U.S. patent application number 13/057578 was filed with the patent office on 2011-06-16 for austenitic stainless steel, and hydrogenation method thereof.
This patent application is currently assigned to Nat'l Institute of Advanced Industrial Science and Technology. Invention is credited to Toshihiko Kanezaki, Yoji Mine, Yukitaka Murakami.
Application Number | 20110139321 13/057578 |
Document ID | / |
Family ID | 41663601 |
Filed Date | 2011-06-16 |
United States Patent
Application |
20110139321 |
Kind Code |
A1 |
Murakami; Yukitaka ; et
al. |
June 16, 2011 |
AUSTENITIC STAINLESS STEEL, AND HYDROGENATION METHOD THEREOF
Abstract
Disclosed are an austenitic stainless steel, and a hydrogenation
method thereof, in which occurrence of fatigue cracks and growth of
fatigue cracks are suppressed by charging the austenitic stainless
steel with hydrogen. In particular, focusing on the amount of
diffusible hydrogen and non-diffusible hydrogen, which cause
hydrogen embrittlement in austenitic stainless steel, the fatigue
strength characteristics of austenitic stainless steel are improved
by bringing the amount of diffusible hydrogen and non-diffusible
hydrogen contained in the austenitic stainless steel to 0.0030 wt %
(30 wt ppm) or higher. The austenitic stainless steel is subjected
to a thermal treatment at a heating temperature of 200 to
500.degree. C. for up to 460 hours in a hydrogen environment. The
hydrogen (H) contained in the austenitic stainless steel is brought
thereby to 0.0030 wt % (30 wt ppm) or higher.
Inventors: |
Murakami; Yukitaka;
(Fukuoka, JP) ; Mine; Yoji; (Fukuoka, JP) ;
Kanezaki; Toshihiko; (Fukuoka, JP) |
Assignee: |
Nat'l Institute of Advanced
Industrial Science and Technology
Tokyo
JP
|
Family ID: |
41663601 |
Appl. No.: |
13/057578 |
Filed: |
July 17, 2009 |
PCT Filed: |
July 17, 2009 |
PCT NO: |
PCT/JP2009/062970 |
371 Date: |
February 4, 2011 |
Current U.S.
Class: |
148/634 ;
148/320 |
Current CPC
Class: |
C21D 6/004 20130101;
C21D 2211/001 20130101; C22C 38/58 20130101; C22C 38/44 20130101;
C22C 38/46 20130101; C22C 38/02 20130101; C22C 38/50 20130101; C22C
38/54 20130101; C22C 38/001 20130101; C22C 38/06 20130101 |
Class at
Publication: |
148/634 ;
148/320 |
International
Class: |
C21D 1/613 20060101
C21D001/613; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 6, 2008 |
JP |
2008-202713 |
Claims
1. 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 a 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 .mu.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.
2. The austenitic stainless steel according to claim 1, wherein the
concentration of the hydrogen (H) in the entirety of the austenitic
stainless steel has a value of 0.0030 wt % (30 wt ppm) or
higher.
3. The austenitic stainless steel according to claim 1, wherein the
concentration of the hydrogen (H) is 0.0050 wt % (50 wt ppm) or
higher.
4. The austenitic stainless steel according to claim 2, wherein the
concentration of the hydrogen (H) in the entirety of the austenitic
stainless steel has a value of 0.0050 wt % (50 wt ppm) or
higher.
5. The austenitic stainless steel according to claim 1, wherein the
Vickers hardness (HV) of the austenitic stainless steel is 1.05 or
higher, 1 being the Vickers hardness (HV) of an austenitic
stainless steel in which the concentration of the hydrogen (H) in
the entirety of the austenitic stainless steel is 0.0005 wt % (5 wt
ppm) or less.
6. The austenitic stainless steel according to claim 1, wherein the
hydrogen is stored in the austenitic stainless steel by subjecting
the austenitic stainless steel to a thermal treatment at a heating
temperature of 80.degree. C. or higher in a hydrogen
environment.
7. The austenitic stainless steel according to claim 6, wherein the
heating temperature ranges from 200.degree. C. to 500.degree.
C.
8. 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.degree. 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 wt %
(30 wt ppm) or higher to be formed to a thickness of 100 .mu.m or
more from the surface of the austenitic stainless steel into the
austenitic stainless steel.
