CARBURIZA4TIONS ENSING METHOD
[Technical Field]
C000lI
The present invention relates to a method for sensing whether
carburization occurs or not on an inner surface of a pipe or tube by an
electromagnetic testing such as an electromagnetic induction testing and
a magnetic flux leakage testing. Hereinafter, "pipe or tube" is referred
to as "tube" when deemed appropriate.
[Background Art]
[00021
It is known that among various steel materials, austenitic stainless
steel is susceptible to carburization. FOP example, a cracking tube,
which is used for the thermal decomposition reaction in an ethylene
manufacturing process of a petrochemical plant, is made of austenitic
stainless steel, and carburiznt.ion occurs on its inner surface after being
used for long hours. iVIoreover, in the manufacturing process of the
cracking tube, carburization occurs when heat treatment is performed in
a poorly degreased condition of lubricant. Since the occurrence of such
cai*burization may cause signiiicant reduction of the life of the cracking
tube, there is n neecl for a~curacelys ensing whether carburization occurs
or not.
[00031
For this reason, conventionally, an electromagnetic test such as an
electromagnetic induction test is carried out on a cracking tube installed
in a plant as a nondestructive inspection across the entire length of the
cracking tube at the time of periodic maintenance of the plant so that
whether carburization occurs or not is sensed based on the magnitude of
the output value thereof. Moreover, also in the manufacturing process of
a cracking tube, whether carburization occurs or not is sensed by
performing an electromagnetic test across its entire length or by cutting
off both ends thereof and performing a microstructure observation
-
thereon.
[00041
In general, if a seamless tube is subjected to drawing-working in a
producing process, the tube has smaller roughness in the inner surface, so
that amount of lubricant adhering on the inner surface becomes smaller.
Hence, heat treatment in a poorly degreased condition causes microscopic
carburization. Particularly, if the clrawing-working is performed in a
high-pressure container, the inner surface of the tube becomes almost
equal to a mirror surface, and thus carburization clue to the poor degrease
becomes extremely microscopic.
[0005J
'There have been suggested various methods of sensing whether
~arburizat~iooncc urs or not including those that have not been put to
17ractical USE! (see Patent Litcratui.~1 to Pateilt Literature 7. for ~sarnple),
but none of these methods can sense the aforementioned microscopic
carburization.
[Citation List]
[Patent Literatwe]
[OOO~]
[Patent Literature 11 JP3-253555A
[Patent Literature 21 JP62-6153A
[Patent Literature 31 JP4- 145358A
[Patent Literature 41 JP6-888078
[Patent Literature 51 JP2000-266727A
[Patent Literature 61 JP2004-279054A
[Patent Literature 71 JP2004-279055A
[Summary of Invention]
[Technical Problem]
[oflo71
,An object of the present invcntlon, which has been made in order to
solve the problems :iccording to the conventionti1 art, is to provide a
carburization sensing tnethotl capable of sensing even inicroscopic
cai.burization that is difficult to be sensed by conventional cni.burization
.;ensing methods.
[0008l
In order to solve the above problems, as described in JP 2010-
197222A suggested by the present inventors, a ferrite meter was
oppositely disposed to an outer surface of a tube having microscopic
carburization on an inner surface thereof so as to measure magnetic
strength (amount of ferrite) at its carburized portion with this ferrite
meter, but no effective indicated values could be obtained. Specifically,
the magnetic strength was measured at ten positions of the tube where it
was confirmed through a microstructure observation that microscopic
carburization had occurred on the inner surface thereof, but all the
indicated values of the ferrite meter were 0.01 Fe% or less. It may be
estimated that such small magnetic strength results from small amount
of production of an oxide magnetic material caused by the carburization.
