Note: Descriptions are shown in the official language in which they were submitted.
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~his invention relates to an apparatus and a method for detec-
ting localized flaws in a tube of a ferromagnetic material. More
specifically, the invention relates to a ferromagnetic tube flaw detec-
tion technique which utilizes magnetic saturation and a transmit-
receive eddy current probe for measurement.
BACKGROUND OF THE INVENTION
In the past, bodies of ferromagnetic mat~rial have been inspec-
ted by a method such as the leakage flux method as taught, for example,
in United States Patent Nos. 3,091,733, May 28, 1963 Fearer et al and
4,602,212, July 22, 1986, Hiroshima et al. In this method, the metal
is magneti~ed in a direction parallel to its surface. At defects or
where regions of the metal body are thinner, some magnetic flux passes
into the air and may be detected by sensor, thus giving an indication
of the presence of faults.
United States Patent No. 4,107,605, August 15, 1978 Hudgell
discloses an eddy current technique for testing of pipelines of ferro-
magnetic material. The probe includes spiral sensing coils placed with
their axes normal to the surface of the pipeline wall and connected on
four legs of an AC bridge, thus compensating for lift-off. Biasing
electromagnetic fields permit distinguishing internal from external
defects in weakly ferromagnetic tubes by comparing outputs from systems
with and without biasing fields.
In United States Patent Nos. 2,992,390, July 11, 1961 Dewitte
and 3,940,689 February 24, 1976 Johnson, Jr., speclal ways of gener-
ating magnetic fields are taught in connection with the eddy current
testing in that DeWitte uses uniquely designed core and Johnson, Jr.
employs a solenoid wound about a core of a substantial length. United
States Patent No. 4,292,589 Sept. 29, 1981 Bonner on the other hand
teaches the use of unique coil arrangements for a differential receiver
of a remote-field eddy current probe. However, his arrangement
requires long probes and low test frequency, thus limiting inspection
speed. United States Patent Nos. 3,952,315 April 20, 1976 Cecco and
1~3(~2~
2,964,699 Dec. 6, 1960 Perriam describe probes for use of testing
weakly ferromagnetic tubes. They use impedance type sensLng circuit
but are not sensitive to circumferential cracks nor are they circum-
ferentially compensating either.
All the prior art instruments suffer from various shortcomings
such as high cost, requirement of specially designed instrumentation,
bulky electromagnetizers for high magnetic saturation, insensitivities
for certain kinds of defects, no defect sensitivity close to weld
joints etc.
OBJECTS OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide
an eddy current probe for inspecting ferromagnetic tubes which is small
and less expensive to manufacture and to use.
It is another object of the present invention to provide an
eddy current probe having specially designed transmit and receive coils
which permit circumferential compensation while retaining high sensi-
tivity to localized defects.
SUMMARY OF THE IN~ENTION
Briefly stated, in accordance with the present invention, an
eddy current probe for detecting localized flaws in a tube made of a
ferromagnetic material includes a transmit coil assembly and at least
one receive coil assembly, all positioned with respect to each other in
a probe housing. The probe further has magnet means for magnetizing
the tube to magnetically saturate the tube near the transmit coil
assembly and t~le receive coil assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
In a more comp~ete understanding of the present invention and
for further objects and advantages thereof, references may be made to
the following description taken in conjunction with the accompanying
drawirlgs in which:
Figure 1 is a schernatic view of a prior art eddy current
probe;
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Figure 2(a), 2(b) and 2(c) show a ferromagnetic stainless steel
test tube and signals obtained by the probe shown in Figure 1;
Figure 3 is a schematic view of an eddy current probe of the
present invention according to one embodiment;
Figure 4 is a schematic view of an eddy current probe of the
present invention according to another embodiment;
Figure 5 is a schematic view of an eddy current probe of the
present invention according to still another embodiment; and
Figure 6 is a schematic view of an eddy current probe of the
present invention according to still another embodiment
showing radial saturation.
Figure 7(a), 7(b), 7(c) and 7(d) illustrate various magnetic
saturation configurations according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
_
Conventional eddy current testing detects changes in eddy cur-
rent induced in an object under test. The eddy current is indirectly
measured by a probe coil located near the surface of the object which
monitors the magnetic flux created by the eddy current. However, when
an eddy current probe is used for ferromagnetic tube inspection, the
magnetic permeability of the ferromagnetic material affects the probe
coils inductance as well as depth of eddy current penetration into the
material. The magnetic permeability strongly depends on factors such
as:
- thermal processing history;
- mechanical processing history;
- chemical composition;
- internal stresses; and
- temperature (if close to curie temperature).
The large variations in permeability make conventional eddy
current testing for defects in magnetic materials very difficult.
The best solution to eddy current testing of a magnetic
material for defects is to bring it to a condition where
~r = 1-0- Relative incremental or recoil permeability, ~r~ is defined
as ~r = ~B/~H where ~B is the change in flux density which accompanies
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a change in magnetizing force, ~H created for example by an eddy cur-
rent coils' alternating current.
A few slightly magnetic materials can be heated above their
curie temperature to make them nonmagnetic. Monel (trademark) 400
heated to between 50 and 70C has been tested in this manner. Most
materials, however, have too high a curie temperature to be tested by
this approach. The only other way to decrease ~r to unity is by
magnetic saturation.
Figure 1 shows a probe known in the art as the saturation probe
which incorporates a permanent magnet configuration designed to maxi-
mize the saturation field over the test coil.
