; however, the reduced depth of penetration would have reduced the effective tube (conductor) cross section and compromised the validity of Eqs. (7) through (10).
At first look, the AC and DC measurements are very close and compare well with the references. Considering the standard DC technique as more accurate reference, the relative error in most of the cases was 1% or less with some maximum error up to 2.8%. The electrical conductivity data is shown in Fig. 2 to better illustrate the comparison between the AC, DC, and different tube length measurements. The electrical conductivity measurements shown in Fig. 2 indicate small but very consistent differences in L1, L2, and length difference (L1 L2) single and average estimates for all four tubes and both DC and AC methods. One possible source of these consistent variations might be some small errors in tube lengths used to calculate the conductivities in Eq. (8) through (10). Further investigation of these small differences was not conducted. Future efforts might address the sources of these small systematic errors and techniques for error compensation if necessary. All measurements were conducted in laboratory conditions with temperature maintained in the range from 26 1C to 27 1C to eliminate any temperature effects. The entire set of measurements was repeated over a month period to verify whether uncontrollable time related factors would interfere with the measurements repeatability. The measurements were very consistent and repeatable with variability in the expected range observed (Fig. 2) for a single time interval (3- to 4-h duration) needed to measure the conductivity of all four tubes. It is worth noting, that the electrical resistance measured in this study was close to the limits of the impedance analyzer HP4294A working with the 4 TP ‘‘user-fabricated’’ fixture. The AC measurement error estimates were not possible to obtain using the HP4294A operation manual procedures and plots. The electrical conductivity values found in the literature [3–5] were not suitable for reliable error estimates either because information was not
68
E. Todorov / NDT&E International 48 (2012) 63–69
Table 5 DC and AC resistance and conductivity. Tube material
Method
635-(mm) Length (L1)
381-(mm) Length (L2)
L1 L2
Reference
Resistance (mX)
Conductivity (MS/m)
Resistance (mX)
Conductivity (MS/m)
Conductivity (MS/m)
Conductivity (MS/m)
SAF 2205
DC 50 Hz 100 Hz 150 Hz 200 Hz AC average
5.6707 0.038 5.700 5.737 5.803 5.749 5.747
1.281 1.274 1.266 1.252 1.264 1.264
3.3957 0.027 3.368 3.447 3.481 3.409 3.426
1.284 1.294 1.264 1.252 1.279 1.272
1.277 1.246 1.269 1.251 1.242 1.252
1.353 [3]
Type 439
DC 50 Hz 100 Hz 150 Hz 200 Hz AC average
9.7207 0.059 9.839 9.759 9.817 9.835 9.813
1.679 1.658 1.672 1.662 1.659 1.663
5.8107 0.039 5.886 5.814 5.898 5.895 5.873
1.685 1.663 1.684 1.660 1.661 1.667
1.669 1.651 1.654 1.665 1.656 1.657
1.587 [4]
Sea-Cure 1 0.035
DC 50 Hz 100 Hz 150 Hz 200 Hz AC average DC 50 Hz 100 Hz 150 Hz 200 Hz AC average
7.3107 0.047 7.416 7.299 7.424 7.341 7.370 9.3707 0.057 9.429 9.313 9.458 9.349 9.387
1.385 1.365 1.387 1.363 1.379 1.373 1.401 1.393 1.410 1.388 1.405 1.399
4.3757 0.032 4.477 4.337 4.446 4.350 4.403 5.6137 0.038 5.663 5.567 5.692 5.580 5.625
1.388 1.356 1.400 1.366 1.396 1.380 1.404 1.391 1.415 1.384 1.412 1.401
1.379 1.378 1.367 1.360 1.353 1.364 1.398 1.395 1.402 1.395 1.393 1.396
1.25 [5]
1.30
SIGac L1
1.28
SIGac L2 SIGac L1-L2
1.26
AvrSIGac L1 AvrSIGac L2 AvrSIGac L1-L2 SIGdc L1 SIGdc L2
1.24
1.70 Conductivity, MS/m
Conductivity, MS/m
Sea-Cure 1- 0.028
SIGdc L1-L2
1.22 0
50
100
150
200
SIGac L1 SIGac L2 SIGac L1-L2 AvrSIGac L1
1.68
AvrSIGac L2 AvrSIGac L1-L2
1.66
SIGdc L1 SIGdc L2 SIGdc L1-L2
1.64 0
250
50
SIGac L2 1.40
SIGac L1-L2 AvrSIGac L1
1.38
AvrSIGac L2 AvrSIGac L1-L2 SIGdc L1
1.36
SIGdc L2 SIGdc L1-L2
1.34 100
150
200
250
1.42
SIGac L1
200
250
Frequency, Hz
Conductivity, MS/m
Conductivity, MS/m
1.42
50
150
Frequency, Hz
Frequency, Hz
0
100
SIGac L1 SIGac L2
1.40
SIGac L1-L2
1.38
AvrSIGac L1 AvrSIGac L2 AvrSIGac L1-L2 SIGdc L1
1.36
SIGdc L2 SIGdc L1-L2
1.34 0
50
100
150
200
250
Frequency, Hz
Fig. 2. Comparison of DC and AC electrical conductivity measurements. (SIGac L1, SIGac L2, SIGac L1–L2, SIGdc L1, SIGdc L2 and SIGdc L1 L2 – AC and DC conductivity with length L1, L2 and L1– L2; AvrSIGac L1, AvrSIGac L2 and AvrSIGac L1 L2 –average AC conductivity for same lengths). (a) SAF 2205 Tube. AC and DC Conductivity measurements. (b) Type 439 Tube. AC and DC Conductivity Measurements. (c) Sea-Cure 1x0.035 Tube. AC and DC Conductivity Measurements. (d) Sea-Cure 1x0.028 Tube. AC and DC Conductivity Measurements.
available on measurement techniques and procedures used to obtain the data. The AC error estimates quoted in this study were obtained by comparing the AC to the DC (reference) measurements while the DC measurement uncertainty (Table 5) was obtained from the Micro-Ohmmeter Model 5600 operation manual. The
small difference between the AC and DC measurements indicated that the design of the custom fixture and the adequate measurement procedures proved very effective in reducing the effect of all sources of possible errors and allowed obtaining of reliable and accurate data.
