U.S. patent application number 12/301707 was filed with the patent office on 2009-10-22 for austenitic stainless steel and process for removing hydrogen therefrom.
This patent application is currently assigned to National Institute of Advanced Industrial Science and Technology. Invention is credited to Toshihiko Kanezaki, Saburo Matsuoka, Yoji Mine, Yukitaka Murakami.
Application Number | 20090263269 12/301707 |
Document ID | / |
Family ID | 39673769 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090263269 |
Kind Code |
A1 |
Murakami; Yukitaka ; et
al. |
October 22, 2009 |
AUSTENITIC STAINLESS STEEL AND PROCESS FOR REMOVING HYDROGEN
THEREFROM
Abstract
By focusing on the non-diffusible hydrogen that causes hydrogen
embrittlement of austenitic stainless steel, the present invention
provides an austenitic stainless steel in which the non-diffusible
hydrogen is removed by maintaining the austenitic stainless steel
in a vacuum of 0.2 Pa or less and heating at a heating temperature
of 200.degree. C. to 500.degree. C. for 460 hours or less to remove
the hydrogen (H) contained therein to a level of 0.00007 mass %
(0.7 mass ppm) or less.
Inventors: |
Murakami; Yukitaka;
(Fukuoka, JP) ; Matsuoka; Saburo; (Fukuoka,
JP) ; Mine; Yoji; (Fukuoka, JP) ; Kanezaki;
Toshihiko; (Fukuoka, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
National Institute of Advanced
Industrial Science and Technology
Tokyo
JP
|
Family ID: |
39673769 |
Appl. No.: |
12/301707 |
Filed: |
October 5, 2007 |
PCT Filed: |
October 5, 2007 |
PCT NO: |
PCT/JP2007/069589 |
371 Date: |
November 20, 2008 |
Current U.S.
Class: |
420/8 ; 148/320;
148/579 |
Current CPC
Class: |
C21D 2211/008 20130101;
C22C 38/44 20130101; C21D 6/004 20130101; C21D 2211/001 20130101;
C21D 3/06 20130101 |
Class at
Publication: |
420/8 ; 148/320;
148/579 |
International
Class: |
C22C 38/00 20060101
C22C038/00; C21D 6/00 20060101 C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2007 |
JP |
2007-022467 |
Claims
1. An austenitic stainless steel having an austenitic phase in
which a crystalline structure is a face centered cubic lattice
structure, wherein diffusible hydrogen and non-diffusible hydrogen,
which are cause of hydrogen embrittlement in the austenitic
stainless steel, are removed therefrom such that the hydrogen (H)
contained in the austenitic stainless steel is removed to a level
of 0.00007 mass % (0.7 mass ppm) or less.
2. The austenitic stainless steel according to claim 1, wherein the
diffusible hydrogen and non-diffusible hydrogen are removed
therefrom such that the hydrogen (H) is removed to a level of
0.00004 mass % (0.4 mass ppm) or less.
3. The austenitic stainless steel according to claim 2, wherein the
diffusible hydrogen and non-diffusible hydrogen are removed
therefrom such that the hydrogen (H) is removed to a level of
0.00001 mass % (0.1 mass ppm) or less.
4. The austenitic stainless steel according to any one of claims 1
to 3, wherein the austenitic stainless steel is heat-treated at a
heating temperature of 200.degree. C. or higher to remove the
diffusible and non-diffusible hydrogen therefrom.
5. The austenitic stainless steel according to claim 4, wherein the
heating temperature is a temperature from 200.degree. C. to
500.degree. C.
6. A process for removing hydrogen from an austenitic stainless
steel, in which the austenitic stainless steel having an austenitic
phase in which a crystalline structure is a face centered cubic
lattice structure is heat-treated in a heat treatment to remove the
hydrogen present in the austenitic stainless steel, wherein the
austenitic stainless steel is heated to a heating temperature of
200.degree. C. or higher to remove the diffusible hydrogen and
non-diffusible hydrogen in the austenitic stainless steel to a
level of 0.00007 mass % (0.7 mass ppm) or less.
7. The process for removing hydrogen from austenitic stainless
steel according to claim 6, wherein the heating temperature is a
temperature from 200.degree. C. to 500.degree. C., and both the
diffusible hydrogen and non-diffusible hydrogen, which are present
in the austenitic stainless steel, diffuse via the stress-induced
martensitic phase brought about by cyclic loading, concentrate in
the cracks undergoing concentrated stress, and cause hydrogen
embrittlement therein, are removed therefrom such that the hydrogen
(H) contained therein is removed to a level of 0.00007 mass % (0.7
mass ppm) or less.
8. The process for removing hydrogen from austenitic stainless
steel according to claim 6, wherein the austenitic stainless steel
is maintained at a temperature from 200.degree. C. to 500.degree.
C., which is a temperature lower than the sensitization temperature
at which chromium (Cr) carbide in the austenitic stainless steel
precipitates due to the heat, for 460 hours or less, and both the
diffusible hydrogen and non-diffusible hydrogen, which cause
hydrogen embrittlement in the austenitic stainless steel, are
removed therefrom such that the hydrogen (H) contained in the
austenitic stainless steel is removed to a level of 0.00004 mass %
(0.4 mass ppm) or less.
9. The process for removing hydrogen from an austenitic stainless
steel according to claim 7 or 8, wherein the hydrogen (H) contained
in the austenitic stainless steel is removed to a level of 0.00001
mass % (0.1 mass ppm) or less.
10. The process for removing hydrogen from an austenitic stainless
steel according to claim 6, wherein the austenitic stainless steel
is maintained at the heating temperature and in a temperature range
below a melting point thereof for 30 seconds or more during the
manufacturing process thereof to remove the diffusible hydrogen and
non-diffusible hydrogen contained therein to a level of 0.00007
mass % (0.7 mass ppm) or less.
