Effect of hydrogen environment on non-propagation and propagation of
fatigue crack in type 304 austenitic stainless steel
H. Matsuno, Y. Aoki, Y. Oda, H. Noguchi
Graduate school of Kyushu University
Kyushu University 744, Moto-oka, Nishi-ku, Fukuoka, 819-0395, Japan
te206325@s.kyushu-u.ac.jp
ABSTRACT
It has been reported that a hydrogen environment enhances the crack growth rates of metallic materials. However, the effects
of hydrogen on the fatigue limit and the fatigue crack growth behavior near the fatigue limit have not been sufficiently clarified.
In this study, the following results are obtained on the basis of bending fatigue tests on type 304 solution-treated austenitic
stainless steel in hydrogen gas, in argon, in nitrogen and in air. (1) At the strain level of the fatigue limit in each environment, a
crack emanating from twin holes stops after propagation to some length. (2) The fatigue limit in hydrogen is slightly higher than
that in air, and the fatigue lives in hydrogen is longer than those in air. (3) The fatigue crack growth rate in hydrogen is lower
than that in argon and in air in the range of low growth rates. This indicates that type 304 austenitic stainless steel does not
show a disadvantage to compare with a type 316L stable austenitic stainless steel which is a promising material. A hydrogen
environment has both acceleration and deceleration effect for fatigue crack growth. It seems that the various effect of
hydrogen environment appear as a summation of the effect of these factors, in a different manner in the various stages of
crack growth or under various environmental conditions; such as, pressure and temperature.
Introduction
Hydrogen is expected to be the next generation energy source because it not only solves the lack of fossil fuel resources but
also reduce carbon dioxide emission. In order to develop the hydrogen society, it is urgent matter to establish the strength
evaluation method for the components exposed to hydrogen in machines and structures, such as fuel-cell vehicles and
infrastructures, because hydrogen has been reported to degrade the strength of materials [1,2]. Our previous study on type
304 austenitic stainless steel shows that the fatigue crack growth rate in a hydrogen gas environment is higher than that in air
atmosphere in the range of high growth rates. The authors explained that this phenomenon is due to the concentration of slip
at the tip of a fatigue crack [3].
However, in the actual fatigue design of many machines and structures the fatigue limit is used. It is very important to study the
effect of hydrogen on a fatigue limit [4] and the behaviour of crack growth near fatigue limit. In this study, the effects of
hydrogen gas environment at low pressure on these phenomena in type 304 solution-treated austenitic stainless steel are
discussed. The results of type 316L austenitic stainless steel are used to aid the understanding in regard to the background of
the phenomena.
Experimental procedure
In-situ Observation System. The system consists of three parts: the fatigue testing machine, the chamber for the environment
and the Scanning Laser Microscope (SLM) for observation. The fatigue testing machine is the bending test machine driven
with the stepping motor, which has the capacity of 5 Nm and the maximum frequency of 10 Hz. The chamber is filled with pure
hydrogen gas or companion gas; pure nitrogen or pure argon. The available maximum pressure of a gas in the chamber is 0.2
MPa in absolute pressure. The temperature in the chamber can be kept constant at a certain value during the test. The SLM is
used for the in-situ observations of the fatigue behavior through a window attached in the chamber. A magnification of up to
3500 times is obtained on the display.
Material and Specimen. The material used in this study is type 304 solution-treated austenitic stainless steel. Type 316L
solution-treated stable austenitic stainless steel is used for comparison. Table 1 shows the chemical compositions and Table 2
shows the mechanical properties of the type 304 and the type 316L austenitic stainless steel. Figure 1 shows the specimen
configuration. Small twin holes with a diameter of 0.05 mm and a depth of 0.1 mm as shown in Figure 1 were introduced at the
center of each specimen as a crack initiation site. After that, in order to remove the work hardened layer, each specimen was
solution treated in vacuum again. Two types of surface finishes were used: One is the electro-polishing, and the other is
buffing with 0.05 µm γ-alumina after electro polishing. Before the fatigue test, each specimen was baked in a vacuum at 373 K
for 1 hour in order to rearrange the hydrogen content in each specimen.
Table 1 Chemical compositions of the type 304 and 316L austenitic stainless steel. (mass%)
C
Si
Mn
P
S
Ni
Cr
Mo
type 304
0.046
0.49
0.84
0.020
0.005
8.70
18.42
0.09
type 316L
0.016
0.71
1.67
0.035
0.002
12.2
17.30
2.14
Table 2 Mechanical properties of the type 304 and 316L austenitic stainless steel.