E. Todorov / NDT&E International 48 (2012) 63–69
4. Conclusions The following conclusions can be made from this study:
Unique and not previously reported magnetic and electrical
properties were acquired at AC and DC excitation for heat exchanger tubes made of SAF 2205, type 439 and Sea-Cure stainless steels. Measurable and significant (Type 439 and Sea-Cure) anisotropy of maximum relative magnetic permeability in A- versus C- direction was observed and reported for the first time. The initial relative magnetic permeability measurements had larger variability than expected. Standard procedures for magnetic property measurements need to be modified (depending on tested material) to increase the reliability of the measurement process when working in weak magnetic fields.
Unexpected differences requiring further investigation were observed between the magnetic properties of the two Sea-Cure tubes with different nominal tube wall thicknesses 0.889 mm (0.035 in.) and 0.7112 mm (0.028 in.).
Acknowledgments Major portion of this study was supported by the Department of Energy under Award Number DE-NE0000279, through the Nuclear Fabrication Consortium (NFC) managed by Edison Welding Institute (EWI). The Electric Power Research Institute (EPRI) co-
69
sponsored the study by providing tube specimens and funding. The author would like to acknowledge the contributions of Kenji Krzywosz (EPRI), Nate Ames (NFC) and Steve Levesque (EWI) for the management support and feedback during the project. The author is also grateful to Greg Umana from KJS Associates Div., Magnetic Instrumentation (MI) Inc. for the patience and diligence in conducting the primary magnetic measurements. References [1] Recommended practice 6: The use of modeling in inspection qualification, Issue 1, European Network for Inspection Qualification (ENIQ), Report No 15. EUR 19017EN. Brussels-Luxembourg: European Commission, 1999. [2] Doctor R. Nuclear power plant NDE challenges – past, present and future, Review of progress in quantitative nondestructive evaluation. AIP 2006;26A:17–31. [3] /http://www.smt.sandvik.com/sandvik/tubeS, Sandvik SAF 2205 duplex stainless steel data sheet, Accessed May 4, 2011. [4] Washko S, Aggen G. Wrought stainless steels. Specialty steels and heatresistant alloys. In: Lampman SR, Zorc TB, editors. Metals Handbook, Vol. 1. Properties and Selection: Irons, Steels, and High Performance Alloys. 10th edn. Materials Park, OH, USA: ASM International; 1990. p. 871. technical. [5] /http://www.sulphuric-acid.com/TechManual/Materials/materials_metals_ cronifer2803mo.htmS, Accessed May 4, 2011. [6] ASTM A 773/A 773M – 01, Standard test method for DC magnetic properties of materials using ring and permeameter procedures with dc electronic hysteresigraphs, ASTM International. [7] ASTM A 927/A 927M – 04, Standard test method for alternating-current magnetic properties of toroidal core specimens using the voltmeter– ammeter–wattmeter method, ASTM International. [8] /http://www.amphenolrf.com/products/CatalogPages/BNC.pdfS. [9] Todorov E, Levesque S, Ames N, Krzywosz K. Measurement of Magnetic Properties of Ferromagnetic Tubes for Heat Exchangers, 104. Hollywood, FL, USA: Summary abstract in Transactions of the American Nuclear Society; 2011 pp. 297–298.