11. The process for removing hydrogen from austenitic stainless
steel according to claim 10, wherein the heating temperature in the
heat treatment to remove the hydrogen present therein is
700.degree. C. or more and also in a temperature range below the
melting point of the austenitic stainless steel.
12. The process for removing hydrogen from austenitic stainless
steel according to claim 11, wherein the heating temperature in the
heat treatment to remove the hydrogen present therein is
920.degree. C. or higher, and the diffusible hydrogen and
non-diffusible hydrogen therein are removed to a level of 0.00004
mass % (0.4 mass ppm) or less.
Description
TECHNICAL FIELD
[0001] The present invention concerns an austenitic stainless steel
with reduced hydrogen embrittlement and a process for removing
hydrogen therefrom. More specifically, the present invention
concerns an austenitic stainless steel wherein the effect of
hydrogen present therein on the growth of fatigue cracks occurring
therein is reduced, and a process for removing hydrogen
therefrom.
BACKGROUND
[0002] From the standpoint of global environmental concerns, the
use of hydrogen as a next generation energy source has received
considerable attention, and R&D on this topic is quite active.
Particularly, one important subject that has received emphasis is
the development and practical application of stationary fuel cells,
fuel cell-powered vehicles, etc., that utilize hydrogen as fuel.
The use of stainless steel as a material for high pressure hydrogen
tanks and parts thereof, piping, and the like in this fuel cell
system has already been investigated (for example, see patent
document 1).
[0003] The components of typical austenitic stainless steel are
shown in Table 1. The first column in this Table 1 lists the names
of stainless steels and the heat-resistant steels as defined in the
Japanese Industrial Standards (JIS). The last column of Table 1
shows the Vickers hardness of the stainless steel (hereinafter,
HV). Other columns are the chemical compositions of the stainless
steel, and the amounts of the components are expressed in units of
mass %. However, the content of hydrogen (H) is expressed as mass
ppm (parts per million by mass).
TABLE-US-00001 TABLE 1 (Units: mass %, *ppm by mass) 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 -- Remainder 2.2 -- 176 SUS304 (B) 0.02 0.35 1.02 0.028 0.007
9.06 18.06 -- Remainder 1.1 -- -- SUS304 (C) 0.05 0.47 0.99 0.032
0.005 8.14 18.21 -- Remainder 2.6 -- -- SUS304 (D) 0.05 0.58 1.24
0.025 0.003 8.09 18.54 -- Remainder 2.2 -- 176 SUS316 (A) 0.05 0.27
1.31 0.030 0.028 10.15 17.01 2.08 Remainder 3.4 -- 161 SUS316 (B)
0.05 0.29 1.37 0.030 0.026 10.05 16.89 2.01 Remainder 1.2 -- --
SUS316 (C) 0.02 0.53 0.98 0.021 0.001 10.15 16.21 2.08 Remainder
1.5 -- 164 SUS316L (A) 0.019 0.78 1.40 0.037 0.010 12.08 17.00 2.04
Remainder 2.6 -- 157 SUS316L (B) 0.010 0.53 0.77 0.023 0.001 12.13
17.16 2.86 Remainder 1.5 -- 145 SUS310S (A) 0.02 0.34 1.12 0.023
0.001 19.22 24.02 -- Remainder 2.8 -- 132 SUS310S (B) 0.01 0.34
1.07 0.024 0.001 19.22 24.05 -- Remainder 2.4 -- -- SUS310S (C)
0.04 0.42 0.38 0.019 <0.001 20.31 24.69 -- Remainder 4.7 -- 151
SUH660 (A) 0.04 0.05 0.42 0.016 0.001 24.30 13.59 1.09 Remainder
1.2 V = 0.26, Al = 0.17, -- Ti = 2.22, B = 0.003
[0004] It is known that hydrogen diffuses through metallic
materials and decreases both the static strength and fatigue
strength thereof (non-patent documents 1 and 2). Various processes
for removing this hydrogen and methods for predicting the effect of
hydrogen have been proposed. In patent document 2, for example,
after the plating process austenitic stainless steel is
heat-treated by maintaining it at a temperature of 270 to
400.degree. C. for 10 minutes or more to remove the hydrogen and
prevent hydrogen embrittlement. Patent document 3 discloses a
method wherein the extent of the hydrogen embrittlement of
austenitic stainless steel is predicted and judged based on the
chemical composition thereof.
[0005] Non-patent document 1 presents fatigue testing results for
austenitic stainless steels in compliance with SUS304, SUS316, and
SUS316L. This fatigue testing was conducted by comparing these
austenitic stainless steels with their hydrogen-charged
counterparts. The fatigue crack growth rate of hydrogen-charged
SUS304 and SUS316 was faster than in the corresponding uncharged
steels. However, no clear difference was seen with SUS316L.
[0006] In addition, non-patent document 1 presents fatigue test
results for JIS SUS304 and the SUS316L austenitic stainless steels
after the test piece was prestrained and a microhole of about 100
.mu.m was formed therein. The fatigue crack growth rate was
accelerated ten-fold in hydrogen-charged SUS304 compared with the
uncharged steel counterpart. Likewise, the fatigue crack growth
rate was accelerated two-fold in SUS316L.
[0007] However, even semi-stable austenitic stainless steel can
undergo stress-induced martensitic transformation due to
cold-working and cyclic stress. Persons skilled in the art in this
industry, including groups of researchers in academic societies,
have commonly believed that hydrogen had almost no effect on the
fatigue crack growth rate in the austenitic stainless steels such
as JIS SUS316L. Non-patent document 1 presents results that reverse
this common belief, and because these results were obtained by
applying cyclic loading at a low frequency of 5 Hz or less, this
finding is very significant.