σ0.2 [MPa] σB [MPa]
Ψ [%]
type 304
263
646
54.1
type 316L
263
567
56
σ0.2 : Proof
0.2% stress
proof stress
σB : Ultimate tensile strength
ψ : Reduction of area
Thickness=3
Figure 1 Shape and dimensions (in mm) of the specimen
Testing Method. The fatigue tests were carried out under displacement-controlled fully reversed bending in air, in hydrogen
gas at a pressure of 0.18MPa (in absolute pressure), and in inert gas (nitrogen gas and argon gas) at the same pressure as in
the case of the hydrogen gas. The temperatures of the three environments were kept constant at 313 K during the fatigue test.
The testing frequency was set at 10 Hz to avoid hysteretic heating of the specimen. In order to determine the loading condition,
a strain gauge was placed on the back surface of the specimen. ∆εt denotes the total strain range. The fatigue process was
monitored using the SLM. Before fatigue test in hydrogen was carried out, each specimen was exposed in 1.0 MPa hydrogen
gas environment at 393 K for the 100 hours for the sake of hydrogen introduction.
Results
Fatigue crack growth rate da/dN [m/cycle]
Our previous study shows that the fatigue crack growth rate in pure hydrogen gas is higher than those in air and in pure
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nitrogen in the range of relatively high growth rate (about and over 10 m /cycle). Figure 2 shows the fatigue growth rate at
∆εt=0.55% [3]. If this acceleration effect of hydrogen appears in the range of extremely low growth rates, it is feared that
hydrogen will lower the fatigue limit.
10
10
-6
-7
-8
10
Crack length l
Figure 2. Fatigue crack growth rate at ∆εt=0.55%
Figure 3 shows the ∆εt-N diagram for type 304 austenitic stainless steel obtained in the present study. At the fatigue limit a
crack emanating from twin small holes stops after propagation to some length in each environment. Figure 4 shows the profile
of the non-propagating crack after the fatigue test at ∆εt=0.23% in each environment. The fatigue limit in hydrogen is slightly
higher than that in air, and the fatigue lives in hydrogen is longer than that in air. The fatigue limit is ∆εt=0.24% (equivalent to
219 MPa) in hydrogen and ∆εt=0.23% (equivalent to 207 MPa) in air. If the hydrogen environment enhances the fatigue crack
growth rate, the fatigue limit may decrease and fatigue lives may shorten, but the realty is contrary to this behaviour.
Total strain range ∆εt [%]
0.3
0.2
0.1
in Air
in H2
not broken
0 5
10
10 6
107
10 8
Number of cycles to failure Nf
Figure 3. ∆εt-N diagram in type 304 austenitic stainless steel
N = 4.2×106 cycles in Air (EP)
N = 1.2×107 cycles in H2 (Buffed)
N = 8.4×106 cycles in N2 (Buffed)
50µm
Figure 4. Profiles of non-propagating cracks after fatigue test
Figure 5 shows the fatigue crack growth rate at ∆εt=0.28% in hydrogen, in argon, and in air. Figure 6 shows the profile of the
propagating crack with a length of about 300 µm at ∆εt=0.28% in each environment. The fatigue crack growth rate in the range
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below 10 m /cycle in hydrogen is lower than that in argon and in air. Our previous study showed that the absence of oxygen
and water vapour retards crack initiation in hydrogen or in nitrogen in comparison with the case in air [5]. The retardation of
fatigue crack initiation and the fatigue crack growth rate relationship between air and hydrogen explain the longer fatigue lives
under the hydrogen than under air in Figure 3.
In order to investigate whether fatigue crack growth rate in the range of low growth rate relationship as shown in Figure 5 is a
special case in type 304 austenitic stainless steel, fatigue crack growth tests were carried out on a type 316L austenitic
stainless steel under hydrogen, under nitrogen, and under air in the same manner as a type 304 austenitic stainless steel
expect no pre-exposure to hydrogen gas. Figure 7 shows fatigue growth rate at ∆εt=0.30% in each environment. The fatigue
growth rate in the range of low growth rate in type 316L austenitic stainless steel exhibits the same tendency as that in type
304 austenitic stainless steel except for the amount of differences in crack growth rate among environments.
Crack length 2a
Fatigue crack growth rate da/dN [m/cycle]
Fatigue crack growth rate da/dN [m/cycle]
10-8
10-9
10-10
∆εt=0.28%
in Nitrogen
in Air
in Hydrogen
10-11
10-8
10-9
10-10
εt=0.30%
in Nitrogen
in Air
in Hydrogen
10-11
1
0.3
0.5
Crack length 2a [mm]
Figure 5. Fatigue crack growth rate at ∆εt=0.28% for
type 304 austenitic stainless steel
Crack length 2a
1
0.5
Crack length 2a [mm]
Figure 7. Fatigue crack growth rate at ∆εt=0.30%
0.3
for type 316L austenitic stainless steel.