Contents lists available at SciVerse ScienceDirect
NDT&E International journal homepage: www.elsevier.com/locate/ndteint
Measurement of electromagnetic properties of heat exchanger tubes Evgueni Todorov n EWI, 1250 Arthur E. Adams Dr., Columbus, OH 43221, USA
a r t i c l e i n f o
abstract
Article history: Received 30 August 2011 Received in revised form 20 February 2012 Accepted 23 February 2012 Available online 5 March 2012
Electromagnetic properties of heat exchanger tubes made of SAF 2205, Type 439 and Sea-Cure stainless steels were determined with magnetic and electrical techniques. The standard magnetic measurements were conducted at direct (DC) and alternate (AC – 200 Hz) current excitation. The magnetic permeability, coercivity, retentivity, and other magnetic parameters were measured in tube axial and circumferential direction, in open and closed magnetic circuits and in wide range of magnetic field strengths from 10 A/m to 82 kA/m. The electrical conductivity was determined with custom-designed fixture in axial direction only at DC and AC (50 – 200 Hz). The unique set of results revealed that a certain anisotropy of properties existed between the two geometric directions. The agreement between the measurements from the different techniques was good where comparison was possible. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Eddy current testing Heat exchanger tubes Electrical conductivity Magnetic permeability Steel electromagnetic properties
1. Introduction
Tube 1. Duplex stainless steel SAF 2205: Actual 19.028-mm outside diameter (OD) by 1.596-mm wall thickness (WT). Nominal 0.75-in. OD by 0.065-in. WT. Tube 2. Type 439 stainless steel: J Actual 15.778-mm OD by 0.829-mm WT. Nominal 0.625-in. OD by 0.035-in. WT. Tube 3. Sea-Cure: J Actual 25.283-mm OD by 0.816-mm WT. Nominal 1-in. OD by 0.035-in. WT. Specimen designation ‘‘Sea-Cure 1 0.035’’. Tube 4. Sea-Cure: J Actual 25.208-mm OD by 0.626-mm WT. Nominal 1-in. OD by 0.028-in. WT. Specimen designation ‘‘Sea-Cure 1 0.028’’. J
Important part of any modern structural integrity program is the use of nondestructive evaluation (NDE) techniques to ensure that fabrication and service-induced flaws and conditions would not affect the safety and service life of critical components such as airframes, jet engines, pipelines, bridges, heat exchangers, rail cars, and others. In addition to NDE personnel experience and training, performance demonstration studies are conducted with large number of representative flaws and flaw conditions to quantify the performance of NDE techniques in terms of probability of detection (POD) and flaw sizing accuracy. The practical trials with specimens are very expensive and time consuming. Computer modeling has been suggested to optimize the NDE technique performance and reduce the number of physical specimens and trials as part of the so-called technical justification [1]. The extensive use of computer modeling approach for NDE is also envisioned for the new generation of nuclear reactors and heat exchangers [2]. Good knowledge of material properties is required, however, to build representative computer models. The use of new ferritic and duplex stainless steel (both magnetic materials) tubing for heat exchangers provided opportunity to consider the computer modeling approach for optimization of inservice NDE eddy current techniques and procedures. The industry was interested in investigating tubes made of three new ferromagnetic materials. Four tubes with length 914.4 mm (3 ft) were submitted for testing as follows:
n
Tel.: þ1 6146885268; fax: þ1 6146885001. E-mail address: [email protected]
0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2012.02.007
The material chemical composition (Table 1) was obtained on samples cut from each tube. It complies with the standard specifications provided in Table 1 for reference. As already discussed, knowledge of material properties is needed to build representative computer models. For eddy current technique modeling, reliable and accurate estimates of tube electrical conductivity, magnetic permeability and other magnetic parameters are required. While some general reference data for the tube electrical conductivity was available in the public domain literature [3–5], the magnetic properties were unknown. Then, the first step in implementing the modeling approach was to reliably determine the electrical conductivity and initial magnetic permeability with computer models and other independent techniques. Further, the initial objective was to determine these two electromagnetic parameters with nondestructive model-based eddy current technique where the independent magnetic and electrical conventional destructive techniques would be used to validate the eddy current measurement models. It was later
64
E. Todorov / NDT&E International 48 (2012) 63–69
to amplitudes.
Table 1 Chemical composition (%). Element
Carbon (C) Manganese (Mn) Phosphorus (P) Sulfur (S) Slicon (Si) Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Nitrogen (N) Aluminum (Al) Titanium (Ti) Iron (Fe) a
SAF 2205 (UNS Type 439 (UNS S31803/ S43035) S32205a)
Sea-Cure Sea-Cure 1 0.035 (UNS 1 0.028 S44660) (UNS S44660)
0.018 1.38
0.022 0.285
0.028 0.302
0.031 0.405
0.024
0.020
0.026
0.025
o 0.001 0.37 22.4
0.004 0.367 17.2
0.006 0.404 26.3
0.004 0.424 25.8
5.61 3.18
0.157 –
1.94 3.71
1.87 3.74
0.18 –
0.0084 0.014
0.018 –
0.015 –
– Balance
0.333 Balance
0.096 Balance
0.168 Balance
realized that with little additional efforts, the independent magnetic measurements would provide significantly more data especially at stronger magnetic fields and along different directions of the tube geometry which would be of value to the future inspection and modeling efforts. Thus, the results from the comprehensive tube characterization with magnetic and electrical independent techniques is presented and discussed in this paper.
2. Methods and theory 2.1. Magnetic measurements
ð1Þ
where B – magnetic flux density or magnetic induction in Tesla (T), m0 ¼ 4p107 – magnetic permeability (constant) of free space or air in Henry/meter (H/m), H – magnetic filed strength or intensity in ampere/meter (A/m), M – material magnetization in A/m, I – magnetic polarization in T, and mr – relative magnetic permeability (dimensionless). The relative magnetic permeability depends on the magnetic field strength H. There are many specific types of the relative magnetic permeability used to describe material magnetic property behavior for various practical purposes. The amplitude or peak relative magnetic permeability is frequently quoted and shown in Eq. (2). The amplitude or peak values of B and H are used to calculate it. B
m0 H
DB
ð3Þ
m0 DH
B H þ 4pM
ð4Þ
where B is expressed in Gauss (G), H – in Oersted (Oe), M¼ I – in electromagnetic unit/cubic centimeter (emu/cm3), and m – magnetic permeability in G/Oe. The magnetic permeability expressed in cgs–emu units in Eq. (4) is equal in value to the relative magnetic permeability equations (2) or (3) as applicable. Except for the original vendor reports, all magnetic measurement data was converted, processed and reported in SI units. Equations to convert cgs–emu into SI units can be found elsewhere in the open literature. Sometime, magnetic measurements are performed in open magnetic circuit where the magnetic field strength in the sample Hin is not equal to the external magnetic field Hext. It is then said that the external magnetic field in the sample is reduced by a demagnetizing field Hdm created by the magnetic poles at the sample ends [Eq. (5)]. Hin ¼ Hext Hdm
Two major sets of magnetic measurements in closed and open magnetic circuit were conducted to determine the magnetic permeability, residual magnetization, coercivity, retentivity, and others. The governing equation (in International System of Units – SI) expressing the magnetization of a magnetic material when placed in external magnetic filed is shown below:
mrampl ¼
dB
m0 dH
The differential relative magnetic permeability is used to describe material behavior when relatively weak alternating magnetic field is superimposed on strong permanent biasing magnetizing field. The differential relative magnetic permeability is also used to monitor the degree of saturation in strong fields where it approaches 1. The amplitude and differential relative magnetic permeabilities are identical in weak field when the material is magnetized in the linear portion of the initial magnetization curve. Some of the vendor magnetic measurements in this study were reported in centimeter–gram–second (cgs–emu) system of units. Eq. (4) is used to calculate the material magnetic permeability in cgs–emu system:
m¼
Material standard specification for reference.