9. The austenitic stainless steel hydrogenation method according to
claim 8, wherein the hydrogen is stored in the austenitic stainless
steel by keeping the austenitic stainless steel for up to 460 hours
at a temperature ranging from 200.degree. C. to 500.degree. C.,
being a temperature lower than a sensitization temperature at which
chromium (Cr) carbides in the austenitic stainless steel
precipitate by heating.
10. The austenitic stainless steel hydrogenation method according
to claim 8, wherein an overall concentration of the hydrogen (H)
contained in the austenitic stainless steel is 0.0030 wt % (30 wt
ppm) or higher.
11. The austenitic stainless steel hydrogenation method according
to claim 8, wherein the local concentration of the hydrogen (H)
contained in the austenitic stainless steel is 0.0050 wt % (50 wt
ppm) or higher.
12. The austenitic stainless steel hydrogenation method according
to claim 10, wherein the overall concentration of the hydrogen (H)
contained in the austenitic stainless steel is 0.0050 wt % (50 wt
ppm) or higher.
Description
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 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 %. The content of hydrogen (H) is expressed as weight ppm at
the last component column of Table 1.
TABLE-US-00001 TABLE 1 C Si Mn P S Ni Cr Mo Fe H* Other HV SUS304
0.06 0.36 1.09 0.030 0.023 8.19 18.66 -- Balance 2.2 -- 176 (A)
SUS304 0.02 0.35 1.02 0.028 0.007 9.06 18.06 -- Balance 1.1 -- --
(B) SUS304 0.05 0.47 0.99 0.032 0.005 8.14 18.21 -- Balance 2.6 --
-- (C) SUS304 0.05 0.58 1.24 0.025 0.003 8.09 18.54 -- Balance 2.2
-- 176 (D) SUS316 0.05 0.27 1.31 0.030 0.028 10.15 17.01 2.08
Balance 3.4 -- 161 (A) 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 0.019 0.78 1.40
0.037 0.010 12.08 17.00 2.04 Balance 2.6 -- 157 (A) SUS316L 0.010
0.53 0.77 0.023 0.001 12.13 17.16 2.86 Balance 1.5 -- 145 (B)
SUS310S 0.02 0.34 1.12 0.023 0.001 19.22 24.02 -- Balance 2.8 --
132 (A) SUS310S 0.01 0.34 1.07 0.024 0.001 19.22 24.05 -- Balance
2.4 -- -- (B) SUS310S 0.04 0.42 0.38 0.019 <0.001 20.31 24.69 --
Balance 4.7 -- 151 (C) SUH660 0.04 0.05 0.42 0.016 0.001 24.30
13.59 1.09 Balance 1.2 V = -- (A) 0.26, Al = 0.17, Ti = 2.22, B =
0.003 (Component units: wt %, *ppm by weight)
[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.degree. 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
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
.mu.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.
[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: "(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
[0010] Patent document 2: Japanese Patent Application Laid-open No.
H10-199380
[0011] Patent document 3: Japanese Patent Application Laid-open No.
2005-9955
[0012] 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 Meeting of JSME/MMD, M&M
2005 (Nov. 4 to 6, 2005, Fukuoka) P86, pp. 595-596.
[0013] 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).
[0014] 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: Nov. 25, 2007; online publication: Apr. 1,
2008)
[0015] 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
fatigue crack growth rate in a material, have not been sufficiently
elucidated.
[0016] 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.
[0017] 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 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.degree. 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 .degree. C. to
several tens of .degree. C., and cycles ranging from sub-seconds to
several hours.
DISCLOSURE OF THE INVENTION
[0018] The present invention is based on the above technical
background, and attains the following objects.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] Definition of Terms
[0023] 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 a manufacturing or working process. Cyclic
loading refers to cyclic loading acting on holes or the like that
are artificially drilled in the material.
[0024] 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.degree. C.,
in 100% pure iron (Fe), having a face-centered cubic lattice
structure (hereinafter, FCC structure).
[0025] 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 like, on austenitic-phase stainless steel at ordinary
temperature.
[0026] 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.degree. C.
[0027] The present invention achieves the above objects on the
basis of the following means.
