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Effect of oxygen addition on fretting fatigue strength in hydrogen of JIS SUS304 stainless steel
Tribology International 76 (2014) 92–99
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Effect of oxygen addition on fretting fatigue strength in hydrogen of JIS SUS304 stainless steel Ryosuke Komoda a,n, Masanobu Kubota b,c,d,e, Yoshiyuki Kondo b, Jader Furtado f a
Graduate School of Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan Department of Mechanical Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan c Air Liquide Industrial Chair on Hydrogen Structural Materials and Fracture, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan d International Institute of Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan e Research Center of Hydrogen Industrial Use and Storage (HYDROGENIUS), 744 Motooka, Fukuoka 819-0395, Japan f Air Liquide R&D Centre de Recherches Claude Delorme-Paris Saclay, 1 chemin de la porte des Loges, Les Loges-en-Josas Jouy-en-Josas 78354, France b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 June 2013 Received in revised form 23 January 2014 Accepted 28 February 2014 Available online 12 March 2014
Fretting fatigue test of SUS304 austenitic stainless steel was performed in air, in hydrogen gas, and in oxygen–hydrogen mixture. The fretting fatigue strength is more significantly reduced in hydrogen as compared to air. An increase in the fretting fatigue strength was found in the mixture. The mechanisms were investigated focusing on crack initiation. As the result, the crack initiation limit was significantly lower in hydrogen than in air, and increased in the mixture. The tangential force coefficient in the mixture is similar to that in air. The morphology of the fretting damage in the mixture was similar to that in air. These results indicated that the adhesion between contacting surfaces was prevented by addition of oxygen. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Fretting fatigue Hydrogen Oxygen Crack nucleation
1. Introduction Hydrogen is the most promising new energy carrier in the very near future. Hydrogen will contribute to a deployment of the use of renewable energy and to carbon dioxide emission reduction. However, hydrogen deteriorates the material strength not only in static loading but also in fatigue loading. The interaction of hydrogen and materials is under active investigation in order to achieve the safe use of hydrogen at low cost [1–9]. Fretting fatigue is one of the most important issues in the design of mechanical components involving contact, because fretting can cause a significant reduction in the fatigue strength of the contact part. In hydrogen equipment, the importance of fretting fatigue may be even higher, since hydrogen can influence both fatigue and fretting. The authors determined that the fretting fatigue strength of several kinds of materials in hydrogen is significantly lower than that in air [10–15]. One of the causes of the reduced fretting fatigue strength in hydrogen gas is the local adhesion between the contacting surfaces and subsequent crack nucleation [14]. Since no oxidation occurs in hydrogen and fretting removes the surface
oxide layer, the adhesion can be the first step in causing the fretting damage in hydrogen similar to the fretting in a vacuum [16] and other non-oxidative environments [17]. From a practical perspective of the use of hydrogen based on fuel cell technology, the effect of impurities on the fretting fatigue strength is also important. For example, PEM fuel cell needs humidification of hydrogen for proton conductivity. The objectives of this study are to characterize the effect of oxygen addition to hydrogen as an impurity on fretting fatigue properties, and to elucidate the mechanism. In this study, the effect of hydrogen and addition of oxygen on crack initiation were highlighted.
2. Experimental procedure 2.1. Material The test material was JIS SUS304 austenitic stainless steel. The chemical composition is shown in Table 1. The material was solution heat treated by heating at 1303 K for 3.9 ks followed by rapid cooling. The mechanical properties are shown in Table 2. 2.2. Fretting fatigue test using bridge pad
n
Corresponding author. Tel.: þ 819 280 232 29. E-mail addresses: [email protected] (R. Komoda), [email protected] (M. Kubota). http://dx.doi.org/10.1016/j.triboint.2014.02.025 0301-679X/& 2014 Elsevier Ltd. All rights reserved.
A fretting fatigue test using bridge pad was carried out in air, in hydrogen and in oxygen–hydrogen mixture. The oxygen–hydrogen
R. Komoda et al. / Tribology International 76 (2014) 92–99
mixture contained 100 vol. ppm oxygen. The pressure of the hydrogen and oxygen–hydrogen mixture was 0.2 MPa in absolute pressure. The detail of the fretting fatigue test method and the configurations of the specimen and contact pad can be seen in Ref. [13].