B ¼ m0 H þ m0 M ¼ m0 H þI ¼ m0 mr H,
mrdif f ¼
ð2Þ
Another type of relative magnetic permeability is the so called differential relative magnetic permeability shown in Eq. (3). It is expressed as a first derivative of B–H curves (initial and hysteresis loops) or ratio of small increments of B and H as opposed
ð5Þ
The external magnetic field strength can only be measured. For cases where the demagnetization is strong (e.g., open magnetic circuit), a relative magnetic permeability of the sample shape is defined as shown in Eq. (6) which is smaller than the material (or intrinsic) relative magnetic permeability:
mrshape ¼
B
m0 Hext
o
B
m0 Hin
¼
B
m0 ðHext NMÞ
¼ mrmaterial
ð6Þ
The demagnetizing factor N (Eq. (6)) is dimensionless and constant in the linear portion of the initial magnetization curve. It depends on the magnetic field strength if magnetization is conducted in the non-linear portion of the initial magnetization curve. The problem with the open circuit magnetic measurements is that the demagnetizing factor N is rarely known or it is difficult to estimate and measurement of material magnetic permeability is impossible. Magnetic measurements in closed magnetic circuits (N ¼0) or with significantly reduced demagnetization (localized magnetization) are used to obtain the material properties. 2.1.1. Vibrating sample magnetometer (VSM) method. Open magnetic circuit A magnetic material specimen is placed in a constant magnetic field. The specimen is magnetized and magnetic poles are formed. The specimen is then vibrated causing change in the magnetic field surrounding the specimen. The changes of the magnetic field surrounding the specimen are sensed with coils in all three directions in space. The signals in the sensor coils will depend on the magnetic properties of the material and the specimen shape. Thus, the magnetic properties of the specimen (and material) can
E. Todorov / NDT&E International 48 (2012) 63–69
be estimated in different direction of magnetization. The VSM measurements are conducted in open magnetic circuit. Four half-ring-shaped specimens with height approximately 25.4 mm were cut from each tube for this test. The vendor cut a rectangular shape specimen 4.75 4.75 mm from each halfring to conduct the test. The measurements were acquired in two different directions – axially (A) along the tube length and circumferentially (C) along the tube circumference. The vendor used an ADE/DMS Model 880 Vibrating Sample Magnetometer manufactured by MicroSense, LLC. 2.1.2. Standard DC and AC methods. closed magnetic circuit The DC and the AC measurements in closed magnetic circuit are the standard [6,7] methods for determining the magnetic properties of the materials. Eight specimens – four short tubes/bars (75-mm length) and four rings (6-mm height) were cut from each of the tubes to conduct the A- and C-direction measurements. An electrical discharge machining (EDM) cutting process was used to provide very tight tolerance in the range of 70.05 mm for the length and parallel end-surfaces. The EDM cutting was conducted in such a way that no effect on magnetic measurements was anticipated due to the machining. The short tubes and rings dimensions were measured with precision mechanical gauges before the magnetic measurements were initiated. Each ring sample has a primary (H) and secondary (B) coil wound with 24AWG and 30AWG magnet wire, respectively. A secondary (B) coil only was wound on each short tube with 30AWG magnet wire over insulating tape with thickness approximately 0.1 mm. The short tube was then placed in a KJS Associates Model YOKE-100 electromagnet and clamped into a closed magnetic test circuit. A calibrated Hall probe was placed at the surface of the coil to measure the applied field strength H. The search coil winding was connected to the system fluxmeter to determine the flux density in the sample. All measurements were made with a KJS Associates, Inc. Model SMT-700 computer-automated soft magnetic hysteresigraph system. Two measurement sets – DC and AC excitation were acquired with the rings samples, while only DC measurements were acquired with the short tube specimens.
