Note: Descriptions are shown in the official language in which they were submitted.
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I
DESCRIPTION
AUSTENITIC STAINLESS STEEL, AND HYDROGENATION METHOD THEREOF
TECHNICAL FIELD
[0001]
The present invention relates to an austenitic stainless
steel, and to a hydrogenation method thereof. More
particularly, the present invention relates to an austenitic
stainless steel having reduced hydrogen embrittlement and
exhibiting superior fatigue characteristics, and to a
hydrogenation method of such an austenitic stainless steel. In
particular, the present invention relates to an austenitic
stainless steel in which occurrence of fatigue cracks and
growth of fatigue cracks in the austenitic stainless steel can
be suppressed by causing 30 wt ppm or more of hydrogen to be
stored in the surface of, or throughout, the austenitic
stainless steel.
BACKGROUND ART
[0002]
The use of hydrogen as a next-generation energy source
has received considerable attention from the standpoint of
global environmental concerns. Hence, development and research
on this topic are quite active. In particular, the development
and practical application of stationary fuel cells, fuel cell-
powered vehicles and the like that utilize hydrogen as fuel
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has become a major target of attention. The use of stainless
steel as a material for high-pressure hydrogen tanks and parts
thereof, as well as piping and the like in such fuel cell
systems, has been explored (for example, Patent document 1).
[0003]
The components of a typical austenitic stainless steel
are set forth in Table 1. The first column in Table 1 lists
the names of stainless steels and heat-resistant steels as
defined in JIS (Japanese Industrial Standards). The last
column of Table 1 shows the Vickers hardness of the stainless
steel (hereinafter, HV). Other columns correspond to the
chemical compositions of the stainless steel, with the units
of the components expressed in weight o. The content of
hydrogen (H) is expressed as weight ppm at the last component
column of Table 1.
Table 1
(Component units: wt-., *ppm by weight)
C Si Mn P S Ni Cr Mo Fe H* Other HV
SUS304
(A) 0.06 0.36 1.09 0.030 0.023 8.19 18.66 - Balance 2.2 - 176
SUS304
(B) 0.02 0.35 1.02 0.028 0.007 9.06 18.06 - Balance 1.1 - -
SUS304
(C) 0.05 0.47 0.99 0.032 0.005 8.14 18.21 - Balance 2.6 - -
SUS304
(D) 0.05 0.58 1.24 0.025 0.003 8.09 18.54 - Balance 2.2 - 176
SUS316
(A) 0.05 0.27 1.31 0.030 0.028 10.15 17.01 2.08 Balance 3.4 - 161
SUS316
0.05 0.29 1.37 0.030 0.026 10.05 16.89 2.01 Balance 1.2 - -
(B)
SUS316
0.02 0.53 0.98 0.021 0.001 10.15 16.21 2.08 Balance 1.5 - 164
(C)
SUS316L
(A) 0.019 0.78 1.40 0.037 0.010 12.08 17.00 2.04 Balance 2.6 - 157
SUS316L 0.010 0.53 0.77 0.023 0.001 12.13 17.16 2.86 Balance 1.5 - 145
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(B)
SUS310S
(A) 0.02 0.34 1.12 0.023 0.001 19.22 24.02 - Balance 2.8 - 132
SUS310S
(B) 0.01 0.34 1.07 0.024 0.001 19.22 24.05 - Balance 2.4 - -
SUS310S
(C) 0.04 0.42 0.38 0.019 <0.001 20.31 24.69 - Balance 4.7 - 151
V =
0.26,
Al =
SUUH660 0.04 0.05 0.42 0.016 0.001 24.30 13.59 1.09 Balance 1.2 Ti
2.22,
B =
0.003
[0004]
As is known, hydrogen penetrates into metallic materials
and reduces both the static strength and fatigue strength of
the material (for instance, Non-patent documents 1 and 2).
Various processes for removing such hydrogen, and methods for
predicting the effect of hydrogen, have been proposed. In the
method disclosed in Patent document 2, for example, austenitic
stainless steel is thermally treated, after a plating process,
by being kept at a temperature of 270 to 400 C for 10 minutes
or longer, to remove hydrogen thereby, in order to prevent
hydrogen embrittlement. Patent document 3 discloses a method
wherein the extent of hydrogen embrittlement of austenitic
stainless steel is predicted and determined based on the
chemical composition thereof.
[0005]
Non-patent document 1 discloses fatigue test results for
austenitic stainless steels according to SUS304, SUS316, and
SUS316L. The fatigue tests are conducted by comparing these
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austenitic stainless steels charged with hydrogen versus
austenitic stainless steels not charged with hydrogen. The
results of such a comparison shows that the fatigue crack
growth rate of hydrogen-charged SUS304 and SUS316 is faster
than in the corresponding uncharged steels. However, no clear
difference is seen in the case of SUS316L.
[0006]
In addition, Non-patent document 1 discloses fatigue
test results for JIS SUS304 and SUS316L austenitic stainless
steels after a test piece is prestrained and a microhole of
about 100 m is formed therein. The fatigue crack growth rate
is accelerated ten-fold in hydrogen-charged SUS304 compared
with an uncharged case. The fatigue crack growth rate is
accelerated two-fold in SUS316L.
[0007]
However, even meta-stable austenitic stainless steel can
undergo mechanically-induced martensitic transformation due to
cold-working and cyclic stress. Practitioners in this industry,
including groups of researchers in academic societies, have
commonly believed that hydrogen has almost no effect on the
fatigue crack growth rate in austenitic stainless steels such
as JIS SUS316L. Non-patent document 1 discloses results that
defy this common belief. This is the more significant in that
the results were obtained by applying cyclic loading at a low
frequency of 5 Hz or less.
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[0008]
Specifically, it has been shown that the growth rate of
fatigue cracks is accelerated by low-frequency cyclic loading
in austenitic stainless steel such as SUS316L. Meanwhile, Non-
patent document 2 points out the following: 11(3) The
martensitic phase resulting from transformation in austenitic
stainless steel becomes a pathway for hydrogen diffusion
throughout the material, and the diffusion coefficient of
hydrogen is increased thereby" (page 130). Non-patent document
3 indicates that growth of fatigue cracks in austenitic
stainless steels SUS304 and SUS316L can be suppressed by
removing non-diffusible hydrogen in ordinary steelmaking
processes.
[0009]
Patent document 1: Japanese Patent Application Laid-open
No. 2004-339569
Patent document 2: Japanese Patent Application Laid-open
No. H10-199380
Patent document 3: Japanese Patent Application Laid-open
No. 2005-9955
Non-patent document 1: Toshihiko KANEZAKI, Chihiro
NARAZAKI, Yoji MINE, Saburo MATSUOKA, and Yukitaka MURAKAMI:
"The effect of hydrogen on fatigue crack growth of pre-
strained austenitic stainless steel". The Japan Society of
Mechanical Engineers [No. 05-9] Proceedings of the 2005 Annual
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Meeting of JSME/MMD, M&M 2005 (Nov. 4 to 6, 2005, Fukuoka) P86,
pp. 595-596.