2.3. Fretting test to investigate the effect of hydrogen and oxygen addition on the nucleation of fretting fatigue cracks Fig. 1 shows the fretting test method used for determining the role of hydrogen in fretting fatigue crack nucleation. Fretting was induced by applying cyclic displacement to the specimen. To prevent the rotation of the contact pad, the height of the contact part of the contact pad was as short as possible and the supporting point of the pad was as close as possible to the specimen surface. The leaf spring supporting the contact pad is beneficial in order to prevent uneven contact and to precisely apply the contact load. The relative slip range between the specimen and contact pad was measured by a small displacement sensor as shown in Fig. 1. The measurement position of the relative slip range is a few millimeters away from the contact part. This might be a cause of the error in the measurement [18,19]. In this study, the error of the relative slip range between the measured value and true value was confirmed to be within 5%. The tangential force was measured by a load cell. The fretting test was carried out by controlling the tangential force as a constant. To do this, the amplitude of the cyclic displacement applied to the specimen was adjusted during the Table 1 Chemical composition (mass%). Material
C
Si
Mn
P
S
Ni
Cr
SUS304
0.05
0.53
0.91
0.034
0.002
8.10
18.19
Table 2 Mechanical properties and hardness. Material Condition
SUS304
0.2% proof stress
Ultimate tensile strength
237 MPa 771 MPa Solution heat treatment
Elongation at fracture
Reduction of area
Vickers hardness
72.3%
81.2%
HV174
93
testing. The test was interrupted at 105 cycles. Identification of a small crack was then carried out. The test frequency was 5 Hz. Fig. 2 shows the specimen and contact pad. The adhesion during the fretting fatigue test in hydrogen varies in size and randomly occurs. To obtain a high reproducibility of the adhesion, a small contact length, which is 0.2 mm, was used in this study. The contact surfaces of the specimen and pad were finished by chemical mechanical polishing using a silicon wafer polishing machine. The surface roughness parameter of the contact surface, Ra, was 0.053 μm. The material was solution heat-treated JIS SUS304 austenitic stainless steel, which is the same as that used in the fretting fatigue test. The test environments were hydrogen and laboratory air. The hydrogen pressure was 0.13 MPa in absolute pressure. 2.4. Finite element analysis of fretting test The local stress near the contact edge in the fretting test was evaluated by a finite element (FE) analysis. The model and the boundary conditions are shown in Fig. 3(a). A half of the fretting test specimen and contact pad was modeled by symmetry. The analysis was carried out by assuming an elastic body under plane strain condition. The friction model was a modified Coulomb friction provided by the solver, which was Marc 2010. The relative sliding velocity in the modified Coulomb friction model was set to a value equivalent to the relative slip velocity in the fretting test. Fig. 3(b) shows the analysis procedure. Initially, a certain friction coefficient, μ, is provided to the contacting surfaces and a certain displacement, d, is given to the specimen. The displacement applied to the specimen was divided into 20 steps and repeated by two cycles as shown Fig. 3(c) by considering convergence of the result. The tangential force coefficient, ϕ, and the relative slip range, ΔS, under the given conditions were then calculated. If the calculated ϕ and ΔS are different from the experimental values, the calculation is repeated after modifying μ and d. For the case in which an elastic rectangle is pressed into contact with an elastic half-space, the contact pressure reaches a theoretically infinite value at the contact edge [20]. Therefore, the result of the FEM calculation under elastic conditions is significantly influenced by the mesh size. In this study, the stress singularity parameters, which are proposed by Hattori [21], were used for the criterion providing the uniqueness of the solution. The adopted mesh size, which was 0.125 μm, was determined when the stress singularity parameters converged to constant values with decreases in the mesh size.
Cyclic displacement Cyclic displacement Driving shaft Slip sensor
Driving shaft Specimen Specimen Bar spring
Slip sensor
Contact load Tightening bolt Leaf spring
Contact load Contact pad
Contact pad Load cell
Leaf spring
Fig. 1. Fretting test method.
6
35
0.2
SUS304 R = -1 f = 20 Hz pc = 100MPa
250
200
150
In air In 0.2MPa H2 In 0.2MPa oxygen-hydrogen mixture (100 vol ppm O2)
0.3
4
Stress amplitude, σa (MPa)
R. Komoda et al. / Tribology International 76 (2014) 92–99
6
94
6
100
105 106 107 Number of cycles to failure, Nf
6 Fig. 2. Configurations of specimen (a) and contact pad (b).
Fig. 4. Effect of oxygen addition on fretting fatigue strength of SUS304 in hydrogen.
Contact part Friction coefficient, μ
Contact pad
Cyclic displacement, d Specimen
Start
FE analysis at initial
Tangential force coefficient,φ
0.6 Contact pressure
SUS304 Solution heat-treated R = -1 , f = 20 Hz
0.5
0.4
0.3
0.2 100
In air In 0.2MPa H2 In 0.2MPa oxygen-hydrogen mixture (100 vol ppm O2)
200 Stress amplitude, σa (MPa)
300
Fig. 5. Effect of oxygen addition on tangential force coefficient in fretting fatigue test in hydrogen and in air.
Adjust μ, d
No
φ, ΔS Analysis = Measured?
Yes
Dislacement, d
End FE analvsis
Time Fig. 3. Procedure for the finite element (FE) analysis of the fretting test. (a) Model for finite element analysis of fretting test, (b) Flow and (c) Displacement applied to the specimen.