[0028] 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.
[0029] 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 .mu.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.
[0030] 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.degree. 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 wt % (30 wt ppm) or higher to be formed to a
thickness of 100 .mu.m or more from the surface of the austenitic
stainless steel into the austenitic stainless steel.
[0031] 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 wt % (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.
[0032] 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 .mu.m from the surface of an austenitic stainless steel
that has a cross-sectional smallest dimension of 200 .mu.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 dimension from among height, length
and thickness of an austenitic stainless steel material.
[0033] 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 wt % (50 wt ppm) or higher.
[0034] The Vickers hardness of austenitic stainless steel
containing no more than 0.0005 wt % (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.
[0035] 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.degree. C. or higher in a hydrogen
environment. Heating is effective within a heating temperature
range from 200.degree. C. to 500.degree. 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.
[0036] 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.
[0037] 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 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.degree. C.
or higher in a hydrogen environment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] 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;
[0039] FIGS. 2A to 2D are diagrams illustrating schematically an
evaluation method of hydrogen concentration distribution;
[0040] 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;
[0041] 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;
[0042] FIG. 5 is a photograph of fatigue cracks arising from the
artificial microhole after fatigue testing;
[0043] FIGS. 6A and 6B 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. 6A corresponds to SUS304 and FIG. 6B corresponds to
SUS316L;
[0044] 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;
[0045] FIG. 8 is a graph illustrating the relationship between test
stress amplitude .sigma. and fatigue life N.sub.f at which a
fatigue test piece fractures in a SUS304 material having an
artificial microhole;
[0046] 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;
[0047] 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.degree. C. and pressure of 35 MPa or 70 MPa;
and
[0048] FIG. 11 is a diagram illustrating a Vickers hardness ratio
versus hydrogen concentration in austenitic stainless steel.
BEST MODE FOR CARRYING OUT THE INVENTION
[0049] 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.
[0050] 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 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 .mu.m. Cathodic hydrogen charging was carried
out as follows.
[0051] 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.degree. C. The pH of the aqueous
solution of sulfuric acid was 3.5. The current density was 27
A/m.sup.2. 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.
[0052] 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 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.
[0053] 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.
[0054] 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.
[0055] 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 .mu.m to 60 .mu.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.
[0056] For example, FIG. 10 illustrates the hydrogen concentration
distribution predicted for SUS316L on the basis of a hydrogen solid
solubility K.sub.S=4.64 wt ppm/MPa.sup.1/2, and a diffusion
coefficient D=8.42.times.10.sup.-17 m/s. In the case, for instance,
of SUS316L, the local hydrogen concentration at the surface is
about 0.0031 wt % (31 wt ppm) after use over 5 years at 25.degree.
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 .mu.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 .mu.m from the
surface.
[0057] In the case, for instance, of SUS316L, an envisaged use over
5 years at 25.degree. 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 .mu.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 .mu.m from
the surface.
[0058] 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.
[0059] [Test Piece]
[0060] 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.
[0061] In order to facilitate observation of fatigue crack growth,
an artificial microhole 100 .mu.m in diameter and 100 .mu.m 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 120.degree..
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.
[0062] 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.
[0063] [X-Ray Diffraction]
[0064] 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 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 CrK.alpha. radiation. In
SUS304, and SUS316L the content of martensite in the test area
before fatigue testing was about 3%.
[0065] 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.degree. 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.
[0066] [Hydrogen Charging Method]
[0067] 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 solution at pH=3.5, a platinum
anode, and a current density of i=27 A/m.sup.2. Cathodic hydrogen
charging was performed for 672 hours (4 weeks) at a temperature of
50.degree. 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.
[0068] 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.degree. C., 242.degree. C.,
250.degree. C. or 280.degree. 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.
[0069] [Fatigue Test Method]
[0070] 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 1.5 Hz, and
a stress ratio of R=-1. The cycling frequency was adjusted so that
the surface temperature of the test area did not exceed 60.degree.
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).
[0071] 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.
[0072] 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 rate. The vacuum chamber internal
pressure was 1.times.10.sup.-7 to 3.times.10.sup.-7 Pa before the
sample was heated. The temperature rise rate in the vacuum chamber
was 0.33.degree. C./s or 0.5.degree. C./s.