[00091
Based on the above described result, first, the present inventors
attempted to sense microscopic carburization not from the outer surface
of the tube, but from the inner surface thereof. Specifically, using a
common inner coil for flaw inspection, a test of confirmiilg whether it is
possible or not to sense carburization was performed under the following
conditions (1) to (3). For the evaluation, a detection signal (absolut,e
value signal) outputted from the inner coil was amplified, and thereafter
was subjected to synchronous detection, thereby separating and
extracting a first signal component and a second signal cornpoilent each of
which had a diffe~ellpt hase by $10" from each other. Each phase of the
first signal component and the second signal component was rotated
(shifted) by the same predetermined amount, and the first signal
component after the rotation was defined as an X signal, and the second
signal component after the rotation was defined as a Y signal. The
amount of the above rotation (amount of phase shift) was determined
such that, if the X signal and the Y signal are expressed on the X-Y vector
plane, the Y axis direction on the X-Y vector plane corresponds to
variation in liftoff of the tube, and the X axis direction corresponds to
variation in magnetic property of the tube.
(1) Inspection target: 13 steel tubes each having microscopic
carburization on its inner surface, and having an outer diameter of 19
mm, and an inner diameter of 17 mm
(2) Inner coil: outer diameter of 16.5 mm, length of 2 mm, impedance of
50 RI100 kHz
(3) Excitation frequency (inspection frequency): 25 kHz
[00101
Using steel tubes of the same type as that. of the above inspection
targets, and having no carburization, a magnetic tape was wound around
the inner surface of each tube by 2.5 turns and 6 turns, respectively, and
detection signals obtainetl from these lnagiletic tapes welee evaluated in
the s;lnle manner.
loo1 11
Figure 1 is a cliilgr;.\2~s ho~vingrc sults of the :~bovet est (diagr:im
.;bowing the S signals ril~t-lt hc Y signals on [he S-Yv ector plnne). In
Figure 1, data plotted with white rectangles indicate data obtained from
the carburized portion of each inspection target, and data plotted with
black rectangles indicate data obtained fkom the magnetic tapes.
The flaw inspection using the inner coil as described in the above
test senses change in electric resistance due to flaws, and this inspection
is usually carried out with high sensitivity, so that this inspection is
sensitive to variation in magnetic property. In the case of having
variation in magnetic property, the X signal becomes a negative value
depending on the magnitude of the variation in magnetic property (data
is plotted in the negative direction of the X axis). However, except for
the data indicated by the arrows A, B, and C in Figure 1, the data
obtained from the carburized portions of the inspection targets became
positive values, and no carburization could be sensed. Although the data
indicated by the arrows A. B, and C in Figure 1 were negative values,
eve11 data indicating the negative value which has the greatest absolute
value (data indicated by the :4riQowA ) had approximately the same signal
strength of the S signal as that of the S signal of the data obt.ained from
the magnetic tape whose number of windings was 6 turns, which
exhibited extremely weak variation in magnetic property.
[O[J 1'21
It is difficult to sense uliclboscopicc :a~*bul.izatiouns ing a coinmon
inner coil for flaw jnspect.ion 11cc;luse excitation ability (magnetic field
.strength) to be used is weal;. 'Co be specific. [.he ~nagiletization
rha1.;ic:tcri,.;tics of ;-i m:~gnetici 11;i:ericl i+IoPt .y:prc,.;setl 'l~ya B-H curve, and
initial permeability is extremely small if the magnetic field strength is
small, and the magnetic permeability increases as the magnetic field
strength increases. Hence, it has been found that it is impossible for a
common inner coil used for flaw inspection to sense microscopic
carburization that causes only weak variation in magnetic property.
For sensing microscopic variation in magnetic property, it is
preferable to employ a mutual induction method that separately provides
an excitation coil and a detection coil, but in the case of using an inner
coil, it is difficult to employ the mutual induction method using an
excitation coil in a large size for the reason of limitation of the dimension
of a coil to be inserted in a tube. In order to increase the magnetic field
strength, it is required to supply a greater excitation current by
increasing a winding diameter of the excitation coil and a diameter of an
electric cable having a length of dozens of meters for supplying the
excitation current to the excitation coil, but the inner diameter of the tube
is limited. Even if the diameter of the electric cable is increased so as to
increase the excitation current, generated heat of the excitation coil also
increases, which causes variation in t,emperature in the det,ection coil,
and this may make it difficult, to ohtain a stable detection signal (absolute
value signal).