[0008] In other words, it has been verified that the growth rate of
fatigue cracks is accelerated by low-frequency cyclic loading in
austenitic stainless steel such as SUS316L. On the other hand,
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 (see page 130)."
[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 Meeting of JSME/MMD, Vol. 2005 (Nov. 4 to 6, 2005, Fukuoka)
P86 p. 595-596. [Non-patent document 2] Toshihiko KANEZAKI, Chihiro
NARAZAKI, Yoji MINE, Saburo MATSUOKA, and Yukitaka MURAKAMI: Effect
of hydrogen on fatigue crack growth and martensitic transformation
of stainless steels. Transactions of the Japan Society of
Mechanical Engineers A. Vol. 72, No. 723, (November 2006), p.
123-130. (manuscript received: May 1, 2006)
[0009] At present, however, sufficient analysis has not been
performed concerning how non-diffusible hydrogen, which is present
in crystals, 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
involving how diffusible hydrogen and non-diffusible hydrogen
influence changes in the amount of martensitic transformation, the
effect of acceleration in the hydrogen diffusion rate, and the
fatigue crack growth rate in a material have not been sufficiently
clarified.
[0010] Furthermore, when stainless steel is used for equipment and
apparatuses related to hydrogen fuel utilization, it is exposed to
a variety of environmental influences depending on the usage
environment. For example, when stainless steel is used for the high
pressure hydrogen container, piping, and the like in a fuel
cell-powered vehicle, loading and release are repeated in a
relatively slow cycle that involves the filling of the high
pressure hydrogen container with hydrogen gas and the subsequent
consumption thereof. In the past, however, fatigue tests have not
taken this slow cycle into account. In other words, it was thought
that a fatigue test using a load with a long cycle could be
replaced by a fatigue test with a quick repetition rate.
[0011] Moreover, low frequency cyclic loading occurs due to
temperature variations in the outside air temperature and the like.
An example of cyclic loading due to variations in the outside air
temperature is thermal stress resulting from compression and
expansion of the stainless steel itself and of the parts connected
to stainless steel components as a result of temperature
differences between day and night. As for the frequency of the
cycle, the temperature gradient between day and night can range
from only a few degrees to ten degrees centigrade or more, and one
cycle is 24 hours long. This means that high pressure hydrogen
tanks at fuel cell vehicle-related facilities, facilities for
supplying fuel for fuel cells, and the like will have a cycle
measured in single day units as noted above, and the hydrogen fill
time will be long. In addition, a fuel cell-powered vehicle is
dependent on the environment in which it operates, and it will have
a temperature gradient cycle ranging from a few degrees to several
tens of degrees centigrade, and a time cycle expressed in units
ranging from subseconds to several hours.
DISCLOSURE OF THE INVENTION
[0012] The present invention is based on the above technological
background, and it attains the following objects.
[0013] An object of the present invention is to provide an
austenitic stainless steel for reducing the effect of hydrogen on
the growth rate of fatigue cracks that occur in austenitic
stainless steel, and a process for removing the hydrogen
therefrom.
[0014] Another object of the present invention is to focus on both
diffusible hydrogen and non-diffusible hydrogen that cause hydrogen
embrittlement in austenitic stainless steel and provide an
austenitic stainless steel having both removed therefrom, and to
provide a process for removing the hydrogen therefrom.
[0015] Another object of the present invention is to focus on the
diffusible hydrogen and the non-diffusible hydrogen that become a
problem with cyclic loading having a long cycle time and provide an
austenitic stainless steel having both removed therefrom, and to
provide a process for removing the hydrogen therefrom.
[0016] Another object of the present invention is to provide an
austenitic stainless steel wherein the diffusible hydrogen and
non-diffusible hydrogen present in austenitic stainless steel are
removed therefrom during a manufacturing step thereof, and a
process for removing the hydrogen therefrom.
DEFINITION OF TERMS
[0017] The present invention uses the following technical terms in
the meanings defined below. Hydrogen charge means causing hydrogen
to permeate the material. Hydrogen charge method refers to a method
whereby the material is exposed in a high pressure hydrogen
chamber, a method wherein cathodic charging is performed, and a
method wherein the material is immersed in a chemical solution and
the like. Fatigue crack growth refers to the enlargement of defects
and cracks that occur in the material during the manufacturing
process or cracks from holes and the like that are artificially
introduced into the material as a result of cyclic loading.
[0018] Fatigue crack growth rate means the rate at which the
fatigue crack progresses. Austenitic stainless steel refers to
Cr--Ni series steel wherein chromium and nickel are added to iron
to produce a stainless steel in the austenitic phase with increased
corrosion resistance in corrosive environments and the like. Table
1 shows a list of this kind of stainless steel. The austenitic
phase is a phase of iron at a temperature range of 911 to
1392.degree. C. in 100% pure iron (Fe), and it has a face centered
cubic lattice structure (hereinafter, FCC lattice structure).
[0019] The FCC lattice is shown in FIG. 11(a). The austenitic phase
can also exist at room temperature when alloying elements such as
chromium and nickel are added to iron. The martensitic phase is a
conformation obtained by quenching steel at a high-temperature,
stable austenitic phase, and it has a body centered cubic lattice
structure (hereinafter, BCC lattice structure). The BCC lattice is
shown in FIG. 11(b). Moreover, the martensitic phase is caused by
adding stress such as cold-working and the like to austenitic phase
stainless steel at ordinary ambient temperatures.
[0020] This transformation from an austenitic phase with an FCC
structure to a martensitic phase with a BCC structure by cold
working is called stress-induced martensitic transformation.
Diffusible hydrogen refers to hydrogen that is present in the
material and exits the material over time at room temperature. This
diffusible hydrogen causes the hydrogen embrittlement of the
material. Hydrogen that cannot exit the material over time even at
temperatures from room temperature to about 200.degree. C., is
called non-diffusible hydrogen.