[0073] 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.
[0074] [Measured Properties]
[0075] 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.
[0076] 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.
[0077] 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 .mu.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 cathodic hydrogen charging. The results do not depend
on the hydrogen charging method, i.e. cathodic hydrogen
charging.
[0078] 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 .mu.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.
[0079] 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 effect
on fatigue strength characteristics is thus obtained also when the
material is used in a hydrogen environment.
[0080] 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 .mu.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.
[0081] 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
piece where the material is SUS316L, in a case where the test piece
is charged with hydrogen, and a case where not.
[0082] The cycling frequency in the fatigue test is 0.3 Hz until
the crack length 2a reaches about 400 .mu.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 .mu.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).
[0083] 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
.mu.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 .mu.m is about 8 times greater than
that when no hydrogen charging is carried out (SUS316L). This is
indicative of significantly enhanced fatigue crack growth
resistance in the fatigue test piece.
[0084] FIG. 8 illustrates the relationship between the test stress
amplitude .sigma. and the fatigue life N.sub.f 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.
[0085] 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 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.
[0086] 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 .mu.m from the surface and the local hydrogen
concentration further inward is less than 30 wt ppm, as in the
graph of FIG. 1.
[0087] The number of cycles until the crack length 2a reaches 300
.mu.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 .mu.m or
deeper, for a crack length 2a=300 .mu.m. Preferably, the austenitic
stainless steel has a local hydrogen concentration of 50 wt ppm or
higher.
[0088] 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.
[0089] Austenitic Stainless Steel
[0090] 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.
[0091] 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-US-00002 TABLE 2 Composition 1 Composition 2 Component
(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 Mo 2.00 to 3.00 or
3.00 or less less 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 Other Balance Fe and Balance Fe
and unavoidable unavoidable impurities impurities
[0092] Composition of the Austenitic Stainless Steel
[0093] 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 10.0 wt % 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 1 vol % or greater. Mn also has the effect of
improving the solid solubility of N.
[0094] 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.
[0095] Mo is added for improving corrosion resistance and has also
the effect of bringing about a fine dispersion of 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 .delta.-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.
[0096] 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.
[0097] Austenitic Phase
[0098] 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.
[0099] Other Properties
[0100] The average grain size is preferably about 50 .mu.m or less.
In modern materials the average grain size is about 50 .mu.m, but a
smaller average grain size is preferred.
[0101] Hydrogenation Treatment by Heating
[0102] 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.
[0103] The austenitic stainless steel is subjected to a thermal
treatment at a heating temperature of 80.degree. 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 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.
[0104] 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.degree. C. Preferably, the thermal treatment is
performed at a heating temperature not lower than 200.degree. C. in
order to add hydrogen effectively into a surface layer over a
thickness of 100 .mu.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.
[0105] The amount of hydrogen (H) contained in the austenitic
stainless steel after the thermal treatment is preferably 0.0050 wt
% (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 hydrogen contained in austenitic
stainless steel beyond conventional amounts.
OTHER EXPERIMENTAL EXAMPLES 1
[0106] 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.degree. 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.
[0107] 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.
TABLE-US-00003 TABLE 3 Hydrogenation Hydrogen concentration (weight
treatment ppm)/Vickers hardness conditions SUS316(A) SUS316(B)
SUS310S(A) SUH660(A) Remarks 94 MPa 95.7/175 87.7/157
129.1/146.sup. 69.9/-- Example of hydrogen gas, the present heating
invention temperature 280.degree. C., held for 200 hours 102 MPa
79.4/-- 73.6/-- --/-- --/-- Example of hydrogen gas, the present
heating invention temperature 120.degree. C., held for 120 hours
Hydrogen- 3.4/161 1.5/145 2.8/132 1.2/-- Comparative uncharged
example material
OTHER EXPERIMENTAL EXAMPLES 2
[0108] 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.degree. 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 hydrogen concentration of the
fatigue test piece of SUS316L(B) before the hydrogen charging
treatment was 1.5 wt ppm.
[0109] 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.
[0110] 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 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.degree. C.
Similarly, the hydrogen saturation concentration of SUS310S is
about 120 ppm in a hydrogen charging treatment in an environment at
280.degree. 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
[0111] 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, 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.
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