In adclition, the inner coil is supposed t.o he inovcd inside the tube,
hut it is difficult to move the iiiiler coil at a high speed t,hereinside, and it.
is also required to retrieve the iliiler coil inserted in the tube, which is not
suitable for automatic inspection in the production line of the tube.
[00131
Based on the results of the above test, the present inventors have
re-studied on a method of sensing whether carburization occurs or not on
the inner surface of the tube that is the inspection target from the outer
surface of the tube. Specifically, first the present inventors have studied
on whether it is possible or not to detect the magnetic tape attached onto
the inner surface of each tube under the following conditions, using the
method shown in Figure 1 of JP2010-197222A suggested by the present
inventors (referred to as a "conventional method", hereinafter).
Variation in magnetic property caused by microscopic carburization to be
actually sensed is weak; therefore various tapes having various numbers
of windings: 1 turn, 3 turns, and 5 turns were used. The magnetic
strength (amount of ferrite) of each magnetic tape attached on the inner
surface of each tube was measured with a ferrite meter.
(1) Excitation fkequency (inspection frequency): 500 Hz
(2) Excitation current: 0.01 A
(3) Number of windings of excitation coil: 200 turns
(4) Length of excitation coil: '70 mnl
[00 141
The results of the above test are shown in Table 1.
h b l e 11
I Number of Turns (~agneticS trength (Fei) (X Axis Signal (mV) I
I I I
3 I 0.035 Undetectable
1 0.01 or lcss I ljndetectable
[00151
,4s shown in Table 1, in the conveiltional method, it was impossible
to detect the magnetic tapes of which number of windings is 3 turns or
less. In other words, weak variation in magnetic property could not be
sensed under the aforemeiltioned conditions; therefore it may be
considered that microscopic carburization cannot be sensed.
In the method of sensing whether carburization occurs or not on the
inner surface of the tube from the outer surface of the tube, the present
iilventors have further conducted enthusiastic studios on influence of the
excitation ability (magnetic field strength) and the excitation fi-equency
on the sensing performance of the microscopic carburization (weak
variation in magnetic property-) as follows.
LOO 161
(1) Influence of excitation al~iltiy imagne t.ic field strength)
In the case of employing the mutual induction method that
separately provides the excitation coil and the detection coil, as the
luagnet,ic field strci1gt.h (prodt1c:t of the exclitation current and the n ~ u n b e ~
i)l'winclings of the escit:~tlo~(:oi il per unit le~lgth)in cl-eases, the voltage
induced in t.he cle tection coil ;acreases. Hence, it i;i possible t,o i:ccluce
sensitivity of the signal processing section for processing an output signal
from the detection coil (gain of the amplifier included in the signal
processing section), thereby attaining such an advantage that reduces
electric noises. As aforementioned, however, the magnetization
characteristics of a magnetic material are expressed by the B-H curve,
and initial permeability is extremely small if the magnetic field strength
is small, and the magnetic permeability increases until the maximum
value is reached in accordance with increase of the magnetic field
strength. If the magnetic field is further increased, the magnetic flux
density becomes saturated, and the magnetic permeability rather
becomes smaller. Hence it is difficult to sense weak variation in
magnetic property unless an appropriate magnetic field strength is
provided. In other words, if the magnetic permeability is small,
variation of the output signal (output voltage) of the detection coil due to
the variation in magnetic property is so small that weak variation in
magnetic property cannot be sensed. In this case, if the sensitivity of the
signal processing section is increased for correction, electric noises
increase, which hinders appropriate inspection.
Accordingly, the sensing performance of microscopic carburization
(weak variation in magnetic property) depends on the excitation ability
(magnetic field strength) in the light of maximizing the magnetic
permeability.
