U.S. patent application number 10/510639 was filed with the patent office on 2005-07-21 for method of nondestructive examination of chromium-containing nickel-based alloy for grain boundary corrosion and examination apparatus.
Invention is credited to Takahashi, Seiki.
Application Number | 20050155678 10/510639 |
Document ID | / |
Family ID | 29243245 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050155678 |
Kind Code |
A1 |
Takahashi, Seiki |
July 21, 2005 |
Method of nondestructive examination of chromium-containing
nickel-based alloy for grain boundary corrosion and examination
apparatus
Abstract
Discloses is a nondestructive inspection method of
grain-boundary attack due to thermal sensitization in a
chromium-containing nickel-based alloy, such as Inconel 600 alloy.
The method comprises measuring a saturation magnetization
M.sub.s(T.sub.i) of a test piece at each of a plurality of
measuring temperatures defined by equally dividing a given
measuring temperature range in the range of a minimum to a maximum
of Curie temperatures corresponding to respective chromium
concentrations in a chromium impoverished region of the alloy, and
then quantitatively determining an average spatial distribution of
the chromium impoverished region of the test piece, or the
chromium-concentration-specific volume of the chromium impoverished
region adjacent to the crystal grain boundaries of the test piece,
according to a given calculation formula. The present invention can
solve disadvantages in a conventional method of inspecting a
chromium impoverished region of a chromium-containing nickel-based
alloy, such as destruction of the alloy surface caused by an
etching or breaking operation, which is incongruous with the
philosophy of a nondestructive inspection, and poor information
about chromium impoverished region, which is obtainable only in the
alloy surface.
Inventors: |
Takahashi, Seiki; (Iwate,
JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW
SUITE 700
WASHINGTON
DC
20036
US
|
Family ID: |
29243245 |
Appl. No.: |
10/510639 |
Filed: |
March 23, 2005 |
PCT Filed: |
April 10, 2003 |
PCT NO: |
PCT/JP03/04581 |
Current U.S.
Class: |
148/509 ;
266/99 |
Current CPC
Class: |
G01N 17/006 20130101;
G01N 33/2045 20190101; G01N 27/80 20130101 |
Class at
Publication: |
148/509 ;
266/099 |
International
Class: |
C22F 001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 12, 2002 |
JP |
2002-110706 |
Claims
What is claimed is:
1. A method for nondestructive inspection of grain-boundary attack
due to thermal sensitization in a chromium-containing nickel-based
alloy, comprising: measuring a saturation magnetization
M.sub.s(T.sub.i) of a test piece at each of a plurality of
measuring temperatures defined by equally dividing a given
measuring temperature range in the range of a minimum to a maximum
of Curie temperatures corresponding to respective chromium
concentrations in a chromium impoverished region of said alloy; and
calculating vk according to the following formula (1) to
quantitatively determine the volume of the chromium impoverished
region in a divided manner on the basis of the chromium
concentrations: 3 M s ( T i ) = k = 1 i v k M k ( T i ) V , ( 1 )
wherein: vk is the volume of the chromium impoverished region
having a chromium concentration C.sub.k; V is the volume of said
test piece; k is a natural number to be determined in conjunction
with dividing the range of a minimum measuring temperature
T.sub.min to a maximum measuring temperature T.sub.max, into n
equal parts, in conformity to measurement conditions; and M.sub.k
(T.sub.i) is a saturation magnetization at a measuring temperature
T.sub.i in the chromium impoverished region having the chromium
concentration C.sub.k, said saturation magnetization being obtained
in advance based on the following data (a), (b) and (c): (a) the
relationship between saturation magnetization and chromium
concentration at an absolute temperature of 0 (zero) K in the
chromium impoverished region; (b) the relationship between Curie
temperature and chromium concentration in the chromium impoverished
region; and (c) the relationship between saturation magnetization
and measuring temperature in the chromium impoverished region.
2. An apparatus for detecting magnetic characteristics of the test
piece for use in the method as defined in claim 1, comprising: a
cooling-medium tank for containing a cooling medium; a test-piece
housing disposed at the central region of said cooling-medium tank
to receive said test piece therein; an exciting device mounted on
the inner wall of said test-piece housing to excite said test
piece; a support member for supporting said test piece in such a
manner that it is located at the center position of said exciting
device; a magnetic flux detector disposed around said test piece; a
cooling device for supplying a cooling medium to said
cooling-medium tank to circulatingly cool said test piece while
allowing cooling gas generated from said cooling medium to flow
into said test-piece housing; a heating device disposed below said
test piece to heat said test piece; and means for controlling the
measuring temperature of said test piece through the use of said
cooling medium and said heating device.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for nondestructive
inspection of grain-boundary attack due to thermal sensitization in
a chromium-containing nickel-based alloy as typified by Inconel
alloys. The present invention also relates to an inspection
apparatus for use in this method.