3. Result and discussion 3.1. Effect of oxygen addition on the fretting fatigue strength in hydrogen The result of the fretting fatigue test is shown in Fig. 4. The fretting fatigue strength in hydrogen was significantly lower than
that in air. This is the same trend with that in our previous studies. The fretting fatigue strength in the oxygen–hydrogen mixture was higher than that in the hydrogen and lower than that in air. The addition of oxygen partially mitigated the reduction in the fretting fatigue strength in hydrogen. Fig. 5 shows the tangential force coefficient as a function of the stress amplitude. The tangential force coefficient in the oxygen– hydrogen mixture was lower than that in the hydrogen and slightly higher than that in air. Since the increase in the tangential force coefficient in the hydrogen was caused by the adhesion between the contacting surfaces because of absence of oxygen [13,14], the one of the possible reasons for the reduced tangential force coefficient in the oxygen–hydrogen mixture is that oxidized fretting wear particles reduces adhesion. Fig. 6 shows the observation of the contact surfaces after the fretting fatigue test in each environment. There were no oxidized fretting wear particles on the fretted surface in hydrogen. On the other hand, the oxidized fretting wear particles were observed on the fretted surface in the oxygen–hydrogen mixture. The amount of oxidized fretting wear particles in the oxygen–hydrogen mixture was smaller than that in air. The morphology of the fretting fatigue damage, which is along perpendicular to the fretting motion, was clearly different from that in air. It was elucidated that the fretting damage in hydrogen is caused by local adhesion between contacting surfaces [14]. The morphology of the fretting damage in the oxygen–hydrogen mixture was similar to that in air. It can be considered that there are multiple reasons for the reduction in tangential force coefficient such as tribo-chemical reaction, surface roughness, etc. Based on the observation shown
R. Komoda et al. / Tribology International 76 (2014) 92–99
95
Fretting
Fretting
Fretting
100μm
100μm
100μm
Fig. 6. Observation of fretted surfaces. (a) In 0.2 MPa hydrogen (sa ¼ 205 MPa, Nf ¼3.2 105), (b) In 0.2 MPa hydrogen oxygen mixture (100 vol. ppm O2) (sa ¼193 MPa, Nf ¼ 3.3 105) and (C) In air (sa ¼206 MPa, Nf ¼4.2 105).
100μm
100μm
Fig. 7. Observation of fretted surface during the fretting test (pc ¼100 MPa, ϕ¼ 0.7, N ¼105). (a) In air (ΔS¼ 7.56 μm) (b) In H2 (ΔS ¼4.89 μm).
in Fig. 6, the production of oxidized fretting wear particles and prevention of local adhesion were one of possible reasons for the reduced tangential force in the oxygen–hydrogen mixture. 3.2. Fretting test assessing the effect of hydrogen on crack initiation Fig. 7 shows the fretted surface of the specimen in air and in hydrogen. The contact surface in air was covered by an oxide. On the other hand, there were no fretting wear particles on the contact surface in the hydrogen. The area where the fretting damage occurred was almost equivalent to the geometric contact area in both environments. A high reproducibility of the fretting damage was achieved. Fig. 8 shows the relationship between the tangential force coefficient and the relative slip range. The relative slip range is represented by the average value after 104 cycles of fretting. The general trend that the tangential force coefficient increases with the increase in the relative slip range is consistent with other fretting fatigue studies [22,23]. The tangential force coefficient corresponding to a certain value of the relative slip range is higher in the hydrogen than in air. In Fig. 5, the tangential force coefficient is shown as a function of the stress amplitude. Since the increase in the stress amplitude increases the relative slip range, the trend in Fig. 8 is similar to that in Fig. 5. Fig. 9 show the section of the specimen fretted in hydrogen cut along the axial direction. Small cracks, which are approximately 10 μm in length, were observed at the contact edge. Table 3 shows the result of the identification of the small cracks. For the same given loading conditions (tangential force coefficient and contact pressure), the small cracks emanated in hydrogen, whereas there were no cracks in air. This indicates that hydrogen assists with the crack nucleation during fretting. Fig. 10 shows the distributions of the maximum shear stress near the contact edge on the specimen surface in a particular relative slip range for each environment at the contact pressure of 50 MPa with the tangential force coefficient of 0.5 obtained by FE analysis. As the result of the calculation, the friction coefficient to realize the given conditions is higher for the hydrogen environment than for air. The
displacement applied to the specimen was lower in the hydrogen environment than in the air. These results correspond to the fact that adhesion occurs in the hydrogen. As shown in the graph, the distributions of the maximum shear stress corresponding to each environment agreed with each other. This indicates that mechanical stress at the contact part is similar between in hydrogen and in air if the same pc and ϕ are applied to the fretting test in both environments, whereas crack nucleation occurred in the hydrogen but did not occur in air in the fretting test under these conditions. Therefore, this may suggest that hydrogen participates in the facilitation of the crack nucleation. As shown in Fig. 10, a very high stress is generated at the contact edge. Since the FE analysis was conducted under elastic conditions, there is segregation from the real situation that relaxation of the stress occurs by local plastic deformation [24] and fretting wear [25,26]. Even so, the fact still remains that a high stress is generated at the contact edge. This may allow for the fact that the crack nucleation during the fretting test is regarded as a low cycle fatigue phenomenon. During the low cycle fatigue of the same kind of material, hydrogen assists with the crack nucleation [27,28]. Considering the hydrogen diffusion coefficient in an austenitic material, which is very low [29], and the low pressure of the hydrogen in this study, it may raise doubt about hydrogen and the material interaction. However, the transport of hydrogen by mobile dislocations [30,31], the attraction of hydrogen to a region with a higher strain [32], the microstructure transformation from austenite to martensite due to fretting [33] (the diffusion coefficient of hydrogen in martensite is very high compared to that in austenite [29]), the vulnerability of martensite to hydrogen, and removal of the surface oxide due to fretting (the oxide layer prevents hydrogen permeation [34]) are possible causes that hydrogen assisted crack nucleation during fretting. 3.3. Adhesion mimic fatigue test It was described that hydrogen participates in the crack nucleation during the fretting through the fretting test and FE
R. Komoda et al. / Tribology International 76 (2014) 92–99
Tangential force coefficient , φ
Table 3 Result of identification of small crack (N ¼105 cycles).