[OO 171
(2) Iilfluellco of excitation kecluency
In the case of sensing variation in magnetic property caused by
carburization on the inner surface of the tube from the outer surface of
the tube, it is required to set the excitation frequency to be a low
frequency in order to reduce the influence of the skin effect, and increase
the penetration depth. Meanwhile, in the case of employing the mutual
induction method, if the excitation frequency is set to be an excessively
low frequency, the voltage induced in the detection coil becomes small,
and thus it is required to increase the sensitivity of the signal processing
section for processing the output signal of the detection coil (gain of the
amplifier included in the signal processing section). Consequently,
electric noises are increased, which may hinder appropriate inspection.
Accordingly, the sensing performance of microscopic carburization
depends on the excitation fi-equency. To be specific, it may be considered
that the penetration depth roughly has a positive correlation with t.he
excitation frequency to the power of -112, and the sensitivity of the signal
processing section (electric noise) has a negative correlation with the
excitation frequency (in other words, this has a positive correlation with
the excitation fkequency to the power of -1); therefore, it was found that
the sensing performance of microscopic carburization depends on the
excitation frequency to the power of -312.
[00181
The present inventors considered that, based on the results of the
above studies, a parameter K represented by the following Equation (1)
can be an index of the carburiza tion sensing perfoi*mance, where the
current value of an excitation current passing through the excitation coil
is defined as I(&, the length of the excitation coil is defined as L (mm),
the number of windings of the excitation coil is defined as N, and the
fiequency of the excitation current passing through the excitation coil is
defined as F (kHz).
K=',JnN/'i:.F-3, 3 * . * [ : , I
Loo 191
Figure 2 is a diagram showing an example of test results from
investigation on a relation between detection signals obtained from
magnetic tapes attached on inner surfaces of tubes having no
carburization and a parameter K using a mutual induction method. In
Figure 2, the abscissa indicates the parameter K, and the ordinate
indicates the detection signal. To be specific, in this test, the value of
the parameter K was varied by using various conditions (excitation
current, etc.) of the excitation coil 11 using an eddy current testing
apparatus 100 described in Figure 3, which is described below. The
values of the detection signals (to he specific, the S axis signal obtained
i ~pyr ocessing the absolute value signal outputted from the detection cuil
12) clbtainect frolu the magnetic tapes whose number of windings were 1
turn and 3 turns were evaluated.
;Is shown in Figure 2, as the value of the parameter K is increased,
the absolute value of the cletection signal (S axis signal) obtained from
c?;ich magnet.ic tape incrcnses (that is, rhe cr~rburizations ensing
performance is enhanced), which exhibits n relatively preferable
correlation therebetween. From the above results, the present inventors
have confirmed that the parameter K can be an index of the carburization
sensing performance. The present inventors have also found that it is
possible to sense microscopic carburization by appropriately adjusting the
value of the parameter K.
[00201
The present invention has been accomplished based on the above
findings of the present inventors.
Specifically, the present invention includes the following first and
second steps.
(1) First step
A first step is a step of inserting a carburized pipe or tube in which
occurrence of carburization on an inner surface thereof is known into an
excitation coil and into a detection coil, and determining a value of a
parameter K represented by the following Equation (1):
i<=(l=N/L:mF-S/2 . . . ( I j
so as to sense the carburization that has occurred in the carburized pipe
or tube based on an output signal outputted kom the detection coil, where
R current value of an excitation current passing through the excitation
coil is defined as I(X), a length of the excitation coil is defined as L (mm),
a number of windings of the excitation coil is defined as N, and a
frequency of the excitation current passing through the excitation coil is
defined as F (kHz).
(2) Secoiid step
A second step is a step of setting conditions of the excitation coil so
as to obtain the value of the parameter K determined in the first step,
and thereafter, inserting a pipe or tube that is an inspection target into
the excitation coil and into the detection coil and sensing whether
carburization occurs or not on an inner surface of the pipe or tube based
on an output signal outputted from the detection coil.
loo2 11
According to the present invention, in the first step, it is configured
to determine the value of the parameter K so as to sense carburization of
the carburized pipe or tube. As apparent &om Equation (I), this
parameter K is proportional to the magnetic field strength (I NJL), and
also proportional to the excitatioil frequency F to the power of -312. As
described above, the carburization sensing performance depends on the
iilagnetic field strength and the excitation frequency to the power of -312,
and thus it may he considered that the parameter K represented by
Equation (1) is an index indicating the carburization sensing performance.