BACKGROUND ART
[0002] Inconel alloys, which are typical ones of
chromium-containing nickel-based alloys, are a heat-resistant alloy
comprising major components of nickel and about 15 to 23 wt % of
chromium, and some of the Inconel alloys additionally contain at
least one of iron, cobalt and molybdenum. A typical one or Inconel
600 alloy (Ni: 76.0 wt %, Cr: 15.5 wt %, Fe: 7.8 wt %, Mn: 0.4 wt
%, Si: 0.2 wt %, C: 0.08 wt %) is widely used in peripheral
equipment of nuclear reactors, thermal plants, chemical plants,
etc. In these cases, due to thermal sensitization caused by a heat
treatment in a welding process or the like or a high-temperature
condition maintained for a long time of period, chromium carbide
precipitates are formed along the crystal grain boundaries of the
alloy to thereby produce chromium-impoverished region. This
chromium impoverished region is one of the factors causing stress
corrosion cracking.
[0003] Heretofore, such stress corrosion cracking has been
generally inspected through methods based on destructive tests,
such as a method in which a test piece is set in an operating
environment, and periodically taken out to subject it to etching
using a chemical agent and optical-microscopic observation, a
method in which chromium-carbide precipitates in a test piece are
electrochemically etched and checked up, and a Charpy test.
[0004] For example, Japanese Patent Publication No. 02-054501
discloses an grain-boundary attack test method in which
polarization is performed at a potential allowing nickel to cause
active dissolution in an aqueous solution containing nitric acid,
to detect a chromium impoverished region of a chromium-containing
nickel-based alloy.
[0005] It has also been known to use a magnetic measurement method
in detecting high-temperature aging embrittlement or strain damage
in an actual member as an iron-based alloy product, such as
ferrite-containing stainless steel or low-alloy steel (see, for
example, Japanese Patent publication No. 07-006950 and Japanese
Laid-Open Patent publication No. 04-218764).
DISCLOSURE OF INVENTION
[0006] As mentioned above, a chromium-containing nickel-based alloy
typified by the Inconel alloys is thermally sensitized to form
chromium carbide precipitates in the vicinity of the crystal grain
boundaries of the alloy and thereby produce a chromium impoverished
region in the crystal grains. FIG. 1 schematically shows a chromium
concentration distribution in the state after chromium carbide
precipitates are formed in the vicinity of the crystal grain
boundaries of the Inconel 600 alloy. For example, as illustrated, a
portion having 10 wt % or less of chromium concentration is formed
around the crystal grain boundary to cause a high risk of stress
corrosion cracking. This chromium concentration distribution is
caused by chromium carbide precipitates to be formed when a
structural member of the Inconel 600 alloy is subjected to a heat
treatment in a welding process, or held at a temperature of 600 to
700.degree. C. for a long time of period, and the pattern of the
distribution is varied depending on the holding time.
[0007] The conventional method of inspecting a chromium
impoverished region involves a destructive operation, such etching
or breaking of the surface of a test piece as described above,
which is incongruous with the philosophy of a nondestructive
inspection, and can obtain information about the chromium
impoverished region only in the surface of the test piece.
Moreover, in the conventional inspection method, a great deal of
time and effort has to be consumed to quantitatively determine
chromium concentrations in the chromium impoverished region and the
volume of the chromium impoverished region.
[0008] A conventional inspection method based on the measurement of
magnetic susceptibility permits only qualitative observation of
precipitates of chromium atoms and a chromium impoverished region
in the vicinity of the crystal grain boundaries.
[0009] The aforementioned magnetic measurement method for a
Fe-based alloy member is intended to calculate the variation of
saturation magnetization based on the comparison with known data.
Thus, the volume of a chromium impoverished region caused by the
formation of chromium carbide precipitates in a chromium-containing
nickel-based alloy cannot be determined in a divided manner on the
basis of the chromium concentrations in chromium impoverished
region.
[0010] The inventor has developed a nondestructive inspection
method capable of quantitatively measuring the presence of a
chromium impoverished region causing grain-boundary attack cracking
in a chromium-containing nickel-based alloy, by use of a magnetic
means, and an inspection apparatus for use in this method.