0.8
Tangential force coefficient, ϕ
0.6 SUS304 Fretting f = 5 Hz In 0.13MPa H2 In H2 In air
0.4 0.2 0
0
2 4 6 Relative slip range , ΔS (μm)
8
100
(a) In air 0.3 0.5 0.7
Not found Not found Not found
Not found Not found Not found
Not found Not found Not found
(b) In 0.12 MPa H2 0.3 0.5 0.7
Not found Not found Found
Not found Not found Found
Found Found Found
600
0.6 SUS304 Fretting f = 5 Hz In 0.13MPa H2 In H2 In air
0.4 0.2
0
2 4 6 Relative slip range , ΔS (μm)
8
0.6
In air ( d = 2.68 μm , μ = 0.8 , C = 0.032 mm/s ) In H2 ( d = 1.83 μm , μ = 1.2 , C = 0.045 mm/s )
400 300
Contact pad
200 Specimen
100
0
x
5 10 Distance from edge of contact surface , x (μm)
15
Fig. 10. Distribution of maximum shear stress in the vicinity of the contact edge of the specimen.
SUS304 Fretting f = 5 Hz In 0.13MPa H2 In H2 In air
0.4 0.2 0
pc = 50 MPa , φ = 0.5
500
0
0.8
0
2 4 6 Relative slip range , ΔS (μm)
8
Fig. 8. Relationship between tangential force coefficient and relative slip range in hydrogen and in air. (a) Contact pressure, pc ¼ 25 MPa, (b) Contact pressure, pc ¼ 50 MPa and (c) Contact pressure, pc ¼100 MPa.
Edge of contact surface Contact surface Contact surface
10μm
50
0.8
0
Contact pressure, pc (MPa) 25
Maximum shear stress , τmax (MPa)
Tangential force coefficient , φ
Tangential force coefficient , φ
96
Crack
Cutting Observation
Fig. 9. An example of fretting crack (In H2, pc ¼50 MPa, ϕ ¼0.7, N¼ 105).
analysis. However, the oxide particles were not considered into the FE analysis. Furthermore, the adhesion was not correctly modeled. These were imposed on the friction coefficient. Therefore, in order to more clearly consider the role of the hydrogen in the crack nucleation during the fretting in hydrogen, an additional test was carried out. A spot-welded nugget was attempted to emulate the adhered spot during the fretting in hydrogen for an evaluation. Thus, this
test is called the adhesion mimic fatigue test. The test method is basically the same as that used in the fretting test except for welded specimen and contact pad. Fig. 11(a) shows the preparation of the specimen. After assembling the specimen and contact pads, they were welded using a spot welding machine. Since a 0.2 mm contact length used in the fretting test was too short to obtain the same quality weld, the contact length was increased to 1 mm. The spot welding was carried out at a welding current of 5500 A for 2.1 ms at a contact pressure of 40 MPa. A compressive load, which corresponds to the contact load during the fretting test, was applied in order to produce similar stress conditions in the fretting test in hydrogen. The adhesion mimic fatigue test was carried out in three environments, which are air, 0.13 MPa hydrogen and 0.13 MPa oxygen–hydrogen mixture (100 vol. ppm O2), with the stress ratio, R, of 1 at the loading frequency, f, of 5 Hz. The fatigue test was interrupted at 105 cycles. Identification of a small crack was then carried out. As shown in Fig. 11(b), complete welding between the specimen and pad was achieved. The change in the microstructure by the spot welding may cause some difference in the crack nucleation behavior compared to that of unwelded specimen. Fig. 12 shows the microstructure of the welded part after adhesion mimic fatigue test. Since a spot welding was used to weld specimen and pad, the microstructure change is not significant. The fatigue cracks emanated from at the corner of the welded part. The photograph clearly indicates that the crack was created by the stress concentration at the corner rather than the effect of the microstructure. Therefore, it can be considered that the purpose of this experiment which is to understand the effect of hydrogen on the crack initiation under severe stress concentration is achieved.