[0021] The present invention adopts the following means to achieve
the aforementioned objects.
[0022] The inventors of the present invention ascertained the fact
that non-diffusible hydrogen in an austenitic stainless steel is
related to fatigue crack growth.
[0023] The present invention concerns an austenitic stainless steel
having an austenitic phase in which the crystal structure is an FCC
lattice structure and a process for removing the hydrogen
therefrom. The present invention is one wherein the diffusible
hydrogen and the non-diffusible hydrogen that cause hydrogen
embrittlement of austenitic stainless steel are removed, with the
hydrogen (H) contained in the austenitic stainless steel being
removed to a level of 0.00007 mass % (0.7 mass ppm) or less.
[0024] A heat treatment at a heating temperature of 200.degree. C.
or higher is preferred for the removal of diffusible hydrogen and
non-diffusible hydrogen from austenitic stainless steel. Moreover,
the upper limit of the heating temperature for the heat treatment
at this time is preferably no more than 500.degree. C. It is
preferred that the heat treatment to remove the diffusible hydrogen
and non-diffusible hydrogen is performed under vacuum.
[0025] Moreover, the temperature range for the heating temperature
is 200 to 500.degree. C., and this range is preferred for removing
both the non-diffusible hydrogen and the diffusible hydrogen, which
are present in austenitic stainless steel, diffuse via the
stress-induced martensitic phase brought about by cyclic loading,
concentrate in the cracks undergoing concentrated stress, and cause
hydrogen embrittlement therein, to thereby remove the hydrogen (H)
contained therein to the aforementioned amount.
[0026] It is preferred that the heating temperature is a
temperature lower than the sensitization temperature, which is the
temperature at which chromium (Cr) carbide in austenitic stainless
steel precipitates due to the heat. Moreover, it is preferred that
the aforementioned vacuum atmosphere is an environment of 0.2 Pa or
less. In addition, it is preferred that the heat treatment is
performed by maintaining the heating temperature in the
aforementioned vacuum atmosphere for 460 hours or less.
[0027] The amount of hydrogen (H) contained in the austenitic
stainless steel after removing the diffusible hydrogen and the
non-diffusible hydrogen is preferably 0.00004 mass % (0.4 mass ppm)
or less. Furthermore, the amount of hydrogen (H) contained in the
austenitic stainless steel after removing the diffusible hydrogen
and the non-diffusible hydrogen is more preferably 0.00001 mass %
(0.1 mass ppm) or less.
[0028] It is preferable that the heat treatment is performed for
the predetermined time in the process of manufacturing the
austenitic stainless steel, and the hydrogen is removed to adjust
the hydrogen (H) to 0.00007 mass % (0.7 mass ppm) or less. At that
time, the heat treatment temperature is preferably 200.degree. C.
or more, and less than the melting point temperature of the
stainless steel. The heating time for this purpose preferably
ranges from 30 seconds or more to several tens of hours or less. An
inert gas flow atmosphere is preferred for this process. The
manufacturing steps for austenitic stainless steel are the ones
used when manufacturing stainless steel, and they include the steps
of a solution heat treatment and an aging treatment.
[0029] The hydrogen removal treatment in the manufacturing process
can be performed in a vacuum or an atmosphere wherein the hydrogen
partial pressure is low, e.g., an inert gas atmosphere. Moreover,
the heat treatment preferably lasts from a few minutes to several
tens of hours long.
[0030] In the case of a solution heat treatment, the most preferred
temperature for the heat treatment is 920.degree. C. or higher.
[0031] In the case of an aging treatment, the most preferred
temperature for the heat treatment is 700.degree. C. or higher.
[0032] In addition, it is preferred that the austenitic stainless
steel is one of the aforementioned austenitic stainless steels or
heat resistant austenitic steels shown in Table 1.
[0033] The present invention provides the following advantageous
effect. In the present invention austenitic stainless steel is
heat-treated at a temperature of 200.degree. C. or higher to remove
the non-diffusible hydrogen and the diffusible hydrogen, thereby
making it possible to provide an austenitic stainless steel that is
highly resistant to fatigue crack growth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a line drawing showing the fatigue test piece,
FIG. 1(a) is a drawing showing the shape of the fatigue test piece,
and FIG. 1(b) is a drawing showing the shape of the artificial
microhole formed in the fatigue test piece;
[0035] FIG. 2 shows a line drawing of the test area in the fatigue
test piece and the fatigue crack starting at the small artificial
hole and progressing therefrom;
[0036] FIG. 3 is a schematic diagram of the procedure for applying
the prestrain to the fatigue test piece;
[0037] FIG. 4 is a photograph of the fatigue cracks emanating from
the artificial microhole after fatigue testing;
[0038] FIG. 5 shows results of x-ray examination of the austenitic
phase and martensitic phase in the test area surface before fatigue
testing and the fatigue cracked surface after fatigue testing
wherein FIG. 5(a) shows the measurement results for SUS304, FIG.
5(b) shows the measurement results for SUS316, and FIG. 5(c) shows
the measurement results for SUS316L;
[0039] FIG. 6 is a graph showing the relationship between the
length of the cracks caused by fatigue testing and number of cycles
wherein FIG. 6(a) shows the results for SUS304, FIG. 6(b) for
SUS316, and FIG. 6(c) for SUS316L;
[0040] FIG. 7 shows photographs of fatigue cracks in SUS304,
SUS316, and SUS316L observed by the replica method;
[0041] FIG. 8 is a graph showing the results of fatigue testing of
SUS316L;
[0042] FIG. 9 is a graph showing the result of the fatigue testing
of SUS316L;
[0043] FIG. 10 is a conceptual drawing showing the circumstances
wherein the diffusible hydrogen and the non-diffusible hydrogen
diffuse through the transformed martensitic phase;
[0044] FIG. 11 is a schematic drawing showing the lattices of the
crystal structures of the austenitic phase and martensitic phase
wherein FIG. 11(a) shows the face centered cubic lattice structure
(FCC) of the austenitic phase, and FIG. 11(b) shows the body
centered cubic lattice structure (BCC) of the martensitic
phase;
[0045] FIG. 12 is a graph showing the results of Additional
Experimental Example 1; and
[0046] FIG. 13 is a graph showing the results of Additional
Experimental Example 2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0047] Below the mode of the present invention is explained through
examples. First of all, the way hydrogen affects the growth rate of
fatigue cracks in austenitic stainless steel will be explained.