[0011] Specifically, the present invention provides a method for
nondestructive inspection of grain-boundary attack due to thermal
sensitization in a chromium-containing nickel-based alloy. The
method comprises: measuring a saturation magnetization
M.sub.s(T.sub.i) of a test piece at each of a plurality of
measuring temperatures defined by equally dividing a given
measuring temperature range in the range of a minimum to a maximum
of Curie temperatures corresponding to respective chromium
concentrations in a chromium impoverished region of the alloy; and
calculating vk according to the following formula (1) to
quantitatively determine the volume of the chromium impoverished
region in a divided manner on the basis of the chromium
concentrations: 1 M s ( T i ) = k = 1 i v k M k ( T i ) V , ( 1
)
[0012] wherein: vk is the volume of the chromium impoverished
region having a chromium concentration C.sub.k; V is the volume of
the test piece; k is a natural number to be determined in
conjunction with dividing the range of a minimum measuring
temperature T.sub.min to a maximum measuring temperature T.sub.max,
into n equal parts, in conformity to measurement conditions; and
M.sub.k (T.sub.i) is a saturation magnetization at a measuring
temperature T.sub.i in the chromium impoverished region having the
chromium concentration C.sub.k. The saturation magnetization is
obtained in advance based on the following data (a), (b) and (c):
(a) the relationship between saturation magnetization and chromium
concentration at an absolute temperature of 0 (zero) K in the
chromium impoverished region; (b) the relationship between Curie
temperature and chromium concentration in the chromium impoverished
region; and (c) the relationship between saturation magnetization
and measuring temperature in the chromium impoverished region.
[0013] The present invention also provides an apparatus for
detecting magnetic characteristics of the test piece for use in the
above method. The apparatus comprises: a cooling-medium tank for
containing a cooling medium; a test-piece housing disposed at the
central region of the cooling-medium tank to receive the test piece
therein; an exciting device mounted on the inner wall of the
test-piece housing to excite the test piece; a support member for
supporting the test piece in such a manner that it is located at
the center position of the exciting device; a magnetic flux
detector disposed around the test piece; a cooling device for
supplying a cooling medium to the cooling-medium tank to
circulatingly cool the test piece while allowing cooling gas
generated from the cooling medium to flow into the test-piece
housing; a heating device disposed below the test piece to heat the
test piece; and means for controlling the measuring temperature of
the test piece through the use of the cooling medium and the
heating device.
[0014] While the present invention will be specifically described
in connection with Inconel 600 alloy known as a typical one of
chromium-containing nickel-based alloys, an object to be inspected
by the method of the present invention is not limited to the
Inconel 600 alloy. Inconel alloys include Inconel 600, Inconel 601,
Inconel 625, Inconel 690 and Inconel 617. The Inconel 600 consists
of about 74 wt % of nickel, about 16 wt % of chromium and about 10
wt % of iron. The term "chromium-containing nickel-based alloy" in
the present invention means a nickel-based alloy containing
chromium in an amount to the extent of causing the grain boundary
precipitation of chromium carbide as in these Inconel alloys.
[0015] The Inconel 600 alloy has a Curie temperature (magnetic
transition temperature) of 109 K. The mechanism of thermal
sensitization and the temperature causing the thermal sensitization
in the Inconel 600 alloy has been investigated through various
researches. Heretofore, the verification of the thermal
sensitization has been conducted based on etching using a chemical
agent or electrochemical etching, and it has been reported that the
thermal sensitization is accelerated at a temperature of 600 to
700.degree. C.
[0016] In a chromium-containing nickel-based alloy, chromium
carbide precipitates are formed in the vicinity of the crystal
grain boundaries of the alloy, and the Curie temperature of a
chromium impoverished region is dependent on the chromium
concentration of the chromium impoverished region. FIG. 2 shows the
relationship between Curie temperature and chromium concentration
in a chromium impoverished region of the Inconel 600 alloy. While
the gradient of the line illustrated in FIG. 2 is slightly changed
at a chromium concentration of 14 wt %, the Curie temperature is
increased approximately in proportion to decrease in the chromium
concentration. The lowest chromium concentration in the previous
researches is 6 wt %, which corresponds to a Curie temperature of
450 K.
[0017] In the inspection method of the present invention, a
saturation magnetization at a measuring temperature T.sub.i in a
chromium impoverished region having a chromium concentration
C.sub.k is obtained in advance based on the above relationship
between the Curie temperature and the chromium concentration, and a
saturation magnetization of a test piece of chromium-containing
nickel-based alloy is actually measured at each of a plurality of
measuring temperatures defined by equally dividing a given
measuring temperature range in the range of 109 K to 450 K or a
minimum to a maximum of Curie temperatures corresponding to
respective chromium concentrations in a chromium impoverished
region of the alloy. Then, based on the pre-determined saturation
magnetization and the actually measured saturation magnetization,
an average spatial distribution of the chromium impoverished region
of the test piece, or the chromium-concentration-specific volume of
the chromium impoverished region adjacent to the grain boundaries
of the test piece, is quantitatively determined according to a
given calculation formula.
[0018] According to the method of the present invention, the volume
of the chromium impoverished region of the chromium-containing
nickel-based alloy adjacent to the crystal grain boundaries having
chromium carbide precipitates formed during a welding process or
high-temperature long-term use can be quantitatively determined in
a divided manner on the basis of the chromium concentrations in the
chromium impoverished region.