Non-patent document 2: Toshihiko KANEZAKI, Chihiro
NARAZAKI, Yoji MINE, Saburo MATSUOKA, and Yukitaka MURAKAMI:
"Martensitic transformation and effect of hydrogen on fatigue
crack growth in stainless steels". Transactions of the Japan
Society of Mechanical Engineers A. Vol. 72, No. 723, (November
2006), pp. 123-130 (manuscript received: May 1, 2006).
Non-patent document 3: Yukitaka MURAKAMI, Toshihiko
KANEZAKI, Yoji MINE, Saburo MATSUOKA: "Hydrogen Embrittlement
Mechanism in Fatigue of Austenitic Stainless Steels",
Metallurgical and Materials Transactions A, 39A(2008-6), pp.
1327-1339 (Manuscript received: November 25, 2007; online
publication: April 1, 2008)
[0010]
At present, however, sufficient analysis is still
lacking on how non-diffusible hydrogen, which is present in
grains, and diffusible hydrogen, which is charged from the
outside, are related to the aforementioned fatigue crack
growth rate in austenitic stainless steels. In addition, the
relationships according to which diffusible hydrogen and non-
diffusible hydrogen exert an influence on changes in the
amount of martensitic transformation, on the effect of
acceleration of the hydrogen diffusion rate, and on the
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fatigue crack growth rate in a material, have not been
sufficiently elucidated.
[00111
When used in equipment and devices related to hydrogen
fuel utilization, moreover, stainless steel is exposed to a
variety of environmental influences, depending on the usage
environment. When stainless steel is used, for instance, in
high-pressure hydrogen containers, piping and the like in a
fuel cell-powered vehicle, filling of the foregoing with
hydrogen gas and release through consumption of the hydrogen
gas are carried out repeatedly. In other words, hydrogen gas
loading and release cycles are repeated in the high-pressure
hydrogen container, piping and the like for fuel cell-powered
vehicles. These repeated cycles are accompanied by changes in
temperature in, for instance, the high-pressure hydrogen
container or piping for fuel cell-powered vehicles. It is
thought that hydrogen intrudes and diffuses thereupon into the
material to a degree greater than the equilibrium level at
room temperature.
[00121
Low-frequency cyclic loading occurs also due to, for
instance, temperature variations in the outside air
temperature. Conceivable examples of cyclic loading due to
variations in the outside air temperature include, for
instance, compression and expansion of the stainless steel
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itself, as a result of temperature differences between day and
night as well as thermal stress resulting from compression and
expansion of parts connected to stainless steel components. As
for the frequency of the cycle, the temperature difference
between day and night can range from only a few degrees to 10 C
or more, one cycle being thus 24 hours long. This means that,
for instance, high-pressure hydrogen tanks, equipment for
supplying fuel for fuel cells and the like in facilities
related to fuel-cell powered vehicles have a cycle measured in
single day units as noted above, and the hydrogen fill time is
accordingly long. In addition, a fuel cell-powered vehicle is
dependent on the environment in which it operates, and
experiences hence temperature differences of several C to
several tens of C, and cycles ranging from sub-seconds to
several hours.
DISCLOSURE OF THE INVENTION
[0013]
The present invention is based on the above technical
background, and attains the following objects.
It is an object of the present invention to provide an
austenitic stainless steel for suppressing the occurrence of
fatigue cracks and growth of fatigue cracks in the austenitic
stainless steel, and to provide a hydrogenation method of such
an austenitic stainless steel.
[0014]
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Another object of the present. invention is to provide an
austenitic stainless steel wherein the formation of fatigue
cracks, and/or fatigue crack growth, is slowed down through
addition of hydrogen to 30 wt ppm or more, by focusing on the
amount of diffusible hydrogen and non-diffusible hydrogen that
cause hydrogen embrittlement in austenitic stainless steel,
and to provide a hydrogenation method of such an austenitic
stainless steel.
It is yet another object of the present invention to
provide an austenitic stainless steel in which the fatigue
crack growth rate can be slowed down during low-frequency
cyclic loading, and to provide a hydrogenation method for such
an austenitic stainless steel.
[0015]
Definition of terms
The present invention uses the following technical terms
in the meanings defined below. Hydrogen charging refers to
causing hydrogen to penetrate into a material. Hydrogen
charging method refers to a method in which a material is
exposed in a high-pressure hydrogen chamber, a method in which
cathodic charging is performed, or a method in which the
material is immersed in a chemical solution or the like.
Fatigue crack growth refers to enlargement of fatigue cracks
in a material through the action of cyclic loading. Fatigue
cracks are defects or cracks generated in the material during
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a manufacturing or working process. Cyclic loading refers to
cyclic loading acting on holes or the like that are
artificially drilled in the material.
[0016]
Fatigue crack growth rate refers to the speed with which
a fatigue crack grows, specifically the length by which
fatigue crack length increases per unit time. Austenitic
stainless steel refers to Cr-Ni steel wherein Cr and Ni are
added to Fe to produce a stainless steel having an austenitic
phase that exhibits increased corrosion resistance in
corrosive environments and the like. Table 1 gives a list of
such stainless steels. Austenitic phase refers to a phase of
iron, at a temperature range of 911 to 1392 C, in 100% pure
iron (Fe), having a face-centered cubic lattice structure
(hereinafter, FCC structure).
[0017]
Fig. 9A illustrates a face-centered cubic lattice. The
austenitic phase can also exist at room temperature when
alloying elements such as Cr and Ni are added to Fe. A
martensitic phase is a structure obtained by quenching steel
from a high-temperature stable austenitic phase. The
martensitic phase has a body-centered cubic lattice structure
(hereinafter, BCC structure). Fig. 9B illustrates a body-
centered cubic lattice. The martensitic phase may arise
through the action of stress, such as cold-working and the
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like, on austenitic-phase stainless steel at ordinary
temperature.
[0018]
The transformation from an austenitic phase having an
FCC structure to a martensitic phase having a BCC structure by
cold working is referred to as mechanically-induced
martensitic transformation. Diffusible hydrogen refers to
hydrogen that is present in the material and escapes from the
material, over time, at room temperature. Non-diffusible
hydrogen refers to hydrogen present in the material and that
cannot escape from the material over time, even at
temperatures from room temperature to about 200 C.
[0019]
The present invention achieves the above objects on the
basis of the following means.