Contact ressure 40MPa Direct current 5500A , 2.1msec Electrode Contact pad Specimen Contact pad Weld
Fatigue load amplitude, Fa (N)
R. Komoda et al. / Tribology International 76 (2014) 92–99
0.5 mm
Specimen
Fig. 11. Preparation of welded specimen and contact pad for adhesion mimic fatigue test. (a) Welding method and (b) Section of weld part.
100μm
Fatigue load amplitude, Fa (N)
Pad
Fatigue load amplitude, Fa (N)
Welding spot
97
pc Crack (MPa) initiated No crack 25 50 100
100
50
0
0
50 100 Contact pressure , pc (MPa)
pc Crack (MPa) initiated No crack 25 50 100
100
50
0
0
50 100 Contact pressure , pc (MPa)
pc Crack (MPa) initiated No crack 25 50 100
100
50
0
0
50 100 Contact pressure , pc (MPa)
Fig. 13. Result of the adhesion mimic fatigue test. (a) In air, (b) In 0.13 MPa hydrogen and (c) In 0.13 MPa oxygen hydrogen mixture (100 vol. ppm O2).
Crack 50μm Fig. 12. Microstructure of the welded part and fatigue crack (In H2, pc ¼ 50 MPa, F¼ 32N, N ¼ 105).
The crack nucleation limit corresponding to each contact pressure in the adhesion mimic fatigue test is shown in Fig. 13. For the same contact pressure, cracks emanated at a lower fatigue load in the hydrogen than in the air. This result clearly indicates that hydrogen plays a major role in the crack nucleation. The crack nucleation limit in the oxygen–hydrogen mixture is higher than that in the hydrogen. This indicated that the addition of oxygen mitigated the hydrogen-assisted crack nucleation. The mechanism that addition of oxygen increased crack initiation limit in the adhesion mimic fatigue test remains to be clarified. For one of the useful reference, Somerday et al. reported the effect of
oxygen addition on the fatigue crack growth behavior [35]. They determined the role of oxygen such that it prevented the hydrogen-assisted crack propagation. An elastic–plastic finite element analysis was performed in order to more clearly understand the result. The analysis of the adhesion mimic fatigue test is relatively more simple than that of the fretting test, because there is no contact and no wear. Therefore, the analysis was carried out under elastic–plastic plane strain condition. The minimum mesh size was 0.5 μm. The fatigue loading, which is divided into 40 steps, was applied for 16 cycles. Fig. 14 shows the crack nucleation in relation to the shear stress range in the vicinity of the crack nucleation site. The shear stress range was obtained in the critical plane where the range of the shear stress takes its greatest value during one cycle of fatigue loading. The result of the adhesion mimic fatigue test was evaluated as a straight line by the maximum range of shear stress regardless of the contact pressure, although the results of experiment shown by fatigue load varied depending on contact pressure as shown in Fig. 13. Therefore, it is presumed that the maximum
R. Komoda et al. / Tribology International 76 (2014) 92–99
Maximum range of shear stress , Δτmax (MPa)
Maximum range of shear stress , Δτmax (MPa)
Maximum range of shear stress , Δτmax (MPa)
98
addition were studied from the view point of the crack nucleation by fretting test and adhesion mimic test. The results are as follows:
1000 pc Crack (MPa) initiated No crack 25 50 100
800 600 400 200 0
0
20 40 60 80 Fatigue load amplitude, Fa (N)
100
1000 pc Crack (MPa) initiated No crack 25 50 100
800 600
(1) The fretting fatigue strength in hydrogen was increased by oxygen addition to the hydrogen. (2) It can be presumed that one of the causes of the increase in the fretting fatigue strength in the oxygen–hydrogen mixture was the mitigation of the adhesion by the generation of oxidized fretting wear particles. (3) Hydrogen participates in the crack nucleation during the fretting test. The reduction in the crack nucleation limit is another cause of the reduced fretting fatigue strength in hydrogen. (4) The addition of oxygen to the hydrogen increased the crack nucleation limit in the adhesion mimic fatigue test. This is one of the possible reasons for the increase in the fretting fatigue strength in the oxygen–hydrogen mixture.
400
Acknowledgments
200
This study was carried out with the support of AIR LIQUIDE, France, and AIR LIQUIDE JAPAN in the framework of the Air Liquide Industrial Chair on Hydrogen Structure Materials and Fracture at the Department of Mechanical Engineering of Kyushu University. This work was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan. The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.
0
0
20 40 60 80 Fatigue load amplitude, Fa (N)
100
1000 pc Crack (MPa) initiated No crack 25 50 100
800 600
References 400 200 0
0
20 40 60 80 Fatigue load amplitude, Fa (N)
100
Fig. 14. Evaluation of the result of the adhesion mimic fatigue test based on the maximum range of shear stress. (a) In air, (b) In 0.13 MPa hydrogen and (c) In 0.13 MPa oxygen hydrogen mixture (100 vol. ppm O2).
range of share stress is the dominant stress component in the crack initiation in the adhesion mimic fatigue test. The critical stress to crack nucleation is significantly lower in hydrogen than in air. It was confirmed that hydrogen assisted the crack nucleation in this experiment. This is one of the mechanisms other than stress concentration due to local adhesion that causes a reduction in the fretting fatigue strength in hydrogen. The critical stress to crack nucleation is higher in the oxygen–hydrogen mixture than in the hydrogen. Although challenges remain in achieving a quantitative understanding, this can be one of the causes of the increase in the fretting fatigue strength in hydrogen by the addition of oxygen and in air.