[0019] In addition, the variation in substantial chromium carbide
precipitates of Inconel 600 structural member can be quantitatively
determined in accordance with the respective calculated volumes
v.sub.1, v.sub.2, - - - v.sub.n of the chromium impoverished
region.
[0020] Furthermore, as to chemical heterogeneity which is one of
factors causing stress corrosion cracking, for example, the volume
of the chromium impoverished region having a chromium concentration
of 10 wt % can be calculated to compute a probability of occurrence
of stress corrosion cracking through simulation techniques or the
like.
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 is a schematic diagram showing a chromium
concentration distribution in chromium carbide precipitates and
chromium-deficient portions adjacent to the crystal grain boundary
of Inconel 600 alloy.
[0022] FIG. 2 is a graph showing the relationship between Curie
temperature and chromium concentration in Inconel 600 alloy.
[0023] FIG. 3 is a graph showing one example of the distribution of
the chromium-concentration-specific volume of a chromium
impoverished region adjacent to the crystal grain boundaries of the
Inconel 600 alloy.
[0024] FIG. 4 is a graph showing the relationship between
saturation magnetization and chromium concentration at an absolute
temperature of 0 (zero) K in the chromium impoverished region of
the Inconel 600 alloy.
[0025] FIG. 5 is a graph showing the relationship between
saturation magnetization and temperature (both the saturation
magnetization and temperature are normalized) in the chromium
impoverished region of the Inconel 600 alloy having a chromium
concentration C.sub.Cr ranging from 14 wt % to 16 wt %.
[0026] FIG. 6 is a graph showing the relationship between
saturation magnetization and temperature (both the saturation
magnetization and temperature are normalized) in the chromium
impoverished region of the Inconel 600 alloy having a chromium
concentration C.sub.Cr ranging from 9 wt % to less than 14 wt
%.
[0027] FIG. 7 is a graph showing the relationship between
saturation magnetization and magnetic susceptibility in the Inconel
600 alloy.
[0028] FIG. 8 is a partially sectional conceptual diagram showing
an apparatus for detecting magnetic characteristics of a test piece
for use in implementing a method of the present invention.
[0029] FIG. 9 is a graph showing a magnetization curve of a test
piece at each measuring temperature, wherein the test piece is
prepared by aging Inconel 600 alloy at 700.degree. C. for 10
hours.
[0030] FIG. 10 is a graph showing one example of inspection results
in which Inconel 600 alloy was subjected to aging at 700.degree. C.
for each of 1 hour and 10 hours to produce a chromium impoverished
region, and the distribution of the chromium-concentration-specific
volume of the chromium impoverished region was actually determined
through the method of the present invention.
[0031] FIG. 11 is a graph showing another example of inspection
results in which Inconel 600 alloy was subjected to aging at
700.degree. C. for each of 10 hours and 100 hours to produce a
chromium impoverished region, and the distribution of the
chromium-concentration-specific volume of the chromium impoverished
region was actually determined through the method of the present
invention.
[0032] FIG. 12 is a graph showing still another example of
inspection results in which the thickness d of the chromium
impoverished region in FIG. 10 was determined with respect to each
of the chromium concentrations.
[0033] FIG. 13 is a graph showing yet another example of inspection
results in which the thickness d of the chromium impoverished
region in FIG. 11 was determined with respect to each of the
chromium concentrations.
BEST MODE FOR CARRYING OUT THE INVENTION
[0034] A process for determining the volume vk of a chromium
impoverished region having a chromium concentration C.sub.k in a
chromium-containing nickel-based alloy is programmed according to
the following analysis procedure.
[0035] When Inconel 600 alloy as a typical one of
chromium-containing nickel-based alloy has a chemically homogeneous
composition free of the formation of carbide precipitates, the
saturation magnetization of the alloy is zero at its Curie
temperature (109 K) or more. In contrast, when a chromium
impoverished region exists therein, the alloy has a certain
saturation magnetization depending on a chromium concentration in
the chromium impoverished region even at a temperature of 109 k or
more. In cases where the saturation magnetization of a test piece
is measured at a measuring temperature T.sub.i of 109 K or more,
the saturation magnetization M.sub.s (T.sub.i) can be determined
according to the following calculation formula (1): 2 M s ( T i ) =
k = 1 i v k M k ( T i ) V ( 1 )
[0036] In the above formula, vk is the volume of the chromium
impoverished region having a chromium concentration C.sub.k, and V
is the volume of the test piece. k is a natural number to be to be
determined in conjunction with dividing the range of a minimum
measuring temperature T.sub.min to a maximum measuring temperature
T.sub.max, into n equal parts, in conformity to measurement
conditions. A larger numerical value of n allows the distribution
to be measured with a higher degree of accuracy. Conversely, an
excessively small value of n permits only a rough analysis of the
distribution. The minimum measuring temperature T.sub.min may be
set at 105 K or 100 K, because the measurement may be fundamentally
initiated at any temperature less than the Curie temperature of a
chromium-containing nickel-based alloy. The maximum measuring
temperature T.sub.max is preferably set at 450 K. If it is set at a
room temperature of 300 K, a chromium concentration distribution
ranging from 16 wt % to 9 wt % will be measured in the chromium
impoverished region. The following description will be made in
connection with the case where T.sub.min and T.sub.max are set at
100 K and a room temperature of 300 K, respectively.
[0037] M.sub.k (T.sub.i) is a saturation magnetization at a
measuring temperature T.sub.i in the chromium impoverished region
having the chromium concentration C.sub.k. The saturation
magnetization is obtained in advance based on the following data
(a), (b) and (c).