Accordingly, in order to sense microscopic carburization, it is only
1.equired to prepare a pipe or tube having microscopic carburization as the
carburized pipe or tube, and determine the value of the parameter K, that
is, adjust the ~a~burizatiosenn sing performance so as to sense this
carburization.
According to the present invention, in the second step, after the
conditioils of the excitatioii coil are so set as to obtain the value of the
parameter Ei determillet1 in the first step, it is configured to sense
whether the carburization occurs or not on the inner surface of the pipe or
tube that is the inspection target. As aforementioned, since the value of
the parameter K is so determined as to sense the carburization of the
carburized pipe or tube in the first step, it may be expected that only by
inspecting a pipe or tube that is an inspection target after the conditions
of the excitation coil are adjusted such that the determined value of the
parameter K is obtained, the carburization in this pipe or tube, which is
substantially equal to the carburization in the carburized pipe or tube
used for determining the value of the parameter K, can also be sensed.
[00221
The present inventors have conducted studies on sensing
microscopic carburization, and to be specific, they have found that it is
preferable to set the value of the parameter K to satisfy 4 5 K 5 8.
That is, in the second step, t,he' conditioils of the excitat.ion coil are
set such that the value of the parameter K satisfies 4 I K 5 8.
[Xdvantageous Effect of Invention]
[ooasl
,According to the carburization sensing method of the present
invention, it is possible to sense microscopic carburization that is difficult
to he sensed by convention:ii c:~rburizations ensing methods.
[Rricf Description of D1.awi11gsI
ro024.1
Figure 1 is a diagram showing results of a test carried out using an
inner coil by the present inventors.
Figure 2 is a diagram showing an example of test results from
investigation on a relation between detection signals obtained from
magnetic tapes attached on inner surfaces of tubes having no
carburization and a parameter K using a mutual induction method.
Figure 3 is a schematic diagram showing an outline configuration
of an eddy current testing apparatus used for a carburization sensing
method according to an embodiment of the present invention.
Figure 4 is a schematic diagram representing on an X-Y vector
plane an X signal and Y signal outputted from a phase rotator included in
the eddy current testing apparatus shown in Figure 3.
Figure 5 is a diagram showing an example of test results fiom
investigation regarding a relation between detection signals obtained
from plural carburized tubes using the eddy current testing apparatus
shown in Figure 3, and the parameter K.
Figure 6A is a diagram showing data satisfying 4 5 R < 8, which are
extracted from data shown in Figure 5, and results from investigation on
a relation between detection signals and carburized depth. Figure 6B is
a diagram plotting data shown in Figure 6A excluding data of K = 8 (4 <
I< 5 G after the exclusion).
[Description of Embodiment]
~00251
Hereafter, with reference to the appended drawings, an
embodiment of the present invention will be described taking an example
of a case in which a tube is a steel tube, and an eddy current test is
performed as the electromagnetic test.
[0026]
Figure 3 is a schematic diagram showing an outline configuration
of an eddy current testing apparatus used for a carburization sensing
method according to an embodiment of the present invention.
As shown in Figure 3, an eddy current testing apparatus 100 of the
present embodiment includes a- detection sensor 1 and a signal processing
section 2. In Figure 3, the detection sensor 1 is shown in a cross
sectional view.
[00271 '
The detection sensor 1 is configured to apply an alternating
magnetic field to a steel tube P thereby inducing an eddy current, and
detect the eddy current induced in the steel tube P. To be specific, the
detection sensor 1 of the present embodiment includes an excitation coil
11 that applies an alternating magnetic field to the inserted steel tube P,
and a detection coil 12 that detects the eddy current induced in the
inserted steel tube P.