4. Conclusions The effect of oxygen addition as impurity in hydrogen on fretting fatigue strength of SUS304 austenitic stainless steel in hydrogen was characterized. Also the mechanisms of the reduced fretting fatigue strength in hydrogen and the effect of oxygen
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[28] Michler T, Naumann J, Sattler E. Influence of high pressure gaseous hydrogen on S–N fatigue in two austenitic stainless steels. Int J Fatigue 2013;51:1–7. [29] Sakamoto Y, Katayama H. The electrochemical determination of diffusivity and solubility of hydrogen in an austenitic type 304 steel. J Jpn Inst Metal Mater 1982;46(8):805–14. [30] Nagao A, Kuramoto S, Kanno M, Shiraga T. Visualization of hydrogen diffusion promoted by stress gradient and plastic deformation in steels. Iron Steel Inst Jpn 2000;86(1):24–31. [31] Horikawa K, Ando N, Kobayashi H, Urushihara W. Visualization of hydrogen gas evolution during deformation and fracture in SCM440 steel with different tempering conditions. Mater Sci Eng, A 2012;534:495–503. [32] Lewis FA, Kandasamy K, Tong XQ. Lattice strain gradient effects on hydrogen diffusion parameter calculations. Int J Hydrogen Energy 2002;27(6):687–94. [33] Kubota M, Shiraishi Y, Komoda R, Kondo Y, Furtado J. Consideration the mechanisms causing reduction of fretting fatigue strength by hydrogen. In: Proceedings of 19th European conference on fracture; 2012. paper ID: 281. [34] Legrand E, Bouhattate J, Feaugas X, Garmestani H. Computational analysis of geometrical factors affecting experimental data extracted from hydrogen permeation tests: II- consequences of trapping and an oxide later. Int J Hydrogen Energy 2012;37(18):13574–82. [35] Somerday BP, Sofronis P, Nibur KA, Marchi CS, Kirchheim R. Elucidating the variables affecting accelerated fatigue crack growth of steels in hydrogen gas with low oxygen concentration. Acta Mater. 2013;61(16):6153–70.
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of oxygen addition on fretting fatigue strength in hydrogen of JIS SUS304 stainless steel Ryosuke Komoda a,n, Masanobu Kubota b,c,d,e, Yoshiyuki Kondo b, Jader Furtado f a
Graduate School of Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan Department of Mechanical Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan c Air Liquide Industrial Chair on Hydrogen Structural Materials and Fracture, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan d International Institute of Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan e Research Center of Hydrogen Industrial Use and Storage (HYDROGENIUS), 744 Motooka, Fukuoka 819-0395, Japan f Air Liquide R&D Centre de Recherches Claude Delorme-Paris Saclay, 1 chemin de la porte des Loges, Les Loges-en-Josas Jouy-en-Josas 78354, France b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 June 2013 Received in revised form 23 January 2014 Accepted 28 February 2014 Available online 12 March 2014
Fretting fatigue test of SUS304 austenitic stainless steel was performed in air, in hydrogen gas, and in oxygen–hydrogen mixture. The fretting fatigue strength is more significantly reduced in hydrogen as compared to air. An increase in the fretting fatigue strength was found in the mixture. The mechanisms were investigated focusing on crack initiation. As the result, the crack initiation limit was significantly lower in hydrogen than in air, and increased in the mixture. The tangential force coefficient in the mixture is similar to that in air. The morphology of the fretting damage in the mixture was similar to that in air. These results indicated that the adhesion between contacting surfaces was prevented by addition of oxygen. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Fretting fatigue Hydrogen Oxygen Crack nucleation
1. Introduction Hydrogen is the most promising new energy carrier in the very near future. Hydrogen will contribute to a deployment of the use of renewable energy and to carbon dioxide emission reduction. However, hydrogen deteriorates the material strength not only in static loading but also in fatigue loading. The interaction of hydrogen and materials is under active investigation in order to achieve the safe use of hydrogen at low cost [1–9]. Fretting fatigue is one of the most important issues in the design of mechanical components involving contact, because fretting can cause a significant reduction in the fatigue strength of the contact part. In hydrogen equipment, the importance of fretting fatigue may be even higher, since hydrogen can influence both fatigue and fretting. The authors determined that the fretting fatigue strength of several kinds of materials in hydrogen is significantly lower than that in air [10–15]. One of the causes of the reduced fretting fatigue strength in hydrogen gas is the local adhesion between the contacting surfaces and subsequent crack nucleation [14]. Since no oxidation occurs in hydrogen and fretting removes the surface
oxide layer, the adhesion can be the first step in causing the fretting damage in hydrogen similar to the fretting in a vacuum [16] and other non-oxidative environments [17]. From a practical perspective of the use of hydrogen based on fuel cell technology, the effect of impurities on the fretting fatigue strength is also important. For example, PEM fuel cell needs humidification of hydrogen for proton conductivity. The objectives of this study are to characterize the effect of oxygen addition to hydrogen as an impurity on fretting fatigue properties, and to elucidate the mechanism. In this study, the effect of hydrogen and addition of oxygen on crack initiation were highlighted.