After a conventional heat treatment (solution heat treatment) is
performed, austenitic stainless steels such as SUS304, SUS316, and
SUS316L shown in Table 1 contain 1 to 4.7 mass ppm of
non-diffusible hydrogen. In the past persons skilled in the art
believed that this non-diffusible hydrogen had no effect on
hydrogen embrittlement.
[0048] However, the fatigue tests described below have determined
that non-diffusible hydrogen affects hydrogen embrittlement.
Hydrogen embrittlement resulting from non-diffusible hydrogen has
been verified at a low frequency fatigue testing speed of about
0.0015 Hz in (approximately 11 minutes as the repetition time of
one cycle) in particular. The inventors of the present invention
performed the following tests and observed how non-diffusible
hydrogen affected the growth rate of the fatigue cracks. One
example of such testing is shown herein.
Test Piece
[0049] The materials used were the SUS304, SUS316, and SUS316L(A)
(hereinafter, simply called SUS316L) austenitic stainless steels
shown in Table 1. A solution heat treatment was performed on the
SUS304, SUS316, and SUS316L steels used. The shape of the fatigue
test piece is shown in FIG. 1(a). The surface of the test piece was
finished by buffing after polishing with # 2000 emery paper.
[0050] As shown in FIG. 1(b), an artificial microhole 100 .mu.m in
diameter and 100 .mu.m deep was opened in the center of the fatigue
test piece in the lengthwise direction with a drill having a radial
tip angle of 120.degree. to facilitate observation of fatigue crack
growth. The artificial microhole was inserted in the center of the
test area of the test piece. The test area was a cylindrical part
at the center of the test piece, and the length of the cylinder was
approximately 20 mm. The top and bottom surfaces of the cylinder
were mutually parallel, and lay perpendicular to the lengthwise
axis of the test piece. FIG. 2 illustrates an outline of the test
piece and the shape of the inserted artificial microhole. In the
case of a hydrogen-charged fatigue test piece, the piece buffed
immediately after the end of hydrogen charging and the artificial
microhole was drilled.
X-Ray Diffraction
[0051] The amount of the martensite in the test area of the fatigue
test piece of austenitic stainless steel was measured by x-ray
diffraction. The x-ray diffraction was performed using a miniature
x-ray stress measurement apparatus PSPC-RSF/KM manufactured by
Rigaku Corporation (Akishima, Tokyo). Quantitative analysis was
determined from the integrated intensity ratio of the diffraction
peaks of the austenitic phase {220} plane and the martensitic phase
{211} plane using CrK .alpha.-rays. In SUS304, SUS316, and SUS316L
the content of martensite in the test area before fatigue testing
was about 3%.
[0052] The content of martensite in the hydrogen-charged test areas
was also about 3%. The content of martensite was measured in two
places before the artificial microhole was inserted. 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 defined by rotating the lengthwise axis of the
test piece 180.degree. 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 cylinder from the first
measurement region.
Hydrogen Charging Method
[0053] Hydrogen charging was performed using the cathodic charging
method. The hydrogen charging conditions were an aqueous sulfuric
acid solution at pH=3.5, platinum anode, and current density i=27
A/m.sup.2. Hydrogen charging was performed for 672 hours (4 weeks)
at a solution temperature of 50.degree. C. (323 K) and 336 hours (2
weeks) at a solution temperature of 80.degree. C. (353 K). The
sulfuric acid solution was replaced once a week to minimize changes
in the sulfuric acid concentration resulting from evaporation.
Prestrained Material
[0054] To investigate the relationship between the acceleration in
fatigue crack growth rate resulting from hydrogen and the amount of
martensitic transformation, prestraining was performed on SUS304
and SUS316L, and martensite-transformed test pieces therefrom were
used. FIG. 3 shows a chart illustrating the prestraining procedure.
Because prestraining promotes martensitic transformation, it was
performed in -70.degree. C. ethanol. After prestraining, the test
piece was worked into the shape shown FIG. 1(a). For the SUS304,
prestraining was applied at a plastic strain (true strain) of
.epsilon..sub.p=0.28, and for SUS316L prestraining was applied at a
plastic strain of .epsilon..sub.p=0.35
[0055] When the Vickers hardness was measured after the prestrain
was applied (measurement load of 9.8 N), it was HV=426 (10 point
average) for SUS304, and HV=351 (10 point average) for SUS316L. The
variation was within .+-.4%. After the test piece was polished, the
amount of martensite in the test area after prestraining was
measured by x-ray diffraction. The martensite content in SUS304 was
65% to 69% by specific volume, and in SUS316L was 26% to 28% by
specific volume. The amount of martensite was measured at two
locations before the artificial microhole was drilled. The
measurement regions were 1 mm in diameter centered on the spot
where the artificial microhole was to be drilled and a spot defined
by rotating the lengthwise axis of the test piece 180.degree. from
the spot where the artificial microhole was to be drilled.