50µm
In Air; 2a=305µm
50µm
In H2; 2a=321µm
50µm
In Ar; 2a=318µm,
Figure 6 Profile fatigue crack at 2a=300µm for type 304 austenitic stainless steel
Figure 8 shows crack growth plots for type 304 austenitic stainless steel. Fatigue crack growth in hydrogen is fastest among
the three environments at ∆εt=0.55%, while at ∆εt=0.28% it is slowest in hydrogen.
Figure 9 shows crack growth plots in the type 316L austenitic stainless steel; at (a) ∆εt=0.60%, (b) ∆εt=0.38%, and (c)
∆εt=0.30%. The order of the fatigue crack growth plots among the three environments changes as the test strain level changes
(The order of the fatigue crack growth plots for air and nitrogen does not change). Fatigue crack growth in hydrogen is fastest
at ∆εt=0.60%, while it is slowest at ∆εt=0.30%. The small difference between the crack growth plots for hydrogen and nitrogen
at ∆εt=0.30% seems to be due to the relatively large ∆εt and the absence of transformation-related phenomena compared with
the case in type 304 austenitic stainless steel at ∆εt=0.28%
2
1
0.8
0.6
∆εt=0.55%
Crack length 2a [mm]
Crack length 2a [mm]
2
in Nitrogen
in Air
in Hydrogen
0.4
0
10000
20000
1
0.8
0.6
∆εt=0.28%
0.4
in Argon
in Air
in Hydrogen
0.3
0
30000
200000
400000
600000
Number of cycles N-N2a=0.3mm
Number of cycles N-N2a=0.4mm
(b)∆εt=0.28%
(a)∆εt=0.55%
Figure 8 Fatigue crack growth plots for type 304 austenitic stainless steel
2
Crack length 2a [mm]
1
0.8
0.6
∆εt=0.60%
0.4
in Nitrogen
in Air
in Hydrogen
0.3
0
10000
20000
30000
1
0.8
0.6
∆εt=0.38%
0.4
in Nitrogen
in Air
in Hydrogen
0.3
0
40000
Number of cycles N-N2a=0.3mm
(a)∆εt=0.60%
100000
1
0.8
0.6
∆εt=0.30%
0.4
in Nitrogen
in Air
in Hydrogen
0.3
0
200000
Number of cycles N-N2a=0.3mm
(b)∆εt=0.38%.
2
Crack length 2a [mm]
Crack length 2a [mm]
2
200000
400000
600000
Number of cycles N-N2a=0.3mm
(c)∆εt=0.30%
Figure 9 Fatigue crack growth plots for type 316L austenitic stainless steel
It has been reported that strength of stable type 316L austenitic stainless steel is generally superior to that of metastable type
304 austenitic stainless steel for the use in hydrogen environment [6,7]. However, according to the present results in the range
of low crack growth rate a type 304 stainless steel is not inferior to than type 316L austenitic stainless steel in hydrogen
environment.
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The deceleration of fatigue crack growth rates in hydrogen in the range below 10 m /cycle seems to be induced by crack
closure. Various factors of crack closure have been suggested; plasticity [8], oxide film [9], fracture asperity, debris, and
transformation. The oxide film is present in air but not in hydrogen and argon. Debris was not observed in each environment
based on Figure 6. Asperity is observed in air and argon, but not in the hydrogen. Within the limits of this experiment,
plasticity-induced crack closure and transformation-induced crack closure (including closure due to diffusion enhancement
after transformation in metastable austenitic stainless steel) seems to play important roles in hydrogen environment. Therefore
it is plausible that the fatigue limit of type 304 in hydrogen is higher than that in air.
Conclusions
On the basis of a study of the characteristics of the fatigue limit and the fatigue crack propagation in a type 304 solution treated
austenitic stainless steel, tested in low-pressure hydrogen gas, low-pressure comparison gas; namely, argon, nitrogen, and in
air, following conclusions are obtained.
1) The fatigue limit in the hydrogen gas environment is slightly larger than that in air.
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2) The fatigue crack growth rate of less than 10 m /cycle in hydrogen is lower than that in argon and in air.
3) The hydrogen gas environment seems to have both the effects of acceleration and deceleration for fatigue crack growth.
Within the limits of this experiment, type 304 stainless steel is suitable for use in the hydrogen gas environment.
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