65
the two lengths was calculated [Eq. (10)].
sd ¼
2ðL1 L2 Þ
ð10Þ
pðR1 R2 ÞWTðOD þ IDÞ
The purpose of this third conductivity value was to identify whether the resistance measurements had a systematic error. If a systematic error existed, it would cause a significant difference between the sd and the other two conductivities. Eqs. (7) through (10) are based on the assumptions that the current density along the tube tested length and through the tube cross section is uniform (no ‘‘skin effect’’), the conductor cross-section is uniform along the conductor length and the tube resistivity or conduc tivity is uniform along the conductor length and through the cross-section. Based on literature references [3–5], the resistance of the tubes was expected to be in the range from 5 to 15 mO. For comparison, the resistance of a center contact in a typical widely popular connector pair (plug-to-jack) as BNC is approximately 1.5 mO [8]. These are extremely low values that pose unique challenges to the measurement technique, circuit and instrumentation. Initially, a 4-terminal pair (4 TP) AC configuration with precision impedance analyzer was selected to eliminate the effects of the contact resistance and thermal electromotive forces. Unfortunately, an off-the-shelf fixture to implement this measurement technique was not commercially available. A special fixture was designed and built shown in Fig. 1. The AC measurements, however, were slow and very demanding in terms of calibration and implementation. Conventional DC measurements were later considered and conducted using the same fixture and a classical 4-terminal (4 T) Kelvin circuit. The DC measurements were fast and very consistent. All of the contact DC and AC electrical conductivity measurements were conducted in A-direction only. A Micro-Ohmmeter Model 5600 (Chauvin Arnoux, Inc.) was used for the DC while an Impedance Analyzer HP 4294 A (Agilent) was used for the AC measurements as shown in Fig. 1.
3. Results and discussion 3.1. Magnetic measurements in open and closed magnetic circuit
2.2. Electrical measurements: four-terminal circuit The material resistivity or conductivity is another parameter that affects the nondestructive testing of the materials with the electromagnetic or eddy current techniques. The main formula that links a material electrical conductivity or resistivity with the conductor length and cross section is shown in Eq. (7):
s¼
1
r
¼
L , RS
ð7Þ
where s – electrical conductivity in Siemens/meter (S/m), r – electrical resistivity in Ohm meter (Om), L – conductor length in meter (m), and S – conductor cross sectional area in m2. Further, two lengths of each tube were measured – L1 and L2. The electrical conductivities s1 and s2 for each of the lengths were calculated using formulas (8) and (9):
s1 ¼ s2 ¼
2L1
,
ð8Þ
2L2 , pR2 WTðOD þ IDÞ
ð9Þ
pR1 WTðOD þ IDÞ
Two sets of data were acquired with the VSM in open magnetic circuit for each tube and direction – major hysteresis loops and initial magnetization curves (16 raw data files). As previously discussed, the data represented the magnetic properties of the specimen shape for field strengths lower than the saturation levels. The material magnetic properties were not possible to determine due to the strong demagnetizing effect. The vendor conducted a second set of measurements (additional 16 raw data files) in an attempt to estimate the demagnetization factor N and correct the magnetic field strength as shown in Eq. (6). The raw data files from the second set were processed in-house to determine the initial and the maximum relative magnetic permeability. The data was approximated (smoothed) using a second and
Micro-Ohmmeter
Impedance Analyzer
Fixture
where R1 and R2 – measured resistances in O, WT – tube wall thickness in m, OD – outer diameter in m, and ID – inner diameter in m. In addition, the conductivity sd for the difference between
Fig. 1. Equipment arrangement for DC and AC resistance measurements.
66
E. Todorov / NDT&E International 48 (2012) 63–69
a third order polynomials followed by analytical differentiation to determine the differential relative magnetic permeability at different field strengths. The estimates were inconsistent, scattered, and inaccurate compared to the more accurate closed magnetic circuit measurements. A discussion with the VSM manufacturer confirmed that the procedure used by the VSM vendor to estimate the demagnetization factor was not suitable for this type of specimens. In summary, the VSM data in weak and medium fields was not adequate to determine the material magnetic properties due to the strong demagnetization in open magnetic circuit. However, the VSM data (first set) acquired at very strong saturating field (no demagnetization) was actually comparing very well to the closed circuit measurements discussed later. A total of 36 data sets were acquired in closed magnetic circuit with DC and AC excitation in both directions of magnetization. The measurements were first conducted to cover the entire range from the weak to the maximum field strength of 75 kA/m (tubes) and 35 kA/m (rings). The magnetic properties of the materials in medium and strong fields were well defined. The magnetic properties in weak fields, however, were difficult to obtain due to the lack of resolution (small number of data points). Subsequent data sets were acquired at reduced maximum field strengths to improve the resolution at weak field strengths. The AC measurements were acquired at two excitation frequencies – 200 Hz and 15 kHz. The relative magnetic permeability measurements versus H at 15 kHz (not discussed in this paper) were unreliable due to the unaccounted and strong effect of the induced eddy currents in the specimens. A similar procedure as discussed above was used to smooth the experimental measurements and calculate the differential relative magnetic permeability at different field strengths. The amplitude or peak [Eq. (2)] mr,in and mr,max measurements extracted from different data sets are summarized in Table 2. The permeability values are reported along with the H field at which the permeability was calculated. Regarding the DC measurements, the mr,max for SAF 2205 tube is smaller compared to the other three tubes. The mr,max is similar for 439 and Sea-Cure tubes. Except for the steel SAF 2205, significant anisotropy in mr,max is observed between the A- and C-directions. The mr,max is almost two times larger in C-direction compared to the A-direction. Except for type 439, the mr,in is less than 100. Such high values of the mr,in for Type 439 are questionable based on previous experience and confirmed by other experimental results discussed further in this study. The AC measurements in C-direction are also presented in Table 2. The mr,in values for the Type 439 and Sea-Cure 1 0.035 are similar while the mr,in for Sea-Cure 1 0.028 is 10% larger. Although having different wall thickness, this difference in the mr,in