Fatigue Test Method
[0056] The fatigue test used a hydraulic servo-controlled tension
and compression fatigue testing machine "Servopulser EHF-ED30KN"
manufactured by Shimadzu Corporation (Chukyo-ku, Kyoto), with an
repetition rate of 0.0015 to 5 Hz, and a stress ratio of R=-1. The
repetition rate was adjusted so that the surface temperature of the
test area did not exceed 60.degree. C. during the fatigue test. The
fatigue cracks were observed using the replica method, and the
length of the fatigue cracks measured.
[0057] Observation of the fatigue cracks by the replica method
revealed the following. After a 0.034 mm thick acetyl cellulose
film (hereinafter, called the replica film) had been immersed in
methyl acetate liquid for a short time, it was applied to
observation site. Two or three minutes elapsed after the replica
film was applied, and the replica film was then peel off after it
had dried. Gold was deposited on the recovered replica film, and
the fatigue cracks in the test area were observed with a
metallurgical microscope.
[0058] Therefore, even if the test piece was not observed directly,
the location of a target fatigue crack could be observed. In the
case of hydrogen-charged material, a sample 7 mm in diameter and
0.8 mm in thickness was cut out from the test area immediately
after the end of fatigue testing, maintained in a vacuum chamber,
and heated at a constant heating rate. The vacuum chamber internal
pressure was 1.times.10.sup.-7 Pa to 3.times.10.sup.-7 Pa before
the sample was heated. The temperature was raised up to 800.degree.
C. at a heating rate of 0.5.degree. C./sec.
[0059] Heating the sample in the vacuum chamber caused hydrogen to
escape therefrom, and the amount of escaped hydrogen was measured
with a quadrupole mass analyzer type thermal desorption
spectrometer (hereinafter, called TDS). The TDS used for
measurement was a thermal desorption spectrometer model
EMD-WA1000S/H manufactured by ESCO, Ltd. (Musashino, Tokyo). The
precision of the TDS measurement was 0.01 mass ppm.
Measured Properties
[0060] FIG. 4 is a photograph of fatigue cracks that developed from
the artificial microhole drilled in hydrogen-charged SUS304 after
fatigue testing. From the photo it can be confirmed that the
fatigue cracks spread from the artificial microhole. These fatigue
cracks developed bilaterally from the artificial microhole, and it
is clear that they grew in a roughly symmetrical manner.
[0061] FIG. 5 shows results of x-ray examination of the austenitic
phase and martensitic phase in the test area surface before fatigue
testing and the fatigue cracked surface after fatigue testing. The
dotted line in FIG. 5 shows the results of measuring the surface of
the test area before fatigue testing. The solid line shows the
results of measuring fatigue cracked surface after fatigue testing.
FIG. 5(a) shows the measurement results for SUS304, and from this
measurement it is clear that after the fatigue testing the
austenitic phase has decreased and the martensitic phrase has
increased compared with before the fatigue testing.
[0062] FIG. 5(b) shows the measurement results for SUS316, and from
this measurement it is clear that after fatigue testing the
austenitic phase has decreased a little and the martensitic phrase
has increased compared with before fatigue testing. FIG. 5(c) shows
the measurement results for SUS316L, and from this measurement it
is clear that after fatigue testing the martensitic phrase has
increased compared with before fatigue testing. Almost no change in
the austenitic phase was seen for SUS316L.
[0063] FIG. 6 is a graph showing the relationship between the
length of the cracks caused by fatigue testing and number of
cycles. FIG. 6(a) shows the results for SUS304, FIG. 6(b) for
SUS316, and FIG. 6(c) for SUS316L. For each material (SUS304,
SUS316, and SUS316L) the measurement results are shown for
hydrogen-charged pieces and uncharged pieces. The repetition rate
was 1.2 Hz for SUS304 and SUS316, and 5 Hz for SUS316L.
[0064] From this graph it is clear that the crack growth rate is
accelerated in hydrogen-charged SUS304 and SUS316 compared with the
uncharged material. For example, in hydrogen-charged material the
number of cycles N until crack length 2a reaches 400 .mu.m is lower
than in uncharged material. In these cases, the fatigue crack
growth rate is approximately twice as fast in the hydrogen-charged
pieces. On the other hand, for SUS316L the fatigue crack growth
rate is slightly higher in the hydrogen-charged material than in
the uncharged material, but no clear difference is seen.
[0065] FIG. 7 shows photographs of fatigue cracks in SUS304,
SUS316, and SUS316L observed by the replica method. As can be seen
in the photograph in FIG. 4, the fatigue cracks essentially grow
symmetrically, and therefore the photographs in FIG. 7 show only
one side of the microhole. From the photographs it can be observed
that the fatigue cracks in hydrogen-charged material tend to grow
more linearly than in uncharged material. Whereas in the
hydrogen-charged material the slip bands occur over a broad region,
it is clear that in the uncharged material the slip bands are
localized near the fatigue cracks.
[0066] FIG. 8 is a graph showing the results of fatigue testing of
SUS316L. This figure shows the fatigue test results of materials
with a hydrogen content of 0.4 mass ppm and 2.6 mass ppm when
uncharged, and after the material with a hydrogen content of 2.6
mass ppm was charged with hydrogen to raise the content to 3.9 mass
ppm. The repetition rate until the fatigue crack reached a length
of 200 .mu.m was 1.5 Hz. When the length of the fatigue crack
became 200 .mu.m, the repetition rate was changed from 1.5 Hz to
0.0015 Hz. The fatigue cracks grew in material with a hydrogen
content of 2.6 mass ppm and mass ppm.
[0067] However, fatigue cracks grew only slightly in the material
with a hydrogen content of 0.4 mass ppm. FIG. 9 is a graph showing
the result of the fatigue testing of SUS316L. This figure shows the
fatigue test results of materials with a hydrogen content of 0.4
mass ppm and 2.6 mass ppm when uncharged, and results after the
material with a hydrogen content of 2.6 mass ppm was charged with
hydrogen to raise the content to 3.9 mass ppm and 5.1 mass ppm. The
repetition rates were 1.5 Hz and 0.0015 Hz.