The inventors of the present invention found that the
fatigue strength characteristics of austenitic stainless steel
can be markedly improved if 30 wt ppm or more of diffusible
hydrogen and non-diffusible hydrogen are present in the
austenitic stainless steel. The present invention relates to
an austenitic stainless steel having an austenitic phase the
crystalline structure of which is a face-centered cubic
lattice structure, and to a hydrogenation method of the
austenitic stainless steel.
[0020]
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The austenitic stainless steel of the present invention
is an austenitic stainless steel having an austenitic phase
the crystalline structure of which is a face-centered cubic
lattice structure, wherein the austenitic stainless steel
undergoes a manufacturing process such that a region in which
the concentration of hydrogen (H) comprising diffusible
hydrogen and non-diffusible hydrogen contained in the
austenitic stainless steel is locally 0.0030 wt% (30 wt ppm)
or higher has a thickness of 100 m or more from the surface
of the austenitic stainless steel into the austenitic
stainless steel, and wherein occurrence of fatigue cracks in
the austenitic stainless steel is delayed, and/or growth of
the fatigue cracks is slowed down.
[0021]
The austenitic stainless steel hydrogenation method of
the present invention is an austenitic stainless steel
hydrogenation method of adding hydrogen to an austenitic
stainless steel, in order to increase the concentration of
hydrogen in an austenitic stainless steel having an austenitic
phase the crystalline structure of which is a face-centered
cubic lattice structure, the method comprising the step of
heating the austenitic stainless steel at a heating
temperature of 80 C or higher in a hydrogen environment, to
cause thereby a region in which a local concentration of the
hydrogen contained in the austenitic stainless steel is 0.0030
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wt*-. (30 wt ppm) or higher to be formed to a thickness of 100
m or more from the surface of the austenitic stainless steel
into the austenitic stainless steel.
[0022]
The concentration of the hydrogen (H) in the entirety of
the austenitic stainless steel may have a value of 0.0030 wt%
(30 wt ppm) or higher. The fatigue strength characteristics of
the austenitic stainless steel are markedly enhanced when the
concentration of hydrogen (H) contained in the austenitic
stainless steel is 0.0030 wto (30 wt ppm) or higher throughout
the austenitic stainless steel. The concentration of hydrogen
throughout the austenitic stainless steel will be referred to
hereafter as overall concentration.
[0023]
The fatigue strength characteristics of the austenitic
stainless steel are markedly enhanced when the concentration
of hydrogen (H) contained throughout a region having a
thickness of at least 100 m from the surface of an austenitic
stainless steel that has a cross-sectional smallest dimension
of 200 m or greater, is 0.0030 wt% (30 wt ppm) or higher.
Hereafter, the concentration of hydrogen throughout a region
of a predetermined thickness from the surface of austenitic
stainless steel will be referred to as local concentration.
The cross-sectional smallest dimension denotes the smallest
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dimension from among height, length and thickness of an
austenitic stainless steel material.
[0024]
In the case of an austenitic stainless steel round bar
material, for instance, the cross-sectional smallest dimension
denotes the diameter. In the case of plate-like austenitic
stainless steel, the cross-sectional smallest dimension
denotes the plate thickness. Preferably, the local
concentration or overall concentration of hydrogen (H)
contained in the austenitic stainless steel is 0.0050 wto (50
wt ppm) or higher.
[0025]
The Vickers hardness of austenitic stainless steel
containing no more than 0.0005 wto (5 wt ppm) of hydrogen is
defined herein as a Vickers hardness of 1. Austenitic
stainless steel manufactured in accordance with conventional
processes comprises 5 wt ppm or less of hydrogen, as set forth
in Table 1. In other words, the above Vickers hardness
corresponds to state where hydrogen is unavoidably taken up
during a conventional manufacturing processes, i.e. a
hydrogen-uncharged state. The Vickers hardness of austenitic
stainless steel in a region containing 30 wt ppm or more of
hydrogen is 1.05 or higher.
[0026]
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To add diffusible hydrogen and non-diffusible hydrogen,
the austenitic stainless steel may be subjected to a thermal
treatment at a heating temperature of 80 C or higher in a
hydrogen environment. Heating is effective within a heating
temperature range from 200 C to 500 C. The heating temperature
may be lower than a sensitization temperature, which is the
temperature at which chromium (Cr) carbides in the austenitic
stainless steel precipitate by heating. In the thermal
treatment, the austenitic stainless steel may be kept at the
above-described heating temperature for up to 460 hours in a
hydrogen environment.
[0027]
Addition of diffusible hydrogen and non-diffusible
hydrogen to austenitic stainless steel may rely, for instance,
on a method that involves exposure in a high-pressure hydrogen
chamber, a cathodic hydrogen charging method, or a method of
immersion in a chemical solution. Preferably, the hydrogen
environment is a chamber filled with hydrogen gas at 1 MPa or
higher.
[0028]
The present invention elicits the following effects.
Specifically, the present invention allows realizing an
austenitic stainless steel where fatigue crack occurrence and
fatigue crack growth are slowed down by bringing the
concentration of non-diffusible hydrogen and diffusible
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hydrogen in the austenitic stainless steel to 30 wt ppm or
higher, through a thermal treatment of the austenitic
stainless steel at a temperature of 80 C or higher in a
hydrogen environment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029]
Fig. 1 illustrates a hydrogen concentration distribution
in a 7 mm-diameter round bar material subjected to cathodic
hydrogen charging, as a function of depth from the surface;
Figs. 2A to 2D are diagrams illustrating schematically
an evaluation method of hydrogen concentration distribution;
Figs. 3A to 3C are diagrams illustrating schematically a
fatigue test piece, wherein Fig. 3A is a diagram illustrating
the shape of a fatigue test piece in Example 1, Fig. 3B is a
diagram illustrating the shape of a fatigue test piece in
Example 2, and Fig. 3C is a diagram illustrating the shape of
an artificial microhole formed in a fatigue test piece;
Fig. 4 is a diagram illustrating a relevant test area in
a fatigue test piece, the shape of a drilled artificial
microhole, and fatigue cracks developing at the artificial
microhole and propagating therefrom;
Fig. 5 is a photograph of fatigue cracks arising from
the artificial microhole after fatigue testing;
Figs. 6A and 6B are a graph illustrating the
relationship between number of cycles and the crack length of
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fatigue cracks as a result of fatigue testing of a fatigue
test piece in Example 1, wherein Fig. 6A corresponds to SUS304
and Fig. 6B corresponds to SUS316L;
Figs. 7A and 7B are a graph illustrating the
relationship between number of cycles and the crack length of
fatigue cracks as a result of fatigue testing of a fatigue
test piece in Example 1, wherein Fig. 7A corresponds to SUS304
and Fig. 7B corresponds to SUS316L;
Fig. 8 is a graph illustrating the relationship between
test stress amplitude a and fatigue life Nf at which a fatigue
test piece fractures in a SUS304 material having an artificial
microhole;
Figs. 9A and 9B are a set of conceptual diagrams
illustrating the lattices of the crystalline structures of an
austenitic phase and a martensitic phase, wherein Fig. 9A
shows a face-centered cubic lattice structure (FCC) of an
austenitic phase, and Fig. 9B shows a body-centered cubic
lattice structure (BCC) of a martensitic phase;
Fig. 10 is a graph illustrating an anticipated hydrogen
concentration distribution in SUS316L, from the surface
towards the interior, after 5 years in a hydrogen gas
environment at a temperature of 25 C and pressure of 35 MPa or
70 MPa; and
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Fig. 11 is a diagram illustrating a Vickers hardness
ratio versus hydrogen concentration in austenitic stainless
steel.