[oozsl
The signal processing section 2 is configured to pass an alternating
tcitation current through the detection sensor 1 ancl sense whether
t:arburization occurs or not on r;he inner surface of the steel tube P based
on a detection signal (absolute value signal) outputted from the detection
sensor 1. To be specific, the signal processing section 2 of the present
embodiment includes an oscillator 21, an amplifier 22, a synchronous
wave detector 23, a phase rotator 24, an AID converter 26, and a
determination section 27.
[00291
The oscillator 21 supplies an alternating excitation current to the
detection sensor 1 (to be specific, the excitation coil 11 of the detection
sensor 1). This causes an alternating magnetic field to be applied to the
steel tube P, and the eddy current is induced in the steel tube P as
described above.
[0030]
-An absolute value signal outputted fiom the detection sensor 1 (to
be specific, the detection coil 12 of the detection sensor 1) is amplified by
the amplifier 22, and thereafter outputted to the synchronous wave
detector 23.
lo03 11
The synchroiious wave detector 23 performs synchronous wave
detection of the output si,gnal of the amplifier 22 based on the reference
signal outputted from the oscillator 21. To be specific, a first reference
signal having the same frccluency and the same phase as those of the
e.ucitc?tion currcnt to be supyli~rtto the tlct~ctioiis eiisor 1, and a second
r.eft.rence ~lgnticl ~wf nlch p'rl:i+...c is shiftc~1~1-ly 1)0° ti.onl the phase of the
j'irct referenct~r iz11:il ;+rtl o~ltp~t : btr li f~olntf lc r).-;it;lli~to2r1 to r he
synchronous wave detector 23. Then, the synchronous wave detector 23
separates and extracts a signal component (first signal component) that is
in phase with the phase of the first reference signal and a signal
component (second signal component) that is in phase with the phase of
the second reference sigilal from the output signal of the amplifier 22.
The separated and extracted first and second signal components are
outputted to the phase rotator 24, respectively.
E00321
The phase rotator 24 rotates (shifts) the phases of the first signal
component and the second signal component outputted from the
synchronous wave detector 23 by the same predetermined amount, and
outputs the first signal component as an X signal and the second signal
component as a Y signal to the AID converter 26, for example. It is noted
that the X signal and the Y signal that are outputted fiom the phase
-
rotator 24 correspond to components of a signal waveform projected to the
X axis and the Y axis, respectively in an X-Y vector plane represented by
two mutually orthogonal axes (X axis and Y axis), where the signal
waveform is so-called a Lissajous figure and used for flaw inspection (that
is, an absolute value signal waveform (to be precise, an absolute value
signal waveform after being amplified by the amplifier 22) of the
clet,ection sensor 1 represented by a polar coordinate (Z, 8) where Z is
amplitude and 8 is phase).
[00331
Figure 4 is a schematic diagram representing on an X-Y vector
plane an X signal and Y signal outputted from a phase rotator 24.
In a state in which a steel tube having no carburization on its inner
surface (referred to as a "reference material", hereinafter) is not inserted
into the detection sensor 1, the balancing quantity of a balance circuit
(not shown) which is disposed in the preceding stage of the amplifier 22 is
adjusted such that the X signal and the Y signal become zero (such that a
spot corresponding to the front edge of a vector of which X axis component
and Y axis component are the X signal and the Y signal, respectively is
located at a balance point (an origin point) shown in Figure 4) so that the
&st signal component and the second signal component, which are
outputted from the synchronous wave detector 23, are zero, respectively.
Next, the reference material is inserted into the detection sensor 1
and halted thereat, and the amplificatioil factor of the amplifier 22 and
the phase rotation amount of the phase rotator 24 are adjusted such that
the S signal equals zero and the Y signal equals a predetermined voltage
(for example, 4V) (such that the front edge of a vector is located at the
1.efei.ence point shown in Figure 4).