[0038] (a) The relationship between saturation magnetization and
chromium concentration at an absolute temperature of 0 (zero) K in
the chromium impoverished region
[0039] (b) The relationship between Curie temperature and chromium
concentration in the chromium impoverished region
[0040] (c) The relationship between saturation magnetization and
measuring temperature in the chromium impoverished region
[0041] More specifically, a saturation magnetization M.sub.k (0) of
the chromium impoverished region with the chromium concentration
C.sub.k at an absolute temperature of 0 (zero) K is first
determined. Further, a Curie temperature T.sub.c K of the chromium
impoverished region with the chromium concentration C.sub.k is
determined. Then, according to the relationship (c) between a
normalized saturation magnetization M.sub.k (T.sub.i)/M.sub.k (0)
and a normalized temperature T.sub.i/T.sub.c K, a saturation
magnetization M.sub.k (T.sub.i)/M.sub.k (0) at the temperature
T.sub.i is calculated. Based on this saturation magnetization
M.sub.k (T.sub.i)/M.sub.k (0) and the previously determined M.sub.k
(0), a saturation magnetization M.sub.k (T.sub.i) of the chromium
impoverished region with the chromium concentration C.sub.k at the
temperature T.sub.i can be obtained.
[0042] In this manner, M.sub.k (T.sub.i) in the formula (1) is
determined in advance. Then, a saturation magnetization M.sub.k
(T.sub.i) of a test piece at T.sub.i K is measured. Based on the
pre-determined saturation magnetization and the measured saturation
magnetization, the simultaneous equations of the formula (1) can be
solved to obtain vk.
[0043] FIG. 3 shows the pre-determined values expressing the
relationship between the volume vk and the chromium concentration
C.sub.k in the chromium impoverished region of the Inconel 600
alloy. FIG. 4 shows the pre-determined values expressing the
relationship between the saturation magnetization and the chromium
concentration in the chromium impoverished region of the Inconel
600 alloy at an absolute temperature of 0 (zero) K. Further, FIG. 5
shows the relationship between a saturation magnetization (which is
normalized by the saturation magnetization at an absolute
temperature of 0 (zero) K) and each measuring temperature (which is
normalized by the Curie temperature T.sub.c) in one example where
the chromium impoverished region of the Inconel 600 alloy has a
chromium concentration of 14 wt % or more. In the same manner, FIG.
6 shows the relationship in another example where the chromium
impoverished region has a chromium concentration of less than 14 wt
%.
[0044] A saturation magnetization of the chromium impoverished
region in the chromium concentration C.sub.k at an absolute
temperature of 0 (zero) K can be obtained from FIG. 4. Further, a
Curie temperature in the chromium concentration C.sub.k can be
obtained from FIG. 2. Based on the ratio between this Curie
temperature in the chromium concentration C.sub.k and the measuring
temperature T.sub.i, a saturation magnetization M.sub.k (T.sub.i)
at the measuring temperature T.sub.i in the chromium impoverished
region with the chromium concentration C.sub.k is obtained using
FIG. 5 or 6.
[0045] A measured value of the saturation magnetization M.sub.s
(T.sub.i) of the test piece at the measurement temperature T.sub.i
is obtained by determining a magnetization curve. The magnetization
curve at the measuring temperature T.sub.i has both a paramagnetic
state and a ferromagnetic state. The ferromagnetic state arises
from a region having a low chromium concentration and a Curie
temperature of the measuring temperature T.sub.i or less.
[0046] In the formula (1), except for vk, all of the physical
values are determined through the above process. The number of
unknown values is n, and the number of formulas (1) is n
correspondingly. Thus, the unknown values vk can be obtained by
solving the simultaneous equations.
[0047] In order to obtain v.sub.1, v.sub.2, - - - v.sub.n through
the above process, it is required to obtain an actually measured
M.sub.s (T.sub.i). In cases where M.sub.s (T.sub.i) is obtained
directly through the above process, a cooling medium and a heater
are used to control the test piece to have each of the measuring
temperatures, and a magnetic field is applied to the test piece
from outside, for example, in the range of 0 to 2.times.10.sup.4
Oe, to measure M.sub.s (T.sub.i). The generation of a strong
magnetic field involves a problem, such as increase in size of a
measuring apparatus. In consideration of this point, a process for
simply determining M.sub.s (T.sub.i) under a weak magnetic field
will be described below.