BEST MODE FOR CARRYING OUT THE INVENTION
[0030]
An embodiment of the present invention is explained next
on the basis of experimental examples. An explanation is given
first on how hydrogen affects the growth rate of fatigue
cracks in austenitic stainless steel. After an ordinary
thermal treatment (solution thermal treatment), austenitic
stainless steels such as SUS304, SUS316, and SUS316L shown in
Table 1 contain 1 to 4.7 wt ppm of non-diffusible hydrogen.
Upon placing this heat-treated austenitic stainless steel in a
hydrogen environment, hydrogen penetrates into the austenitic
stainless steel through the surface, and diffuses into the
material.
[0031]
The material exhibits a hydrogen concentration
distribution from the surface towards the interior. Fig. 1
illustrates a measurement example of the measurement of the
hydrogen concentration distribution of a material. Herein, 7
mm-diameter, about 30 mm-long round bars of SUS304, SUS316 and
SUS316L(A) set forth in Table 1 were subjected to cathodic
hydrogen charging. Thereafter, the hydrogen concentration
distribution of the round bars was measured, and the results
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are plotted in the graph of Fig. 1. The ordinate axis of the
graph of Fig. 1 represents hydrogen concentration. The
hydrogen concentration units are weight ppm. The abscissa axis
of the graph of Fig. 1 represents the distance from the
surface of the measurement sample. The units of the distance
from the surface are m. Cathodic hydrogen charging was
carried out as follows.
[0032]
An anode and a cathode were arranged in an aqueous
solution of sulfuric acid, and the anode and the cathode were
connected to a power source. The liquid temperature of the
aqueous solution of sulfuric acid was held at 50 C. The pH of
the aqueous solution of sulfuric acid was 3.5. The current
density was 27 A/m2. A platinum electrode was used in the anode.
Round bars of austenitic stainless steel were used as the
cathode. Cathodic hydrogen charging was carried out over 672
hours. Once the round bars were charged with hydrogen, the
hydrogen concentration distribution of the round bars was
measured in accordance with the procedure described below. The
dashed line in the Figure represents hydrogen concentration in
a round bar not subjected to hydrogen charging.
[0033]
Fig. 2A illustrates an austenitic stainless steel round
bar subjected to cathodic hydrogen charging, depicted herein
before measurement. As illustrated in Fig. 2A, a disc-like
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sample for measurement, about 0.8 mm thick, was sliced from
the austenitic stainless steel round bar having been subjected
to cathodic hydrogen charging. The amount of hydrogen
comprised in the sample was measured by thermal desorption
analysis. Thereafter, the round bars were ground using emery
grinding paper. The solid line in Fig. 2B denotes the round
bar after emery grinding . The dashed line in Fig. 2B denotes
the round bar of Fig. 2A, the surface of which has been
removed through emery grinding. As illustrated in Fig. 2B, a
further disc-like sample was sliced again out of the round bar.
The hydrogen amount in this latter sample was measured.
[0034]
The round bar of Fig. 2B was ground again with emery
grinding paper. The emery-ground round bar is denoted in Fig.
2C by a solid line. The dashed line of Fig. 2C is the round
bar illustrated in Fig. 2B. As illustrated in Fig. 2C, a
further disc-like sample was sliced again out of the round bar.
The hydrogen amount in this latter sample was measured. The
operation of grinding using emery grinding paper, sample
slicing and hydrogen amount measurement was thus repeated as
described above. Fig. 2B and Fig. 2C illustrate a respective
annular portion removed through grinding with emery grinding
paper. The annular portion is the portion between the round
bar depicted with a solid line and the portion depicted with
the dashed line.
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[0035]
The hydrogen concentration of the annular portion was
worked out by dividing the difference between the hydrogen
amount of the sample before emery grinding and the hydrogen
amount of the emery-ground sample, by the weight of the
annular portion. The ground sample volume can be calculated by
subtracting the sample volume after grinding from the sample
volume before grinding, as illustrated in Fig. 2D. The weight
of the annular portion and the hydrogen concentration in the
annular portion can be worked out, accordingly, on the basis
of the ground sample volume. The relationship between the
local concentration of hydrogen and depth from the surface of
a test piece of the round bar can be obtained thus by
repeating the above operation.
[0036]
The results illustrated in Fig. 1 show that the region
at which the hydrogen local concentration is 0.0030% (30 wt
ppm) or higher extends from 5 m to 60 m from the surface.
Thus, the hydrogen concentration of austenitic stainless steel
placed in a hydrogen environment exhibits a gradient derived
from penetration of hydrogen through the surface, and ensuing
diffusion. A gradient such that hydrogen concentration
decreases gradually from the surface towards the interior is
to be expected, given that the austenitic stainless steel is
used in an actual environment of high-pressure hydrogen gas.
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[0037]
For example, Fig. 10 illustrates the hydrogen
concentration distribution predicted for SUS316L on the basis
of a hydrogen solid solubility KS=4.64 wt ppm/ MPa1/'2, and a
diffusion coefficient D=8.42x10-17 mm2/s. In the case, for
instance, of SUS316L, the local hydrogen concentration at the
surface is about 0.0031 wto (31 wt ppm) after use over 5 years
at 25 C and a hydrogen pressure of 35 MPa. The local hydrogen
concentration at the surface is expected to exhibit a gradient
such that the local hydrogen concentration decreases gradually
from the surface towards the interior, up to about 400 m from
the surface. The region in which the local hydrogen
concentration is 30 wt ppm or higher is expected to extend up
to about 5 m from the surface.
[0038]
In the case, for instance, of SUS316L, an envisaged use
over 5 years at 25 C and a hydrogen pressure of 70 MPa is
expected to result in a local hydrogen concentration at the
surface of about 0.0049 wt% (49 wt ppm), and in a hydrogen
concentration gradient such that the local hydrogen
concentration decreases gradually towards the interior up to
about 400 m from the surface. The region in which the local
hydrogen concentration is 30 wt ppm or higher is expected to
extend up to about 80 m from the surface.