was not expected for the same material at the same C-direction. The mr,in although acquired in stronger fields (vendor equipment limitation) is less than 100 (except for Sea-Cure 1 0.028) compared to the DC measurements in Table 2. The mr,max values for the Type 439 and Sea-Cure tubes are very close (less than 4% difference) to those measured at the DC. The mr,in AC value for SAF 2205 is higher than the one at DC, however, the mr,max value is very close (less than 2% difference) to the one at DC. In summary, the AC measurements compare well to the DC measurements shown in Table 2 at stronger fields. The differential mr,in and mr,max calculated in-house along the initial magnetization curve, across various techniques and field strengths is presented in Table 3 [9]. The differential mr,in DC values are more consistent especially for the type 439 compared to Table 2. The differential mr,in values for Type 439 and Sea-Cure 1 0.035 are closer than those between Sea-Cure 1 0.035 and Sea-Cure 1 0.028. Except for SAF 2205, an anisotropy (10% to 20%) is observed for Type 439 and Sea-Cure tubes in weak fields (Table 3). The anisotropy trend in weak fields is opposite (A-direction has higher permeability than C-direction) to the trend in stronger fields where the mr,max was calculated. Similar to the trend in Table 2, significant anisotropy in differential mr,max is observed (Table 3) between the A- and C-directions for type 439 and Sea-Cure with the C-direction having higher permeability than the A-direction. For steel SAF2205, the trend in the differential mr,max was opposite (A-direction has higher permeability than C-direction) and less pronounced ( 11% difference) compared to other two materials. The differential mr,max values were very close between the AC and DC for the ring specimens (C-direction). For C-direction, the AC differential mr,in were larger (except Sea-Cure 1 0.035) than the DC differential mr,in values. The increase of magnetic permeability at AC was actually expected because it was calculated at stronger fields (60 to 75 A/m) determined by equipment limitations. The data provided in ‘‘DC mr at AC field’’ column in Table 3 support this expectation for SAF 2205 (ideal case). The other three values in the same column confirm the expected trend, although, a bit larger than the measured AC values in the ‘‘AC mr,in‘‘ column. The AC and DC differential mr,in, and the DC differential mr,in calculated at the AC field strengths (‘‘DC mr at AC field’’) had somewhat more variability than the differential mr,max for the AC and DC. It is not clear whether the increased variability was simply due to the different measurement techniques as noted in the standards [6] or optimized measurement procedure (outside of study objectives) for weak fields would provide more consistent results. The parameters in open and closed magnetic circuit obtained at strong saturating fields and along major hysteresis loops are summarized in Table 4 for all four tubes and both A- and
Table 2 Amplitude/peak magnetic measurements in closed magnetic circuit. Tube material
Direction
DC
AC
lr,in
lr,max
lr,in
lr,max
A C
53.1940a 19940
103.591118.5 879392.94 8691293
na 34.39105
na 84.691376
Type 439
A C
242.8940 737940
321.991167.1 3089390.7 6149627 6269609
na 90.9968.8
na 604.99635
Sea-Cure 1 0.035
A C
74.2940 na
317.19785.8 3119393.19 5059528 5249512
na 91971.9
na 524.69526
Sea-Cure 1 0.028
A C
56940 72940
321.69677.6 3409394.08 5679414 5559432
na 101.4964.3
na 542.99457
SAF 2205
a
Magnetic field strength (A/m) at which permeability was measured.
E. Todorov / NDT&E International 48 (2012) 63–69
67
Table 3 DC and AC differential relative magnetic permeability in closed magnetic circuit (in-house calculated based on raw B–H data). Tube material
Direction
DC lr,in
AC lr,in
DC lr at AC field
DC lr,max
AC lr,max
SAF 2205
A C
33.690.5a 33.690.5
na 36.8975
na 36.5975
139.69750 125.29890
na 123.99913
Type 439
A C
95.3910 86.495
na 92.5960
na 97.7960
577.59361 1063.29374
na 1048.49388
Sea-Cure 1 0.035
A C
99.590.5 79.191.5
na 66.9970
na 119.6970
582.39373 934.69285
na 917.19296
Sea-Cure 1 0.028
A C
107.290.5 96.292
na 123.8970
na 160970
563.69352 917.19240
na 923.89254
a
Magnetic field strength (A/m) at which permeability was calculated.
Table 4 VSM and DC closed circuit comparison in strong fields and major hysteresis loops. Tube material
Direction
VSM (in-house calculated)
Axial (A)/ Circ. (C)
Hc (A/m)
Br (mT)
Hs (kA/m)
Ms (kA/m)
Bs (T)
Hc (A/m)
Br (mT)
Hmax (kA/m)
Mmax (kA/m)
Bmax (T)
SAF 2205
A C
868.8 1143
7.118 9.111
477.0 712.9
437.291.01a 430.991.01
1.149 1.437
787.6 916.4
178 165
81.06 42.59
439.091.20 393.892.72
0.654 0.548
Type 439
A C
938.7 866.5
10.12 9.142
780.2 744.8
129291.01 132091.01
2.604 2.595
558.6 391.0
378 581
82.13 42.68
130191.05 128592.21
1.738 1.670
Sea-Cure 1 0.035
A C
923.5 819.4
10.59 9.271
673.2 652.1
100291.01 100591.01
2.106 2.082
497.5 340.0
311 438
82.01 39.70
980.791.05 981.391.94
1.335 1.294
Sea-Cure 1 0.028
A C
683.1 575.2
9.029 7.746
738.3 735.1
103091.01 101791.01
2.223 2.201
440.9 293.1
273 394
82.67 43.00
954.991.05 958.391.90
1.304 1.262
a
DC in closed circuit (vendor reported)
Relative differential magnetic permeability where saturation (Ms) or maximum (Mmax) material magnetization was achieved.