[0048] A magnetic susceptibility at the measuring temperature and a
saturation magnetization have a simple relationship therebetween as
expressed by the following formula (2):
.chi.0 (T.sub.i)=A M.sub.s (T.sub.i), (2)
[0049] wherein A is a proportionality multiplier.
[0050] For example, given that a magnetization by 50 Oe is defined
as a magnetic susceptibility .chi.0, .chi.0 (T.sub.i) and M.sub.s
(T.sub.i) are correlated with one another as shown in FIG. 7. As
seen in FIG. 7, .chi.0 (T.sub.i) and M.sub.s (T.sub.i) have a
strong correlation therebetween. The proportionality multiplier A
is a constant almost independent of the measuring temperature and
the chromium concentration. Thus, instead of directly measuring the
saturation magnetization M.sub.s (T.sub.i) at the measuring
temperature T.sub.i, the saturation magnetization M.sub.s (T.sub.i)
can be determined indirectly from the relationship of the formula
(2) based on a magnetization measured under a weak magnetic field.
In this case, the value of A is determined in advance.
[0051] The magnetization characteristic obtained by the above
measurement at each of the measuring temperatures is based on
magnetization under a weak magnetic field of 50 Oe, and thereby the
saturation magnetization values of the test piece cannot be
directly obtained. Thus, it is required to determine in advance a
coefficient for obtaining an intended magnetization characteristic
through a conventional magnetization measurement. However, this
coefficient can be determined in advance by measuring the
magnetization characteristic of a test piece identical to a known
material having actually measured data.
[0052] The magnetic susceptibility value is determined from the
pseudo-magnetization characteristic obtained in the above manner.
Then, based on this value, the presence of substantial thermal
sensitization in the test piece due to precipitation of chromium
atoms can be verified, and then the quantity of the thermal
sensitization can be determined.
[0053] In the above process, the constant A is dependent on the
internal structure of a material. Thus, this constant A is obtained
in advance using a preliminary-test piece having the same material
as that of the actual test piece, and then the relationship between
the magnetic susceptibility and volume fraction of the chromium
impoverished region is determined according to the formula (2)
using the obtained constant. This relationship allows the volume of
the chromium impoverished region transformed by thermal
sensitization to be readily determined correspondingly to the
magnetic susceptibility obtained through the aforementioned
measurement. Thus, any strong magnetic field is not required to
obtain v.sub.1, v.sub.2, - - - v.sub.n.
[0054] As above, according to the nondestructive inspection method
of thermal sensitization in a structural member of Inconel 600
alloy, a magnetic susceptibility value is derived by the
pseudo-magnetization curve obtained from an actual measurement to
accurately determine the volume of the chromium impoverished region
as shown in FIG. 10 without destruction of the structural member.
In addition, through the comparison between the respective states
of a specific material before and after thermally sensitized, an
average spatial distribution of the chromium impoverished region
due to thermal sensitization in a structural member of Inconel 600
alloy can be measured in a nondestructive manner. The above
analysis can be readily performed using a pre-programmed computing
unit.
[0055] FIG. 8 is a partially sectional conceptual diagram of a
magnetic-characteristic detecting apparatus for use in implementing
the method of the present invention. As shown in FIG. 8, a
test-piece housing 2 is disposed at the central region of a
cooling-medium tank 1, and an exciting device 3 composed of an
electromagnet or a superconducting magnet is mounted on the inner
wall of the housing 2. A cooling medium is supplied from a cooling
device 4 to the cooling-medium tank 1 through a supply pipe 5. A
cooling-gas supply pipe 6 is provided at the bottom wall of the
test-piece housing 2 to allow gas generated from the cooling medium
to flow into the test-piece housing 2. After cooling a test piece
8, the gas is circulated from the upper portion of the test-piece
housing 2 to the cooling device 4 through a cooling-gas discharge
pipe 7.
[0056] The test piece 8 is attached to a test-piece support member,
and inserted into the housing 2 in such a manner that it is located
at the central position of the exciting device 3. In FIG. 8, the
test piece 8 is attached at the lower portion of a test-piece
support rod 9 serving as the test-piece support member, and the
test-piece support rod 9 is inserted from the center of the top
wall of the test-piece housing 2 so as to allow the test piece 8 to
be located at the central position of the exciting device 3. A
magnetic flux detector 10 is attached to the test-piece support rod
9 in such a manner that it is located around the test piece 8.
Measurement data from the magnetic flux detecting device 10 are
transferred through a lead wire 11 to a computing unit 12 for
analyzing a magnetization characteristic to calculate the volume of
a chromium impoverished region.