[0039]
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According to conventional knowledge, intrusion of
hydrogen into the austenitic stainless steel gives rise to
hydrogen embrittlement and impairment of fatigue strength
characteristics. However, the below-described fatigue tests
revealed that fatigue strength characteristics improved
noticeably when the amount of hydrogen that intrudes into the
austenitic stainless steel is 30 wt ppm or higher. No hydrogen
embrittlement due to intrusive hydrogen was observed when the
amount of hydrogen was 30 wt ppm or higher. In particular, the
inventors of the present invention carried out the following
experiments to observe how the content of hydrogen influenced
the fatigue crack growth rate. An example of such an
experiment is described next.
[0040]
[Test piece]
The materials used were the SUS304, SUS316, and
SUS316L(A) (hereinafter, simply SUS316L) austenitic stainless
steels set forth in Table 1. A solution thermal treatment was
performed on SUS304, and SUS316L. The shape of a fatigue test
piece made up of these materials is shown in Fig. 3A and Fig.
3B. The surface of the test piece was finished by buffing
after grinding with grade 2000 emery grinding paper.
[0041]
In order to facilitate observation of fatigue crack
growth, an artificial microhole 100 m in diameter and 100 m
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deep was drilled in the radial direction of the fatigue test
piece, at the center of the fatigue test piece in the
lengthwise direction, as shown in Fig. 3C. The drill bit had a
tip angle of 1200. The bottom of the artificial microhole
matched the shape (conical shape) of the tip angle of the
drill bit. The artificial microhole was drilled in the center
of the test area of the fatigue test piece. The test area is a
cylindrical portion at the center of the test piece.
[0042]
The length of the cylindrical portion of the test area,
i.e. the length of the portion having a same outer diameter,
is about 20 mm in the fatigue test piece of Fig. 3A, and of
about 14 mm in the fatigue test piece of Fig. 3B. Fig. 4
illustrates an outline of the test area and the shape of the
drilled artificial microhole. In the case of a hydrogen-
charged fatigue test piece, the piece was buffed again, and
the artificial microhole was drilled, immediately once
hydrogen charging was over.
[0043]
[X-ray diffraction]
The amount of martensite in the test area of the fatigue
test piece of austenitic stainless steel was measured by X-ray
diffraction. X-ray diffraction was performed using a micro X-
ray stress measurement apparatus PSPC-RSF/KM by Rigaku
Corporation (location: Akishima, Tokyo, Japan). Quantitative
24
CA 02733658 2011-02-02
analysis was determined on the basis of the integrated
intensity ratio of the diffraction peaks of the austenitic
phase {220} plane and the martensitic phase {211} plane, using
CrKa radiation. In SUS304, and SUS316L the content of
martensite in the test area before fatigue testing was about
3-0..
[0044]
In SUS304 and SUS316L the content of martensite in the
hydrogen-charged test area before fatigue testing was about 3%.
The content of martensite was measured in two places before
drilling of the artificial microhole. The first measurement
region was a circular region 1 mm in diameter centered on the
spot at which the artificial microhole was to be drilled. The
second measurement region was a region 1 mm in diameter
centered on a spot rotated by 180 about the lengthwise axis,
from the spot where the artificial microhole was to be drilled.
In other words, the second measurement region was located on
the opposite side of the cylindrical portion with respect to
the first measurement region.
[0045]
[Hydrogen charging method]
Hydrogen charging was performed using a cathodic
charging method or a high-pressure hydrogen exposure method.
In the cathodic hydrogen charging method, the cathodic
hydrogen charging conditions included an aqueous sulfuric acid
CA 02733658 2011-02-02
solution at pH=3.5, a platinum anode, and a current density of
i=27 A/m2. Cathodic hydrogen charging was performed for 672
hours (4 weeks) at a temperature of 50 C (323 K) of the aqueous
solution of sulfuric acid. The aqueous solution of sulfuric
acid was replaced once a week to minimize changes in the
sulfuric acid concentration resulting from evaporation.
[0046]
In the case of a high-pressure hydrogen exposure method,
the fatigue test piece was placed in a high-pressure hydrogen
gas environment at a pressure of 10 MPa, 25 MPa, 48 MPa, 74
MPa or 94 MPa, and a temperature of 235 C, 242 C, 250 C or 280 C,
to charge thereby the fatigue test piece with hydrogen. The
fatigue test piece of Fig. 3A was charged with hydrogen by
being kept for 400 hours, 414 hours, 416 hours or 419 hours,
while the fatigue test piece of Fig. 3B was charged with
hydrogen by being kept for 200 hours, in the high-pressure
hydrogen gas environment.
[0047]
[Fatigue test method]
In the fatigue test there was used a hydraulic servo-
controlled tensile and compressive fatigue testing machine
"Servopulser EHF-ED30KN" by Shimadzu Corporation (location:
Kyoto-shi, Kyoto, Japan), and a servo-controlled tensile and
compressive fatigue testing machine "8500" by Instron. The
fatigue test was carried out at a cycling frequency of 0.05 to
26
CA 02733658 2011-02-02
1.5 Hz, and a stress ratio of R=-l. The cycling frequency was
adjusted so that the surface temperature of the test area did
not exceed 60 C during the fatigue test. Fatigue cracks were
observed, and the length of the fatigue cracks was measured in
accordance with the replica method or by using a scanning
electron microscope S-2500CX, by Hitachi (location: Chiyoda-ku,
Tokyo, Japan).
[0048]
The fatigue cracks were observed by the replica method
was performed as follows. An approximately 0.034 mm-thick
acetyl cellulose film (hereinafter, replica film) was immersed
in methyl acetate liquid for a short time, and was then
affixed to the observation site of the fatigue test piece.
Once dried, the replica film was peeled off the fatigue test
piece, two or three minutes after affixing, and was recovered.
Gold was vapor-deposited on the recovered replica film, and
the fatigue cracks in the test area were observed with a
metallurgical microscope.
[0049]
The site of a target fatigue crack could thus be
observed even though the test piece was not observed directly.
In the case of a hydrogen-charged material, a sample 7 mm in
diameter and 0.8 mm thick was sliced from the test area
immediately after the end of fatigue testing, was placed in a
vacuum chamber, and was heated at a constant temperature rise
27
CA 02733658 2011-02-02
rate. The vacuum chamber internal pressure was 1X10-7 to 3x10-'
Pa before the sample was heated. The temperature rise rate in
the vacuum chamber was 0.33 C/s or 0.5 C/s.