[00341
After the above described fidjustment is performed i11 advance, the
steel tube P thi~its a11 inspec~tiont arget is moved in the axial clirection,
ancl inserted into the detection sensor 1. thereby acquiring the S signal
and the Y signal.
L00361
The AID converter 26 performs AID conversion of the output signal
of the phase rotator 24, and outputs it to the determination section 27.
[00361
The determination section 27 senses whether carburization occurs
or not on the inner surface of the steel tube P based on the output data of
the AID converter 26 (that is, digital data obtained through AID
conversion of X signal and Y signal. Hereafter, referred to as X signal
data and Y signal data). As shown in Figure 4, the position of the front
edge of the vector varies in accordance with variation in magnetic
property of the steel tube P, and this variation becomes greater in the X
axis direction than in the Y axis direction. Hence, the determination
section 27 of the present embodiment senses whether carburization
occurs or not using the X signal data of the X signal data and the Y signal
data that have been inputted. To be specific, the determination section
27 of the present embodiment compares the inputted X signal data with a
threshold value which is determined and stored in advance, and
determines that carburization has occurred on the inner surface of the
steel tube P if the X signal data exceeds the threshold value, and
det.ermines that carburization has not occurred on the inner surface of
the steel tube P if the S signal data is within the aforementioned
threshold value .
Lo03 71
In order to sense the carburization on the inner surface of the steel
tube P using the eddy current testing apparatus 100 having the above
described coldiguration, a carburized tube PO in which occurrence of
microscopic carburization on an inner surface thereof is known is inserted
into the excitation coil 11 and the detection coil 12 in advance (see Figure
3). A value of the parameter K represented by the following Equation (1)
is so determined as to sense the carburizatioil that has occurred in the
carburized tube PO based on the output signal (to be specific, X signal
data) outputted from the detection coil 12.
;(= g.N/,; .F--?*" s . . { 7 ;
In the above Equation (I), I represents a current value (A) of the
excitation current passing through the excitation coil 11, L represents a
length (mm) of the excitation coil 11, N represents the number of
windings of the excitation coil 11, and F represents a frequency (kHz) of
the excitation current passing through the excitation coil 11.
[00381
-After the conditions of the excitat.ion coil 11 (the current value of
the excitation current, the length of the excitation coil, the number of
windiilgs of the excitation coils. ;ind the frequency of the excitation
crur~.ent);I re so set as to obt:~in the value of the parameter I< determined
i11 the above znanizer, the steel tube P that is the inspection target is
inserted into the excit.atio11 ooil 11 and tile tletection coil 12 so ;is to sense
;vhet,hei* carbusization c)(:curs or nut 011 the inner surface of the steel tube
P based on the output ~jguill (1.9 '?e tiprcific, S signal data) &oin the
iktectlon cc~il 'LS.
!00:3!11
Figure 5 is a diagram showing an example of test results from
investigation regarding a relation between detection signals obtained
fiom plural carburized tubes PO using the eddy current testing apparatus
100, and the parameter K under the following conditions. In Figure 5,
the abscissa represents the parameter K, and the ordinate represents the
detection signals. To be specific, in this test, it was configured to vary
the value of the parameter K by using various conditions of the excitation
coil 11 of the eddy current testing apparatus 100. The values of the
detection signals (to be specific, X axis signal obtained by signalprocessing
absolute value signals outputted from the detection coil 12)
obtained from the plural carburized tubes PO were evaluated. In Figure
6, data plotted with the identical symbol were obtained from the identical
carburized tube PO. The .carburized portion of each carburized tube PO
had a magnetic strength (amount of ferrite) of 0.01 Fe% or less.
(1) Frequency of excitation current F: 0.3 to 1 kHz
(2) Current value of excitation current I: 0.1 to 1 X
(3) Length of excitation coil L: 70 lllm
(4) Numher of windings of excitation coil N: 200 turns
( G ) Material quality of carburizeci tube: high Ni austenitic stainless steel
((3) Outer diameter of curburized tube: $15 to 25 mm
A
(7) Thickness of carburizect t ubc: 0.9 to 1.25 min
(8) Carburized depth of carhu~izedtu be: 27 to 46 jinl
[00401
As shown in Figure 5, if 4 5 K < 8 is satisfied, the X signal becomes
a negative value, and thus it is possible to sense variation in magnetic
property, which is caused by the carburized portion on the inner surface
of the carburized tube PO.