[0057] A controller (not shown) is operable to supply an exciting
current to the exciting device 3. A heating device 13 is disposed
below the test piece 8 and close to the bottom wall of the
test-piece housing 2 to control a measuring temperature of the test
piece 8. A thermometer 14 is attached on the lower end of the
support rod 9 to measure the temperature of the test piece 8. The
cooling medium may be liquid nitrogen.
[0058] In a measurement using the above magnetic-characteristic
detecting apparatus, the temperature of the test piece 8 is evenly
controlled by a means (not shown) for controlling the measuring
temperature of the test piece through the use of the cooling medium
and the heating device 13. Then, the controller acts to supply an
exciting current to the exciting device 3, and measurement data
about voltage induced in the magnetic flux detector 10 in response
to the exciting current are led to the computing unit 12. The
computing unit amplifies and integrates the measurement data to
obtain the magnetization characteristic of the test piece 8 at the
measuring temperature, and the volume of the chromium impoverished
region calculated according to an analysis program is displayed on
a display unit 15. Subsequently, the measurement of magnetic
characteristic will be repeatedly performed at each of the
equally-divided measuring temperatures. Through the above
operation, a saturation magnetization M.sub.k (T.sub.i) of the test
piece at each of the plurality of measuring temperatures defined by
equally dividing a given measuring temperature range in the range
of a minimum to a maximum of Curie temperatures corresponding to
respective chromium concentrations in the chromium impoverished
region can be measured.
EXAMPLE
Example 1
[0059] Inconel 600 alloy was subjected to aging at 700.degree. C.
for 10 hours, and used as a test piece. The saturation
magnetization of this test piece was measured using the inspection
apparatus as shown in FIG. 9. Measuring temperatures were defined
by equally dividing a temperature range of 100 to 300 K into 10
parts. A magnetization curve at the respective measuring
temperatures was obtained as shown in FIG. 9. A magnetization curve
of a saturation magnetization M.sub.s (T.sub.i) at a measuring
temperature T.sub.i was extrapolated from the magnetization curve
illustrated in FIG. 9, and the saturation magnetization M.sub.s
(T.sub.i) was obtained from the intersecting point between the
extrapolated magnetization curve and the vertical axis (external
magnetic field: zero). Then, the relationship between the measuring
temperature T.sub.i and the saturation magnetization M.sub.s
(T.sub.i) (unit: emu/g) was obtained as shown in Table 1.
1TABLE 1 Saturation magnetization at each temperature in Inconel
600 subjected to aging at 700.degree. C. for 10 hours Temperature
100 K 120 K 140 K 160 K 180 K 200 K 220 K 240 K 260 K 280 K 300 K
M.sub.s (T.sub.i) 11.8 8.42 5.23 2.75 1.27 0.58 0.271 0.13 0.067
0.037 0.015
[0060] vk (k=1, 2, 3, - - -, 10) was obtained by the following
calculation.
[0061] As for M.sub.s (300), Ti=300 and k=1 can be assigned to the
formula (1) to express M.sub.s (300) as the following formula
(3):
M.sub.s (300)=v.sub.1M.sub.1 (300)/V (3)
[0062] A chromium impoverished region with a Curie temperature of
300 k or more has a chromium concentration of 10.2 wt % or less.