[0050]
Heating the sample in the vacuum chamber caused hydrogen
to desorb from the sample. The amount of desorbed hydrogen was
measured using a quadrupole mass analyzer-type thermal
desorption analyzer (hereinafter, TDS). The TDS used for
measurement was a thermal desorption analyzer (hereafter, TDS)
EMD-WA1000S/H by ESCO, Ltd. (location: Musashino, Tokyo,
Japan). The precision of the TDS measurement was 0.01 wt ppm.
[0051]
[Measured properties]
Fig. 5 is a photograph of fatigue cracks that developed
from the artificial microhole drilled in hydrogen-uncharged
SUS304 after fatigue testing. The photograph shows fatigue
cracks spreading from the artificial microhole. These fatigue
cracks develop bilaterally from the artificial microhole, and
propagate in a roughly symmetrical manner. Figs. 6A and 6B are
graphs illustrating the relationship between the number of
cycles in the fatigue test and crack length of the fatigue
cracks in the fatigue test pieces as a result of fatigue
testing. The ordinate axis in the graphs of Figs. 6A and 6B
represents crack length.
[0052]
28
CA 02733658 2011-02-02
The abscissa axis in the graphs illustrated in Figs. 6A
and 6B represents the number of cycles in the fatigue test.
Figs. 6A and 6B correspond to an instance in which there was
used the fatigue test piece having a 7 mm-diameter test area
illustrated in Fig. 3A. Fig. 6A corresponds to an instance
where the material was SUS304. Fig. 6B corresponds to an
instance where the material was SUS316L. The graphs
illustrated in Figs. 6A and 6B depict measurement results for
both hydrogen-charged pieces and hydrogen-uncharged pieces,
for each material SUS304 and SUS316L. The cycling frequency
was 1 Hz or 1.2 Hz for SUS304, and 1 Hz for SUS316L. The
cycling frequency is virtually unaffected by the difference
between 1 Hz and 1.2 Hz.
[0053]
The graph illustrated in Fig. 6A indicates that a
fatigue test piece subjected to cathodic hydrogen-charging and
having a hydrogen concentration gradient identical to that of
Fig. 1 exhibits a faster fatigue crack growth rate than when
not subjected to hydrogen charging, in a test performed in the
atmosphere. For example, the number of cycles N until the
crack length 2a reaches 400 m is smaller in a material
subjected to cathodic hydrogen charging than in an uncharged
material. In these cases, the fatigue crack growth rate is
approximately twice as fast in the pieces subjected to
29
CA 02733658 2011-02-02
cathodic hydrogen charging. The results do not depend on the
hydrogen charging method, i.e. cathodic hydrogen charging.
[00541
In SUS304, where the test was carried out by switching
to a 0.68 MPa hydrogen gas atmosphere when the crack length 2a
reached 200 m, the fatigue crack growth rate became faster as
compared with that when the test was carried out in the
atmosphere, also in the case of a test piece not subjected to
hydrogen charging. By contrast, SUS304 having a total hydrogen
concentration of 70.4 wt ppm and 89.2 wt ppm, through exposure
to hydrogen gas in experimental examples of the present
invention, exhibited a markedly slower fatigue crack growth
rate as compared with the aforementioned hydrogen-uncharged
material and cathodically hydrogen-charged material.
[00551
The Vickers hardness of SUS304, in which the total
hydrogen concentration had been raised to 89.2 wt ppm, was 192,
i.e. 1.09 times the Vickers hardness 176 in the absence of
hydrogen charging. The Vickers hardness herein is measured
under a test load of 9.8 N, at room temperature, in the
atmosphere. In SUS304 having the total hydrogen concentration
raised to 70.4 wt ppm through exposure to hydrogen gas, the
fatigue crack growth rate exhibits no noticeable difference
between a test performed in the atmosphere and a test
performed in a 0.68 MPa hydrogen gas. Sufficient enhancing
CA 02733658 2011-02-02
effect on fatigue strength characteristics is thus obtained
also when the material is used in a hydrogen environment.
[0056]
Fig. 6B illustrates measurement results of SUS316L
subjected to a fatigue test in the atmosphere. Fatigue crack
growth rate is slower in a case where the total hydrogen
concentration of the test piece is 47 wt ppm, as compared with
a test piece not charged with hydrogen. The number of cycles N
until the crack length 2a reaches 400 m is greater in the
case of hydrogen charging to 30 wt ppm or higher, than in the
absence of hydrogen charging. The fatigue crack growth rate is
about 8 times slower in the case of hydrogen charging to 30 wt
ppm or higher.
[0057]
Fig. 7A and Fig. 7B are graphs illustrating the
relationship between crack length in a fatigue test piece and
number of cycles in a fatigue test. The fatigue test piece is
a test piece having a 4 mm-diameter test area illustrated in
Fig. 3B. Fatigue testing of the fatigue test piece was carried
out in the atmosphere. The graph of Fig. 7A illustrates
measurement results of a fatigue test of a test piece where
the material is SUS304, in a case where the test piece is
charged with hydrogen, and a case where not. The cycling
frequency in the fatigue test was 0.3 Hz. The graph of Fig. 7B
illustrates measurement results of a fatigue test using a test
31
CA 02733658 2011-02-02
piece where the material is SUS316L, in a case where the test
piece is charged with hydrogen, and a case where not.
[0058]
The cycling frequency in the fatigue test is 0.3 Hz
until the crack length 2a reaches about 400 m, and 0.05 Hz
thereafter. In the case of Fig. 7A, where the total hydrogen
concentration of the fatigue test piece is 23.8 wt ppm and
there is no region where the local hydrogen concentration is
30 wt ppm or higher (SUS304), the number of cycles N until the
crack length 2a reaches 1000 m is reduced by about 4/5 and
the fatigue crack growth rate becomes faster, vis-a-vis a case
where no hydrogen charging is carried out (SUS304).
[0059]
In the case of a total hydrogen concentration of 98.6 wt
ppm of the present invention (SUS304), by contrast, no fatigue
cracks derived from the artificial microhole are formed by the
stage where the number of cycles at which the crack length 2a
reaches 1000 m is N=11000 in case of no hydrogen charging.
This indicates that development of fatigue cracks is
suppressed. In the fatigue test piece (SUS316L) having a total
hydrogen concentration of 78.9 wt ppm, as illustrated in Fig.
7B, the number of cycles N until the crack length 2a reaches
1000 m is about 8 times greater than that when no hydrogen
charging is carried out (SUS316L). This is indicative of
32
CA 02733658 2011-02-02
significantly enhanced fatigue crack growth resistance in the
fatigue test piece.