C-directions. The degree of saturation was determined by calculating the differential mr. The coercivity (Hc) and retentivity (Br) were calculated from the major hysteresis loops at saturation (VSM) and maximum (closed circuit) magnetic fields. The magnetic saturation parameters Hs, Bs, and Ms were determined at a differential mr threshold value of 1.01. The closed circuit magnetization (Mmax) measurements in strong fields are very close to (differential mr from 1.05 to 2.72) but still smaller than the saturation magnetization (Ms) achieved with the VSM technique (differential mr of 1.01). Much stronger field intensities (up to 30 times) were required, however, to achieve saturation in the open VSM circuit. The difference between the two techniques in Br is large (up to 64 times) as shown in Table 4. The difference in the coercivity between the open and closed circuit measurements is less dramatic – up to 2.5 times. The material retentivity and coercivity should be measured in closed magnetic circuit if the demagnetization effect in open circuit cannot be eliminated. 3.2. Electrical conductivity measurements with four-terminal circuit The resistance and the electrical conductivity measurements at DC and AC for both lengths and length difference are presented in Table 5 along with the conductivity reference data found in the open literature and sources. The DC resistance uncertainty range is also included in the table. The AC measurements were conducted at four frequencies – 50, 100, 150, and 200 Hz. The AC average resistance and electrical conductivity were calculated as well. The higher frequencies might have provided more accurate AC results; however, the reduced depth of penetration would have reduced the effective tube (conductor) cross section and compromised the validity of Eqs. (7) through (10).
At first look, the AC and DC measurements are very close and compare well with the references. Considering the standard DC technique as more accurate reference, the relative error in most of the cases was 1% or less with some maximum error up to 2.8%. The electrical conductivity data is shown in Fig. 2 to better illustrate the comparison between the AC, DC, and different tube length measurements. The electrical conductivity measurements shown in Fig. 2 indicate small but very consistent differences in L1, L2, and length difference (L1 L2) single and average estimates for all four tubes and both DC and AC methods. One possible source of these consistent variations might be some small errors in tube lengths used to calculate the conductivities in Eq. (8) through (10). Further investigation of these small differences was not conducted. Future efforts might address the sources of these small systematic errors and techniques for error compensation if necessary. All measurements were conducted in laboratory conditions with temperature maintained in the range from 26 1C to 27 1C to eliminate any temperature effects. The entire set of measurements was repeated over a month period to verify whether uncontrollable time related factors would interfere with the measurements repeatability. The measurements were very consistent and repeatable with variability in the expected range observed (Fig. 2) for a single time interval (3- to 4-h duration) needed to measure the conductivity of all four tubes. It is worth noting, that the electrical resistance measured in this study was close to the limits of the impedance analyzer HP4294A working with the 4 TP ‘‘user-fabricated’’ fixture. The AC measurement error estimates were not possible to obtain using the HP4294A operation manual procedures and plots. The electrical conductivity values found in the literature [3–5] were not suitable for reliable error estimates either because information was not
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E. Todorov / NDT&E International 48 (2012) 63–69
Table 5 DC and AC resistance and conductivity. Tube material
Method
635-(mm) Length (L1)
381-(mm) Length (L2)
L1 L2
Reference
Resistance (mX)
Conductivity (MS/m)
Resistance (mX)
Conductivity (MS/m)
Conductivity (MS/m)
Conductivity (MS/m)
SAF 2205
DC 50 Hz 100 Hz 150 Hz 200 Hz AC average
5.6707 0.038 5.700 5.737 5.803 5.749 5.747
1.281 1.274 1.266 1.252 1.264 1.264
3.3957 0.027 3.368 3.447 3.481 3.409 3.426
1.284 1.294 1.264 1.252 1.279 1.272
1.277 1.246 1.269 1.251 1.242 1.252
1.353 [3]
Type 439
DC 50 Hz 100 Hz 150 Hz 200 Hz AC average
9.7207 0.059 9.839 9.759 9.817 9.835 9.813
1.679 1.658 1.672 1.662 1.659 1.663
5.8107 0.039 5.886 5.814 5.898 5.895 5.873
1.685 1.663 1.684 1.660 1.661 1.667
1.669 1.651 1.654 1.665 1.656 1.657
1.587 [4]
Sea-Cure 1 0.035
DC 50 Hz 100 Hz 150 Hz 200 Hz AC average DC 50 Hz 100 Hz 150 Hz 200 Hz AC average
7.3107 0.047 7.416 7.299 7.424 7.341 7.370 9.3707 0.057 9.429 9.313 9.458 9.349 9.387
1.385 1.365 1.387 1.363 1.379 1.373 1.401 1.393 1.410 1.388 1.405 1.399
4.3757 0.032 4.477 4.337 4.446 4.350 4.403 5.6137 0.038 5.663 5.567 5.692 5.580 5.625
1.388 1.356 1.400 1.366 1.396 1.380 1.404 1.391 1.415 1.384 1.412 1.401
1.379 1.378 1.367 1.360 1.353 1.364 1.398 1.395 1.402 1.395 1.393 1.396
1.25 [5]
1.30