Given that the chromium impoverished region with a Curie
temperature of 300 k or more is represented by a chromium
impoverished region with a Curie temperature of 350 k. The chromium
impoverished region with a Curie temperature of 350 K has a
chromium concentration of 8.82 wt % as can be read from FIG. 2, and
then a saturation magnetization at an absolute temperature of 0
(zero) K is 38.3 emu/g as can be read from FIG. 4. A Curie
temperature at a chromium concentration of 8.82 wt % is 350 K. As
to a saturation magnetization at 300 K, a normalized saturation
magnetization corresponding to 300/350 (=0.86) is 0.52 as can be
read from FIG. 6, and a saturation magnetization based on the
pre-determined data is 38.3.times.0.52, or M.sub.1 (300)=19.9
emu/g. Then, M.sub.s (300) is 0.015 emu/g as can be read from Table
1. Thus, v.sub.1/V is determined as 7.5.times.10.sup.-4.
[0063] In the same manner, the volume v.sub.2 of the chromium
impoverished region having a Curie temperature of 280 K to less
than 300 K or a chromium concentration between 11..times.wt % or
less and 10 wt % or more can be determined according to the
following formula (4):
M.sub.s (280)={v.sub.1 M.sub.1 (280)+v.sub.2 M.sub.2 (280)}/V
(4)
[0064] M.sub.s (280)=0.037 emu/g can be read from the actually
measured value (Table 1). v.sub.1/V has been determined as
7.5.times.10.sup.-4 in the previous operation. M.sub.1 (280) is a
saturation magnetization at a Curie temperature of 280 K in a
chromium concentration of 8.82 wt %. Thus, a normalized saturation
magnetization corresponding to 280/350=0.80 is 0.602 as can be read
from FIG. 6, and a saturation magnetization based on the
pre-determined data is 38.3.times.0.602 or 23.0 emu/g. Then,
M.sub.2 (280) is a saturation magnetization in a chromium
concentration of 10.7 wt % (Curie temperature: 300 K). A saturation
magnetization of M.sub.2 (280) at an absolute temperature of 0
(zero) K is 33.8 emu/g as can be read from FIG. 4. Thus, a
normalized corresponding to 280/300 (=0.93) is 0.36 as can be read
from FIG. 6, and a saturation magnetization based on the
pre-determined data is 33.8.times.0.36. Based on this value,
M.sub.2 (280) at 280 K is determined as 12.2 emu/g. These values
are assigned to the formula (4), and resulting
0.037=7.7.times.10.sup.-4.times.23.0+12.2 v.sub.2/V is transformed
to v.sub.2/V=1.58.times.10.sup.-3 to obtain V.sub.2.
[0065] The above operations is repeatedly performed to determine
the values of other volume fractions v.sub.3/V, v.sub.4/V, - - -,
v.sub.10/V.
[0066] An actual Inconel 600 includes chromium precipitates even
before aging. In consideration of this situation, the relationship
between the chromium concentration and the volume of a chromium
impoverished region adjacent to the crystal grain boundary of the
alloy is determined as shown in FIGS. 10 and 11.
[0067] FIG. 10 shows an inspection result of a test piece having
precipitates through aging at 700.degree. C. for each of 1 hour and
10 hours. FIG. 11 shows an inspection result of a test piece having
precipitates through aging at 700.degree. C. for each of 10 hour
and 100 hours. These results show the volume of the precipitates
derived by subtracting a pre-aging volume from a post-aging volume.
The result in FIG. 11 is the distribution of chromium precipitates
in the vicinity of the grain boundaries calculated on the
assumption that the chromium precipitates are formed in the crystal
grain boundaries. The volume of a chromium impoverished region is
increased dependent on the aging time, and the deficient region
spreads over after 10 hours (FIG. 10). Then, chromium is supplied
from around the deficient region to be back to normal (FIG. 11).
This phenomenon corresponds to the result of optical microscopic
observation.
[0068] Alternatively, the distribution of the chromium impoverished
region adjacent to the crystal grain boundaries can be determined
through a simple process based on the previously obtained v.sub.1,
v.sub.2, v.sub.3, - - -, on the assumption that the crystal grains
have a spherical shape. Specifically, given that each of the
crystal grains has a spherical shape with a radius r and a volume
V, and the thickness of the chromium impoverished region with a
chromium concentration C.sub.k is d, the thickness d of the
chromium impoverished region can be determined with respect to each
of the chromium concentrations, because v.sub.k/V is obtained from
the following formula: v.sub.k/V=(4.pi.r.sup.2 d)/(4/3
.pi.r.sup.3)=3d/r. The result is shown in FIGS. 12 and 13, wherein
the horizontal axis represents the thickness (nm) of the chromium
impoverished region on the assumption that a crystal grain boundary
is 0 (zero) nm.
[0069] While the temperature range of 110 K to 300 K in the above
example has been divided into 10 parts, the temperature range may
be appropriately set up depending on requited information. Further,
the temperature range may be equally subdivided to obtain
information about the concentration distribution in the chromium
impoverished region more sensitively.
INDUSTRIAL APPLICABILITY
[0070] According to the present invention, a saturation
magnetization is directly measured using a magnetization measuring
apparatus or indirectly obtained using a measured magnetic
susceptibility, so as to determine the volume of a chromium
impoverished region in a chromium-containing nickel-based alloy in
a divided manner on the basis of chromium concentrations of the
alloy, in accordance with the above actually measured saturation
magnetization and a pre-determined saturation magnetization based
on preliminary obtained data about the chromium impoverished
region.
[0071] Thus, the present invention allows the level of thermal
sensitization in a structure made of a chromium-containing
nickel-based alloy typified by Inconel 600, such as nuclear
reactors, or power generators for thermal power plants, to be
reliably inspected in a nondestructive manner before occurrence of
cracks due to grain-boundary attack cracking. In addition, the
inspection can be performed using a simple magnetic-characteristic
detecting apparatus equipped with a small-size cooling device.
[0072] A nondestructive inspection cannot be achieved by any
etching-based inspection method irrespective of type, such as
chemical or electrochemical. Moreover, the conventional method
based on electrochemical etching can obtain poor information about
the chromium impoverished region only in the surface of a test
piece. In contrast, a magnetic method can obtain average
information about the entire test piece including the surface and
inside thereof. The method of the present invention is superior to
the conventional electrochemical-etching-based method in terms of
measurement accuracy.
* * * * *