[0068] From this graph it is clear that the fatigue cracks have
grown in the material with a hydrogen content of 2.6 mass ppm and
in the same material charged with hydrogen to a content of 3.9 mass
ppm and 5.1 mass ppm. It is clear that when the repetition rate was
a low 0.0015 Hz, the fatigue crack growth rate is faster than at a
repetition rate of 1.5 Hz. However, it is also clear that in
material with a hydrogen content of 0.4 mass ppm the fatigue crack
growth rate is slower at repetition rates of both 0.0015 Hz and 1.5
Hz. This illustrates the fact that fatigue cracks do not grow much
when the hydrogen content in the material is 0.4 mass ppm or
less.
[0069] FIG. 10 is a conceptual drawing showing the situation
wherein the diffusible hydrogen and the non-diffusible hydrogen
diffuse through the transformed martensitic phase. In the figure
the tip of the fatigue crack undergoes martensitic transformation,
and the diffusible hydrogen and non-diffusible hydrogen diffuse via
the martensitic phase. In other words, the hydrogen moves using the
martensitic phase with its fast diffusion rate as a passageway, and
it accumulates at the tip of the fatigue crack. This is a
phenomenon related to hydrogen diffusion and migration time. The
rate of diffusion of the hydrogen in the austenitic phase (FCC) is
four decimal places slower than the rate of diffusion in the
martensitic phase (BCC). The fatigue crack periphery undergoes
martensitic transformation, and the surrounding hydrogen diffuses
through this martensitic phase and gathers at the tip of the
fatigue crack.
Involvement of Non-Diffusible Hydrogen
[0070] Therefore, through the above test it was shown that not only
diffusible hydrogen, but also non-diffusible hydrogen, which has
been disregarded in prior art, is involved in fatigue crack growth.
This is a novel finding concerning hydrogen embrittlement, and the
martensitic transformation of the fatigue crack tip (transformation
from FCC to BCC) affects this.
Relationship Between Fatigue Test Rate and Fatigue Crack Growth
Rate
[0071] In addition, from FIG. 9 showing the above test results, it
is clear that the fatigue crack growth rate accelerates in
austenitic stainless steel such as SUS316L if the fatigue test rate
is slow. In a similar manner, as shown in FIG. 6, the fatigue crack
growth rate is faster in hydrogen-charged material such as the test
pieces charged with diffusible hydrogen than in uncharged material.
As shown in FIGS. 8 and 9, fatigue cracks grow very little in
material with a hydrogen content of 0.4 mass ppm or less. Thus, the
effect of slowing the fatigue test rate is a phenomenon related to
hydrogen diffusion and migration time (the diffusion rate is four
decimal places slower in FCC than in BCC).
[0072] Below the alloying components in the austenitic stainless
steel of the present invention, the content thereof, and the
manufacturing steps, etc., stipulated in the manufacturing process
of the present invention are explained.
Austenitic Stainless Steel
[0073] Austenitic stainless steel is also called Cr--Ni stainless
steel, and it involves the addition of chromium and nickel to iron.
The principal components of austenitic stainless steel are iron,
chromium, and nickel with various additives shown in Table 2
below.
[0074] Table 2 below shows preferred examples of the austenitic
stainless steel of the present invention, but the mode of the
present invention is by no means limited thereto.
TABLE-US-00002 TABLE 2 Composition 1 Composition 2 Component (mass
ratio) (mass ratio) C .ltoreq.0.030 .ltoreq.0.08 Si .ltoreq.1.00
.ltoreq.1.50 Mn .ltoreq.2.00 .ltoreq.2.00 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 .ltoreq.3.00
.ltoreq.3.00 Al -- .ltoreq.0.35 N -- .ltoreq.0.50 Ti --
.ltoreq.2.35 V -- .ltoreq.0.50 B -- .ltoreq.0.010 H .ltoreq.0.00007
(0.7 ppm) .ltoreq.0.00007 (0.7 ppm) Other Remainder Fe and
Remainder Fe and inevitable impurities inevitable impurities
Chemical Composition of Austenitic Stainless Steel
[0075] Chromium is added to iron to improve corrosion resistance.
Nickel is added to iron in combination with chromium to increase
corrosion resistance. Nickel and manganese are elements for
insuring nonmagnetic properties after cold rolling. The nickel
content must be 10.0 mass % or more to insure the nonmagnetic
properties after cold rolling. In addition, it is necessary to
adjust the content of nickel according to the content of silicon
and manganese so that a stress-induced martensitic phase of
.gtoreq.1 vol % does not occur. Manganese also has the effect of
improving the solid solubility of nitrogen.
[0076] Carbon is an element for a strong austenite formation. In
addition, carbon is an effective element for enhancing the strength
of stainless steel. When an excess of carbon is added, coarse
chromium carbides precipitate during the recrystallization process,
and that causes a decrease in intergranular corrosion resistance
and fatigue properties. Silicon is added for deacidification and
strengthening of the solid solution. Because generation of the
martensitic phase during cold-working is promoted when the content
of silicon increases, adding only a small amount thereof is
preferred. Nitrogen brings about solution hardening.
[0077] Molybdenum is added for improved corrosion resistance. In
addition, it has the effect of bringing about the fine dispersion
of carbonitrides in the aging treatment. Titanium is an effective
element for precipitation hardening and is added to increase the
strength brought about by the aging treatment. Boron is an
effective alloying component for the prevention of edge cracks in
the hot rolled steel area caused by the difference in the
deformation resistance between the .delta.-ferrite phase in the hot
working temperature region and the austenitic phase. Aluminum is an
element added for deacidification during steelmaking and is
effective in precipitation hardening in a similar manner to
titanium.