The importance of achieving maximum saturation is illustrated
in Figure 2(a), 2(b), and 2(c) which shows results from Type 439 stain-
less steel heat exchanger tube. A 15.9 mm OD by 1.2 mm thick tube with
internal and external calibration defects and a shot peened area was
used to compare the performance of various saturation probes. As shown
in Figure 2(a), the external defects ranged from 20 to 100~ deep.
Figure 2(b) shows the signals obtained with a probe capable of 99~
saturation and Figure 2(c) signals with 95% saturation. The relative
magnetic permeability (~r) at 99~ saturation is approximately 1.15 and
at 95% saturation it is 1.9. At 99g saturation the eddy current sig-
nals from the external calibration holes display the characteristic
phase rotation with depth, that one expects for nonmagnetic materials.
In contrast, wlth only 95% saturation the signals are distorted and
indistinguishable from "change in magnetic permeability" signals. From
similar tests on other ferromagnetic tubes it has been found that at
least 98% saturation is needed (~r c 1.3) for reliable test results.
This requires detailed optimization of the saturation magnet design for
each ferromagnetic tube material. However, even the most optimized
saturation probe cannot completely saturate some tubes especially car-
bon steel tubes or pipes.
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Contrary to the prior belief, the inventors have discovered
that when an eddy current probe having a transmit coil assembly and a
receive coil assembly is used, only partial maer1etic saturation, (e.g.
of less than 50%) would suffice for good sensitivity in thin and thick
tubes of weakly or strongly magnetic material.
According to the present invention, the transmit coil generates
a magnetic field and eddy currents that decrease rapidly with radial
and axial distance. The magnetic field is much weaker but uniform
axially and radially at a few coil diameters away from the transmit
coil. In this periphery region the best signal-to-noise is obtained.
Partial saturation is sufficient to decrease the magnetic permeability
variations and allow the magnetic field to reach the tubes external
defects.
Referring to Figure 3 one of the preferred embodiments is
illustrated to have a transmit coil assembly 1 and a receive coil
assembly 3. The coil assemblies are housed in a probe casing 5 of a
non-ferromagnetic material. And an electrical connection is made at 7.
Four permanent magnets 9 are arranged with their polarities as shown,
that is, two adjacent ones being oppositely polarized. All the polar-
ities of the magnets can lie reversed with the same results. The
transmit coil assembly is of a bobbin coil type in t~1iS embodiment and
is positioned over the second magnet. The receive coil assembly 3
comprises a set of four pancake coils and is positioned over the third
magnet. Magnetic field keeper disks 11 of a high ~ material such as
permendur (Trade ~ ) are placed between the permanent magnets 9.
Four pancake coils of the receive coil assembly 3 are arranged 90
circumferentially apart from each other and oppositely polarized as
shown for circumferential compensation. The distance between the
transmit coil assembly and the receive coil assembly is set in this
embodiment as being about twice the diameter of the bobbin coil which
is roughly same as the diameter of the tube under test. While a bobbin
coil and four pancake coils are illustrated in the figure; other coil
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configurations such as a plurality of pancake coils in a
transmit coil assembly and more than four pancake coils
in a receive coil assembly. The present applicant's U.S.
Patent No. 4,808,924 issued on February 28, 1989,
"Circumferentially Compensating Eddy Current Probe with
Alternately Polarized Transmit Coils and Receive Coils"
and U.S. Patent No. 4,808,927 issued on February 28,
1989, "Circumferentially Compensating Eddy Current Probe"
describe various possible circumferentially compensating
coil configurations.
Referring to Figure 4, another embodiment is
shown in which five permanent magnets are arranged in the
same fashion as in Figure 3. The bobbin transmit coil 21
i5 over the second magnet and two bobbin receive coils
are located over the second and the third magnets. The
distances of the receive coils from the common transmit
coil are about D and 2D as shown in the figure, D being
the diameter of the tube under test. Each receive coil
23, 25 is monitored separately. The receive coil 23 is
sensitive to internal tube defects and the receive coil
25 to internal as well as external tube defects.
Figure 5 shows a different embodiment which
uses three permanent magnets. The transmit coil 31 is of
a bobbin type and the pancake receive coils are four in
number arranged 90 circumferentially apart from each
other. The distance between the transmit and receive
coils is zero.
While all the embodiments described so far
utilize axial magnetic saturation along the tube under
test, for some applications, such as detection of defects
under ferromagnetic support plates, radial saturation is
preferred. Figure 6 shows one of such configurations for
radial saturation. In the figure, three permanent
magnets are arranged axially with magnetic field keeper
disks 41 and 43 between them. Instead of over the
permanent magnets as in the previous embodiments, four
transmit coils and four receive coils are located over
respective keeper disks 41 and 43. A typical tube
support plate 45 is also shown.
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Other coil configurations, e.g. 2 transmit coils and 2 receive
coils over each keeper disk or a bobbin transmit coil and four receive
coils over either separate keeper disks or over a same disk, are also
possible.
In these embodiments, radial saturation increases slightly in
the tube under support plates, as can be seen in the figure, whereas
axial saturation drastically decreases, rendering inspection
impossible.
Figures 7(a), 7(b), 7(c) and 7(d) illustrate schematically
other magnetic configurations in which the permanent magnets are com-
pletely replaced or are partially supplemented by dc magnets (dc
powered electromagnets). In figure 7(a), an electromagnet 51 is posi-
tioned over the middle permanent magnet. The energization of the
electromagnet facilitate finer controlling of the magnetic saturation.
Since the resultant saturation flux density is a function of tube
thickness, controlling the DC saturation allows tubes of various
thickness to be inspected with the same probe at the optimized
saturation conditLon. A high ~r material 53 is also provided. In
figures 7(b), 7(c) and 7(d), electromagnets are provided on a high ~r
material such as permendur (Trade Name).
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