[0060]
Fig. 8 illustrates the relationship between the test
stress amplitude a and the fatigue life Nf at which the
fatigue test piece fractures in the SUS304 material having an
artificial microhole. The ordinate axis of the graph
represents stress amplitude and the abscissa axis represents
fatigue life. In a comparison of fatigue life at a stress
amplitude of 280 MPa, the test piece containing 89.2 wt ppm of
total hydrogen concentration exhibits a fatigue life that is
about 8 times longer than that of a test piece not charged
with hydrogen. In a fatigue test piece containing 109 wt ppm
of total hydrogen concentration, no fatigue cracks develop
even at about 27 times the number of cycles of the fatigue
life of the fatigue test piece that is not charged with
hydrogen.
[0061]
The Vickers hardness of the fatigue test piece having a
total hydrogen concentration of 109 wt ppm was 193, namely
1.10 times the Vickers hardness 176 of the fatigue test piece
not charged with hydrogen. As a characterizing feature of the
invention, the austenitic stainless steel of the present
invention is charged with hydrogen to 30 wt ppm or more.
Occurrence of fatigue cracks in the austenitic stainless steel
33
CA 02733658 2011-02-02
is dramatically reduced by incorporating 30 wt ppm or more of
hydrogen into the austenitic stainless steel. Fatigue crack
growth in the austenitic stainless steel can likewise be
dramatically slowed down by incorporating 30 wt ppm or more of
hydrogen into the austenitic stainless steel.
[0062]
It can be easily inferred that fewer fatigue cracks,
and/or resistance against fatigue crack growth, should allow
prolonging the fatigue life of the austenitic stainless steel,
but fatigue crack growth is accelerated a case where the
region at which the local hydrogen concentration is 30 wt ppm
or higher is located about several tens of m from the surface
and the local hydrogen concentration further inward is less
than 30 wt ppm, as in the graph of Fig. 1.
[0063]
The number of cycles until the crack length 2a reaches
300 m takes up half the fatigue life, as illustrated in Figs.
6A and 6B. Therefore, fatigue strength characteristics can be
effectively enhanced if the local hydrogen concentration is 30
wt ppm or higher in a region at least at a depth of 100 m or
deeper, for a crack length 2a = 300 m. Preferably, the
austenitic stainless steel has a local hydrogen concentration
of 50 wt ppm or higher.
[0064]
34
CA 02733658 2011-02-02
An explanation follows next on the alloying components
in the austenitic stainless steel of the present invention, on
the content of the alloying components, and on a manufacturing
method as prescribed by the manufacturing method of the
present invention.
Austenitic stainless steel
Austenitic stainless steel, also called Cr-Ni stainless
steel, is obtained through addition of Cr and Ni to Fe. The
main components of austenitic stainless steel are Fe, Cr and
Ni, with various additives given in Table 2 below.
[0065]
Table 2 below shows preferred examples of the austenitic
stainless steel of the present invention, but the way in which
the present invention is embodied is in no way limited thereto.
Table 2
Component Composition 1 Composition 2
(weight ratio) (weight ratio)
C 0.030 or less 0.08 or less
Si 1.00 or less 1.50 or less
Mn 2.00 or less 2.00 or less
Ni 12.00 to 15.00 8.00 to 27.00
Cr 16.00 to 18.00 13.50 to 26.00
2.00 to 3.00 or
Mo less 3.00 or less
l
AI - 0.35 or less
N - 0.50 or less
Ti - 2.35 or less
V - 0.50 or less
B - 0.010 or less
H 0.00007 (0.7 ppm) 0.00007 (0.7 ppm)
or less or less
Balance Fe and Balance Fe and
Other unavoidable unavoidable
impurities impurities
CA 02733658 2011-02-02
[0066]
Composition of the austenitic stainless steel
Cr is added to Fe to improve corrosion resistance. Ni is
added to Fe, in combination with Cr, to increase corrosion
resistance. Ni and Mn are elements for securing non-magnetism
after cold rolling. The Ni content must be 10.0wto or higher
to secure non-magnetism after cold rolling. In addition, the
content of Ni must be adjusted in accordance with the content
of Si and Mn, in such a manner so as to preclude formation of
a mechanically-induced martensitic phase of lvolo or greater.
Mn also has the effect of improving the solid solubility of N.
[0067]
C is an element used for forming strong austenite. In
addition, C is an effective element for enhancing the strength
of stainless steel. When an excess of C is added, coarse Cr
carbides precipitate during a recrystallization process, and
intergranular corrosion resistance and fatigue characteristics
are impaired. Si is added for deacidification and
strengthening of the solid solution. Adding only a small
amount of Si is preferred, since formation of the martensitic
phase during cold-working is promoted by the Si content. N
brings solution hardening about.
[0068]
Mo is added for improving corrosion resistance and has
also the effect of bringing about a fine dispersion of
36
CA 02733658 2011-02-02
carbonitrides in an aging treatment. Ti is an effective
element for precipitation hardening, and is added to increase
the strength brought about by the aging treatment. B is an
effective alloying component for preventing edge cracks in the
hot-rolled steel area caused by the difference in deformation
resistance between the 8-ferrite phase in the hot working
temperature region and in the austenitic phase. Al is an
element added for deacidification during steelmaking, and is
effective in precipitation hardening, in a manner similar to
Ti.
[0069]
The present invention can also be embodied by adding
elements such as Nb, Cu or the like, as needed, besides the
elements described in Table 2 above. Nb can serve as a
substitute for Ti.
Austenitic phase
In a preferred austenitic stainless steel, the
austenitic phase is essentially 100% of the total volume.
Austenitic stainless steel having no martensitic phase is
preferred. Instances of abundant martensitic phase with
respect to austenitic phase, for instance as in Non-patent
document 2, do not fall under the definition of austenitic
stainless steel according to the present invention.
[0070]
Other properties
37
CA 02733658 2011-02-02
The average grain size is preferably about 50 m or less.
In modern materials the average grain size is about 50 m, but
a smaller average grain size is preferred.
[0071]
Hydrogenation treatment by heating
An explanation follows next on a hydrogenation treatment
of austenitic stainless steel by heating. Incorporating 30 wt
ppm or more of hydrogen into an austenitic stainless steel is
effective in enhancing resistance against fatigue crack
generation and/or growth. This effect of hydrogen has been
found for the first time by the inventors of the present
invention. To bring that effect about, 30 wt ppm or more of
hydrogen is added into austenitic stainless steel by carrying
out a thermal treatment as follows.
[0072]
The austenitic stainless steel is subjected to a thermal
treatment at a heating temperature of 80 C or higher to add
diffusible hydrogen and non-diffusible hydrogen. The thermal
treatment is carried out in a hydrogen environment. Hydrogen
environments include, for instance, high-pressure and low-
pressure hydrogen gas environments, cathodic hydrogen charging
environments, immersion hydrogen charging environments, as
well as liquid-phase or gas-phase environments having a high
partial pressure of hydrogen. In the thermal treatment, the
austenitic stainless steel is kept at the heating temperature
38
CA 02733658 2011-02-02
for up to 460 hours in the hydrogen environment. The heating
temperature is preferably lower than the sensitization
temperature, which is the temperature at which chromium (Cr)
carbides in the austenitic stainless steel precipitate by
heating.