[0078] The mode for carrying out the present invention can also be
used by adding elements such as niobium, copper, etc., as needed in
addition to the elements described in Table 2 above. Niobium can
serve as a substitute for titanium.
Austenitic Phase
[0079] Austenitic stainless steel wherein the austenitic phase is
essentially 100% of the total volume is preferred. Austenitic
stainless steel having no martensitic phase contained therein is
preferable.
Other Properties
[0080] The average crystalline particle diameter is preferably
about 50 .mu.m or less. In modern materials the average crystalline
particle diameter is about 50 .mu.m, but a smaller the average
crystalline particle diameter is preferred.
Hydrogen Removal Treatment by Heating
[0081] The hydrogen removal treatment involving the heating of
austenitic stainless steel will now be described. The inventors of
the present invention ascertained that non-diffusible hydrogen
takes part in fatigue crack growth, and based upon that discovery,
the heat treatment described below is performed to remove the
non-diffusible hydrogen and the diffusible hydrogen present in
austenitic stainless steel.
[0082] Removal of the diffusible hydrogen and the non-diffusible
hydrogen involves performing a heat treatment on the austenitic
stainless steel at a heating temperature of 200.degree. C. or more.
The heat treatment is performed in a vacuum. The vacuum environment
is 0.2 Pa or less. Moreover, for the heat treatment the austenitic
stainless steel is maintained under vacuum at the heating
temperature for 460 hours or less. The temperature of the heat
treatment is lower than the sensitization temperature, which is the
temperature at which the carbides of chromium (Cr) in the
austenitic stainless steel precipitate due to heating.
[0083] For the austenitic stainless steels shown in Table 1 and
Table 2, for example, the upper limit of the heating temperature is
500.degree. C. As a result, both the non-diffusible hydrogen, and
the diffusible hydrogen (which are present in austenitic stainless
steel, diffuse via the stress-induced martensitic phase brought
about by cyclic loading, concentrate in the cracks undergoing
concentrated stress, and cause hydrogen embrittlement) can be
removed.
[0084] By such a heat treatment it is possible to remove from
austenitic stainless steel the diffusible hydrogen and the
non-diffusible hydrogen that cause hydrogen embrittlement therein,
and thereby adjust the hydrogen (H) contained in austenitic
stainless steel to 0.00007 mass % (0.7 mass ppm) or less. The
preferred content of hydrogen (H) in austenitic stainless steel
after this heat treatment is .ltoreq.0.00004 mass % (.ltoreq.0.4
mass ppm), and .ltoreq.0.00001 mass % (.ltoreq.0.1 mass ppm) is
even more preferred.
[0085] Thus, it is possible to provide an excellent austenitic
stainless steel wherein the content of hydrogen therein is less
than in the prior art austenitic stainless steels, and acceleration
of fatigue crack growth does not occur even with cyclic loading
involving a long cycle time.
Additional Experimental Example 1
[0086] The experiment was performed on a heat-treated test piece of
SUS316. The test piece was a rod 7 mm in diameter. For the TDS
measurement a disk 7 mm in diameter and 0.8 mm in thickness was cut
from the rod. For the experiment the test piece was heat-treated at
800.degree. C. for 20 minutes. The atmospheres during the
experiment were an air atmosphere, a vacuum atmosphere
(approximately 0.006 Pa), and an argon gas atmosphere. The heat
treatment was performed while supplying argon gas thereto. The
heating rate for the TDS measurement was 0.5.degree. C./second up
to 700.degree. C. The escaped hydrogen was measured for heating up
to 700.degree. C.
[0087] The measurement was performed with a thermal desorption
spectrometer model EMD-WA1000S/H manufactured by ESCO, Ltd.
(Musashino, Tokyo). FIG. 12 shows the measurement results. In the
graph the horizontal axis shows the measurement temperature, and
the vertical axis shows the hydrogen intensity. The hydrogen
concentration of the test piece that had not been heat-treated was
1.5 mass ppm. When the heat treatment was performed in air, the
hydrogen concentration of the test piece became 0.7 mass ppm. When
the heat treatment was performed in a vacuum, the hydrogen
concentration of the test piece became 0.4 mass ppm. When the heat
treatment was performed under with the argon gas flow, the hydrogen
concentration decreased to 0.4 mass ppm.
Additional Experimental Example 2
[0088] The experiment was performed on a heat-treated test piece of
SUH660. The test piece was a rod 7 mm in diameter. For the TDS
measurement a disk 7 mm in diameter and 0.8 mm in thickness was cut
from the rod. For the experiment the test piece was heat-treated at
720.degree. C. for 16 hours. The atmosphere during the experiment
was a vacuum atmosphere (approximately 0.006 Pa). The hydrogen
concentration was 1.3 ppm before the aging treatment and 0.6 ppm
after the aging treatment.
[0089] In this manner, an aging treatment and the like was
performed during the manufacturing process of stainless steel, and
the hydrogen contained therein could be removed. The heating rate
for the TDS measurement was 0.33.degree. C./second up to
600.degree. C. The escaped hydrogen was measured for heating up to
600.degree. C. The measurement was performed with a thermal
desorption spectrometer model EMD-WA1000S/H manufactured by ESCO,
Ltd. (Musashino, Tokyo). FIG. 13 shows the measurement results. The
horizontal axis in the graph shows the measurement temperature, and
vertical axis shows the hydrogen escape strength.
INDUSTRIAL APPLICABILITY
[0090] The present invention is good for use in fields where
corrosion resistance and high pressure hydrogen are necessary. More
specifically, the present invention is good for use in metal
gaskets, various types of valves used in automobiles, springs,
steel belts, cutting blade material, fuel cells, and material for
valves, springs, etc., surrounding fuel cell systems.
* * * * *