2. Experimental procedure 2.1. Material The test material was JIS SUS304 austenitic stainless steel. The chemical composition is shown in Table 1. The material was solution heat treated by heating at 1303 K for 3.9 ks followed by rapid cooling. The mechanical properties are shown in Table 2. 2.2. Fretting fatigue test using bridge pad
n
Corresponding author. Tel.: þ 819 280 232 29. E-mail addresses: [email protected] (R. Komoda), [email protected] (M. Kubota). http://dx.doi.org/10.1016/j.triboint.2014.02.025 0301-679X/& 2014 Elsevier Ltd. All rights reserved.
A fretting fatigue test using bridge pad was carried out in air, in hydrogen and in oxygen–hydrogen mixture. The oxygen–hydrogen
R. Komoda et al. / Tribology International 76 (2014) 92–99
mixture contained 100 vol. ppm oxygen. The pressure of the hydrogen and oxygen–hydrogen mixture was 0.2 MPa in absolute pressure. The detail of the fretting fatigue test method and the configurations of the specimen and contact pad can be seen in Ref. [13].
2.3. Fretting test to investigate the effect of hydrogen and oxygen addition on the nucleation of fretting fatigue cracks Fig. 1 shows the fretting test method used for determining the role of hydrogen in fretting fatigue crack nucleation. Fretting was induced by applying cyclic displacement to the specimen. To prevent the rotation of the contact pad, the height of the contact part of the contact pad was as short as possible and the supporting point of the pad was as close as possible to the specimen surface. The leaf spring supporting the contact pad is beneficial in order to prevent uneven contact and to precisely apply the contact load. The relative slip range between the specimen and contact pad was measured by a small displacement sensor as shown in Fig. 1. The measurement position of the relative slip range is a few millimeters away from the contact part. This might be a cause of the error in the measurement [18,19]. In this study, the error of the relative slip range between the measured value and true value was confirmed to be within 5%. The tangential force was measured by a load cell. The fretting test was carried out by controlling the tangential force as a constant. To do this, the amplitude of the cyclic displacement applied to the specimen was adjusted during the Table 1 Chemical composition (mass%). Material
C
Si
Mn
P
S
Ni
Cr
SUS304
0.05
0.53
0.91
0.034
0.002
8.10
18.19
Table 2 Mechanical properties and hardness. Material Condition
SUS304
0.2% proof stress
Ultimate tensile strength
237 MPa 771 MPa Solution heat treatment
Elongation at fracture
Reduction of area
Vickers hardness
72.3%
81.2%
HV174
93
testing. The test was interrupted at 105 cycles. Identification of a small crack was then carried out. The test frequency was 5 Hz. Fig. 2 shows the specimen and contact pad. The adhesion during the fretting fatigue test in hydrogen varies in size and randomly occurs. To obtain a high reproducibility of the adhesion, a small contact length, which is 0.2 mm, was used in this study. The contact surfaces of the specimen and pad were finished by chemical mechanical polishing using a silicon wafer polishing machine. The surface roughness parameter of the contact surface, Ra, was 0.053 μm. The material was solution heat-treated JIS SUS304 austenitic stainless steel, which is the same as that used in the fretting fatigue test. The test environments were hydrogen and laboratory air. The hydrogen pressure was 0.13 MPa in absolute pressure. 2.4. Finite element analysis of fretting test The local stress near the contact edge in the fretting test was evaluated by a finite element (FE) analysis. The model and the boundary conditions are shown in Fig. 3(a). A half of the fretting test specimen and contact pad was modeled by symmetry. The analysis was carried out by assuming an elastic body under plane strain condition. The friction model was a modified Coulomb friction provided by the solver, which was Marc 2010. The relative sliding velocity in the modified Coulomb friction model was set to a value equivalent to the relative slip velocity in the fretting test. Fig. 3(b) shows the analysis procedure. Initially, a certain friction coefficient, μ, is provided to the contacting surfaces and a certain displacement, d, is given to the specimen. The displacement applied to the specimen was divided into 20 steps and repeated by two cycles as shown Fig. 3(c) by considering convergence of the result. The tangential force coefficient, ϕ, and the relative slip range, ΔS, under the given conditions were then calculated. If the calculated ϕ and ΔS are different from the experimental values, the calculation is repeated after modifying μ and d. For the case in which an elastic rectangle is pressed into contact with an elastic half-space, the contact pressure reaches a theoretically infinite value at the contact edge [20]. Therefore, the result of the FEM calculation under elastic conditions is significantly influenced by the mesh size. In this study, the stress singularity parameters, which are proposed by Hattori [21], were used for the criterion providing the uniqueness of the solution. The adopted mesh size, which was 0.125 μm, was determined when the stress singularity parameters converged to constant values with decreases in the mesh size.