[0073]
In the case of the austenitic stainless steels of Table
1 and Table 2, for instance, the upper limit of the heating
temperature is 500 C. Preferably, the thermal treatment is
performed at a heating temperature not lower than 200 C in
order to add hydrogen effectively into a surface layer over a
thickness of 100 m or greater from the surface. By way of
such a thermal treatment, diffusible hydrogen and non-
diffusible hydrogen, which cause hydrogen embrittlement in
austenitic stainless steel, are added to the austenitic
stainless steel in an amount of 30 wt ppm or more. The content
of hydrogen (H) in the austenitic stainless steel becomes thus
0.0030 wt% (30 wt ppm) or greater.
[0074]
The amount of hydrogen (H) contained in the austenitic
stainless steel after the thermal treatment is preferably
0.0050 wto (50 wt ppm) or higher. An austenitic stainless
steel having excellent fatigue strength characteristics, in
which occurrence and/or growth of fatigue cracks are
suppressed, can be provided by increasing thus the amount of
39
CA 02733658 2011-02-02
hydrogen contained in austenitic stainless steel beyond
conventional amounts.
[0075]
Other experimental examples 1
Hydrogenation treatment experiments were carried out
using test pieces made of SUS316(A), SUS316L(B), SUS310S(A)
and SUH660(A). The test pieces were 7 mm-diameter round bars.
In the experiments, the test pieces were subjected to a 200-
hour thermal treatment at a temperature of 280 C in hydrogen
gas at a pressure of 94 MPa. The total hydrogen concentration
and the Vickers hardness of the test pieces after the thermal
treatment were measured. For TDS measurement, the test pieces
were sliced into discs having a diameter of 7 mm and a
thickness of 0.8 mm.
[0076]
The TDS used for measurement was a thermal desorption
analyzer EMD-WA1000S/H by ESCO, Ltd. (location: Musashino,
Tokyo, Japan). The measurement results are given in Table 3.
The hydrogen concentration in the test pieces not subjected to
the hydrogenation treatment ranged from 1.5 to 3.4 wt ppm. The
hydrogen concentration in the test pieces subjected to the
thermal treatment in the hydrogen gas environment ranged from
69.9 to 129.1 wt ppm. The change in Vickers hardness from
before the hydrogenation treatment to Vickers hardness after
the hydrogenation treatment ranged from 1.08-fold to 1.11-fold.
CA 02733658 2011-02-02
Table 3
Hydrogenation Hydrogen concentration (weight
treatment ppm)/Vickers hardness Remarks
conditions SUS316(A) SUS316(B) SUS31OS(A) SUH660(A)
94 MPa
hydrogen gas,
heating Example of
temperature 95.7/175 87.7/157 129.1/146 69.9/- the present
280-C, invention
held for 200
hours
102 MPa
hydrogen gas,
heating Example of
temperature 79.4/- 73.6/- -/- -/- the present
120 C, invention
held for 120
hours
Hydrogen-
uncharged 3.4/161 1.5/145 2.8/132 1.2/- Comparative
material example
[0077]
Other experimental examples 2
Hydrogenation treatment experiments were carried out
using test pieces made of SUS316(A) and SUS316L(B). The
fatigue test pieces were discs having a diameter of 7 mm and a
thickness of 0.2 mm. In the experiments, the test pieces were
subjected to a 120-hour thermal treatment at a temperature of
120 C in hydrogen gas at a pressure of 102 MPa. The total
hydrogen concentration of the fatigue test piece of SUS316(A)
before the hydrogen charging treatment was 3.4 wt ppm. The
total hydrogen concentration of the fatigue test piece after
the hydrogen charging treatment was 79.4 wt ppm. The total
41
CA 02733658 2011-02-02
hydrogen concentration of the fatigue test piece of SUS316L(B)
before the hydrogen charging treatment was 1.5 wt ppm.
[0078]
The total hydrogen concentration of the fatigue test
piece after the hydrogen charging treatment was 73.6 wt ppm.
Fig. 11 illustrates the relationship between hydrogen
concentration in the entire fatigue test piece and Vickers
hardness ratio. The Vickers hardness ratio denotes herein the
ratio of Vickers hardness of austenitic stainless steel
subjected to the hydrogen charging treatment of the present
invention with respect to 1, which is the Vickers hardness of
austenitic stainless steel containing only unavoidable
hydrogen taken up during a conventional manufacturing process.
As Table 1 shows, austenitic stainless steels obtained by
conventional manufacturing methods comprise ordinarily 1 to 5
wt ppm of hydrogen.
[0079]
The concentration of hydrogen in stainless steel can be
thus brought to 30 wt ppm or higher by subjecting the
stainless steel to a thermal treatment in the above-described
hydrogen environments. The hydrogen environment of the present
invention is not limited to a high-pressure hydrogen gas
environment. The hydrogen charging treatment may also be
performed in an environment suitable for hydrogen charging
achieved by controlling the environment at which manufacturing
42
CA 02733658 2011-02-02
processes, for instance a solution thermal treatment, are
carried out. The saturation concentration of hydrogen in a
metallic material such as stainless steel is determined
experimentally and/or theoretically on the basis of, for
instance, the metal material, the hydrogen charging treatment
method, as well as the temperature, pressure and so forth
during the hydrogen charging treatment. For instance, the
hydrogen saturation concentration of SUS316 and SUS316L is
about 100 ppm in a hydrogen charging treatment in an
environment at 100 MPa and 280 C. Similarly, the hydrogen
saturation concentration of SUS310S is about 120 ppm in a
hydrogen charging treatment in an environment at 280 C.
Therefore, charging at or beyond the saturation concentration
of hydrogen is technically meaningless, and hence the hydrogen
charging treatment of the present invention refers to charging
up to the hydrogen saturation concentration.
INDUSTRIAL APPLICABILITY
[0080]
The present invention is good for use in corrosion
resistance and in fields that employ high-pressure hydrogen.
In particular, the present invention may be appropriate for
use in members that may undergo hydrogen embrittlement and/or
delayed fracture due to hydrogen intrusion, for instance metal
gaskets, various types of valves used in automobiles, springs,
piping, flexible hoses, couplings, pressure gauges, diaphragms,
43
CA 02733658 2011-02-02
bellows, pressure containers, bolts, steel belts, cutting
blades, fuel cells, as well as materials for valves, springs
and the like ancillary to fuel cell systems.
44