If 4 > I< is satisfied, because the current value of the excitation
current becomes smaller, or the excitation frequency becomes greater and
the penetration depth becomes smaller, the magnetic field strength on
the inner surface of the carburized tube PO becomes smaller.
Consequently, the magnetic permeability of the carburized tube PO
becomes smaller, and variation in magnetic property due to the
carburization cannot be accurately sensed. On the other hand, if 8 < K is
satisfied, because the fi'eyuency of the excitation current is a low
frequency, so that the penetration depth becomes greater, but the voltage
induced in the detection coil 12 becomes smaller, so that the sensitivity of
t,he signal processing section 2 (gain of the amplifier 22) becomes greater.
Hence, influence of the variation in collductivity becomes greater
(:ompared to the variation in magnetic property. As a result, it may be
considered that the S signal becomes a positive value.
[00411
i"iccordingly. it can be said that it is possible to sense whether the
carburization occurs or not on the inner surface of the steel tube P by
iilspecting the steel tube P that is the inspectioil target after the
c.onditions of the excitation coil 11 are so set as to satisfy 4 5 I< < 8, and
the threshold value stored in the determination section 27 is set to be
zero, for .example.
100421
Figure 6A is a diagram showing data satisfjrlng 4 5 K 5 8, which are
extracted from data shown in Figure 5, and results from investigation on
a relation between detection signals and carburized depth. Figure 6B is
a diagram plotting data shown in Figure 6A excluding data of K = 8 (4 <
K 5 6 after the exclusion). In Figure 6A and Figure 6B, data plotted
with the identical symbol indicates data having the identical I<.
100431
As shown in Figure 6A, there is a relatively preferable correlation
between the detection signals and the carbwized depth. Accordingly, it
is possible to estimate the carburized depth to some extent based on the
magnitude of the detection signal. It is, however, preferable to satisfy 4
5 I< 5 6, as shown in Figure 6B, because the absolute value of the
detection signal may become small in some cases in the case of K = 8
(data surrounded by a dotted line in Figure 6A).
[Reference Signs List]
C00441
1
A Detection sensor
2 Signal processing section
11 Excitation coil
12 Detect,ion coil
21 Oscillator
22 Amplifier
23 Synchronous wave detector
24 Phase rotator
26 AID converter
27 determination section
100 eddy current testing apparatus
P Steel tube
PO Carburized tube
We claim:
[Claim 1]
A method for sensing whether carburization occurs or not on an
inner surface of a pipe or tube by an electromagnetic testing, comprising:
a first step of inserting a carburized pipe or tube in which
occurrence of carburization on an inner surface thereof is known into an
excitation coil and into a detection coil, and determining a value of a
parameter K represented by the following Equation (1):
K=(I=N/L)WF-~/Z = = * (1)
so as to sense the carburization that has occurred in the carburized pipe
or tube based on an output signal outputted from the detection coil, where
a current value of an excitation current passing through the excitation
coil is defined as I@), a length of the excitation coil is defined as L (mm),
a number of windings of the excitation coil is defined as N, and a
frequency of the excitation current passing through the excitation coil is
defined as F (kHz); and
a second step of setting conditions of the excitation coil so as to
obtain the value of the parameter M determined in the first step, and
thereafter, inserting a pipe or tube that is an inspection target into the
excitation coil and into the detection coil and sensing whether
carburization occurs or not on an inner surface of the pipe or tube based
on an output signal outputted from the cletectzon coil.
[Claim 2]
The method of sensing whether carburization occurs or not on an
inner surface of a pipe or tube according to claim 1, wherein
in the second step, the conditions of the excitation coil are set such
that the value of the parameter K satisfies 4 I K I 8.