SIGac L1
1.28
SIGac L2 SIGac L1-L2
1.26
AvrSIGac L1 AvrSIGac L2 AvrSIGac L1-L2 SIGdc L1 SIGdc L2
1.24
1.70 Conductivity, MS/m
Conductivity, MS/m
Sea-Cure 1- 0.028
SIGdc L1-L2
1.22 0
50
100
150
200
SIGac L1 SIGac L2 SIGac L1-L2 AvrSIGac L1
1.68
AvrSIGac L2 AvrSIGac L1-L2
1.66
SIGdc L1 SIGdc L2 SIGdc L1-L2
1.64 0
250
50
SIGac L2 1.40
SIGac L1-L2 AvrSIGac L1
1.38
AvrSIGac L2 AvrSIGac L1-L2 SIGdc L1
1.36
SIGdc L2 SIGdc L1-L2
1.34 100
150
200
250
1.42
SIGac L1
200
250
Frequency, Hz
Conductivity, MS/m
Conductivity, MS/m
1.42
50
150
Frequency, Hz
Frequency, Hz
0
100
SIGac L1 SIGac L2
1.40
SIGac L1-L2
1.38
AvrSIGac L1 AvrSIGac L2 AvrSIGac L1-L2 SIGdc L1
1.36
SIGdc L2 SIGdc L1-L2
1.34 0
50
100
150
200
250
Frequency, Hz
Fig. 2. Comparison of DC and AC electrical conductivity measurements. (SIGac L1, SIGac L2, SIGac L1–L2, SIGdc L1, SIGdc L2 and SIGdc L1 L2 – AC and DC conductivity with length L1, L2 and L1– L2; AvrSIGac L1, AvrSIGac L2 and AvrSIGac L1 L2 –average AC conductivity for same lengths). (a) SAF 2205 Tube. AC and DC Conductivity measurements. (b) Type 439 Tube. AC and DC Conductivity Measurements. (c) Sea-Cure 1x0.035 Tube. AC and DC Conductivity Measurements. (d) Sea-Cure 1x0.028 Tube. AC and DC Conductivity Measurements.
available on measurement techniques and procedures used to obtain the data. The AC error estimates quoted in this study were obtained by comparing the AC to the DC (reference) measurements while the DC measurement uncertainty (Table 5) was obtained from the Micro-Ohmmeter Model 5600 operation manual. The
small difference between the AC and DC measurements indicated that the design of the custom fixture and the adequate measurement procedures proved very effective in reducing the effect of all sources of possible errors and allowed obtaining of reliable and accurate data.
E. Todorov / NDT&E International 48 (2012) 63–69
4. Conclusions The following conclusions can be made from this study:
Unique and not previously reported magnetic and electrical
properties were acquired at AC and DC excitation for heat exchanger tubes made of SAF 2205, type 439 and Sea-Cure stainless steels. Measurable and significant (Type 439 and Sea-Cure) anisotropy of maximum relative magnetic permeability in A- versus C- direction was observed and reported for the first time. The initial relative magnetic permeability measurements had larger variability than expected. Standard procedures for magnetic property measurements need to be modified (depending on tested material) to increase the reliability of the measurement process when working in weak magnetic fields.
Unexpected differences requiring further investigation were observed between the magnetic properties of the two Sea-Cure tubes with different nominal tube wall thicknesses 0.889 mm (0.035 in.) and 0.7112 mm (0.028 in.).
Acknowledgments Major portion of this study was supported by the Department of Energy under Award Number DE-NE0000279, through the Nuclear Fabrication Consortium (NFC) managed by Edison Welding Institute (EWI). The Electric Power Research Institute (EPRI) co-
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sponsored the study by providing tube specimens and funding. The author would like to acknowledge the contributions of Kenji Krzywosz (EPRI), Nate Ames (NFC) and Steve Levesque (EWI) for the management support and feedback during the project. The author is also grateful to Greg Umana from KJS Associates Div., Magnetic Instrumentation (MI) Inc. for the patience and diligence in conducting the primary magnetic measurements. References [1] Recommended practice 6: The use of modeling in inspection qualification, Issue 1, European Network for Inspection Qualification (ENIQ), Report No 15. EUR 19017EN. Brussels-Luxembourg: European Commission, 1999. [2] Doctor R. Nuclear power plant NDE challenges – past, present and future, Review of progress in quantitative nondestructive evaluation. AIP 2006;26A:17–31. [3] /http://www.smt.sandvik.com/sandvik/tubeS, Sandvik SAF 2205 duplex stainless steel data sheet, Accessed May 4, 2011. [4] Washko S, Aggen G. Wrought stainless steels. Specialty steels and heatresistant alloys. In: Lampman SR, Zorc TB, editors. Metals Handbook, Vol. 1. Properties and Selection: Irons, Steels, and High Performance Alloys. 10th edn. Materials Park, OH, USA: ASM International; 1990. p. 871. technical. [5] /http://www.sulphuric-acid.com/TechManual/Materials/materials_metals_ cronifer2803mo.htmS, Accessed May 4, 2011. [6] ASTM A 773/A 773M – 01, Standard test method for DC magnetic properties of materials using ring and permeameter procedures with dc electronic hysteresigraphs, ASTM International. [7] ASTM A 927/A 927M – 04, Standard test method for alternating-current magnetic properties of toroidal core specimens using the voltmeter– ammeter–wattmeter method, ASTM International. [8] /http://www.amphenolrf.com/products/CatalogPages/BNC.pdfS. [9] Todorov E, Levesque S, Ames N, Krzywosz K. Measurement of Magnetic Properties of Ferromagnetic Tubes for Heat Exchangers, 104. Hollywood, FL, USA: Summary abstract in Transactions of the American Nuclear Society; 2011 pp. 297–298.