Cyclic displacement Cyclic displacement Driving shaft Slip sensor
Driving shaft Specimen Specimen Bar spring
Slip sensor
Contact load Tightening bolt Leaf spring
Contact load Contact pad
Contact pad Load cell
Leaf spring
Fig. 1. Fretting test method.
6
35
0.2
SUS304 R = -1 f = 20 Hz pc = 100MPa
250
200
150
In air In 0.2MPa H2 In 0.2MPa oxygen-hydrogen mixture (100 vol ppm O2)
0.3
4
Stress amplitude, σa (MPa)
R. Komoda et al. / Tribology International 76 (2014) 92–99
6
94
6
100
105 106 107 Number of cycles to failure, Nf
6 Fig. 2. Configurations of specimen (a) and contact pad (b).
Fig. 4. Effect of oxygen addition on fretting fatigue strength of SUS304 in hydrogen.
Contact part Friction coefficient, μ
Contact pad
Cyclic displacement, d Specimen
Start
FE analysis at initial
Tangential force coefficient,φ
0.6 Contact pressure
SUS304 Solution heat-treated R = -1 , f = 20 Hz
0.5
0.4
0.3
0.2 100
In air In 0.2MPa H2 In 0.2MPa oxygen-hydrogen mixture (100 vol ppm O2)
200 Stress amplitude, σa (MPa)
300
Fig. 5. Effect of oxygen addition on tangential force coefficient in fretting fatigue test in hydrogen and in air.
Adjust μ, d
No
φ, ΔS Analysis = Measured?
Yes
Dislacement, d
End FE analvsis
Time Fig. 3. Procedure for the finite element (FE) analysis of the fretting test. (a) Model for finite element analysis of fretting test, (b) Flow and (c) Displacement applied to the specimen.
3. Result and discussion 3.1. Effect of oxygen addition on the fretting fatigue strength in hydrogen The result of the fretting fatigue test is shown in Fig. 4. The fretting fatigue strength in hydrogen was significantly lower than
that in air. This is the same trend with that in our previous studies. The fretting fatigue strength in the oxygen–hydrogen mixture was higher than that in the hydrogen and lower than that in air. The addition of oxygen partially mitigated the reduction in the fretting fatigue strength in hydrogen. Fig. 5 shows the tangential force coefficient as a function of the stress amplitude. The tangential force coefficient in the oxygen– hydrogen mixture was lower than that in the hydrogen and slightly higher than that in air. Since the increase in the tangential force coefficient in the hydrogen was caused by the adhesion between the contacting surfaces because of absence of oxygen [13,14], the one of the possible reasons for the reduced tangential force coefficient in the oxygen–hydrogen mixture is that oxidized fretting wear particles reduces adhesion. Fig. 6 shows the observation of the contact surfaces after the fretting fatigue test in each environment. There were no oxidized fretting wear particles on the fretted surface in hydrogen. On the other hand, the oxidized fretting wear particles were observed on the fretted surface in the oxygen–hydrogen mixture. The amount of oxidized fretting wear particles in the oxygen–hydrogen mixture was smaller than that in air. The morphology of the fretting fatigue damage, which is along perpendicular to the fretting motion, was clearly different from that in air. It was elucidated that the fretting damage in hydrogen is caused by local adhesion between contacting surfaces [14]. The morphology of the fretting damage in the oxygen–hydrogen mixture was similar to that in air. It can be considered that there are multiple reasons for the reduction in tangential force coefficient such as tribo-chemical reaction, surface roughness, etc. Based on the observation shown
R. Komoda et al. / Tribology International 76 (2014) 92–99
95
Fretting
Fretting
Fretting
100μm
100μm
100μm
Fig. 6. Observation of fretted surfaces. (a) In 0.2 MPa hydrogen (sa ¼ 205 MPa, Nf ¼3.2 105), (b) In 0.2 MPa hydrogen oxygen mixture (100 vol. ppm O2) (sa ¼193 MPa, Nf ¼ 3.3 105) and (C) In air (sa ¼206 MPa, Nf ¼4.2 105).
100μm
100μm
Fig. 7. Observation of fretted surface during the fretting test (pc ¼100 MPa, ϕ¼ 0.7, N ¼105). (a) In air (ΔS¼ 7.56 μm) (b) In H2 (ΔS ¼4.89 μm).
in Fig. 6, the production of oxidized fretting wear particles and prevention of local adhesion were one of possible reasons for the reduced tangential force in the oxygen–hydrogen mixture. 3.2. Fretting test assessing the effect of hydrogen on crack initiation Fig. 7 shows the fretted surface of the specimen in air and in hydrogen. The contact surface in air was covered by an oxide. On the other hand, there were no fretting wear particles on the contact surface in the hydrogen. The area where the fretting damage occurred was almost equivalent to the geometric contact area in both environments. A high reproducibility of the fretting damage was achieved. Fig. 8 shows the relationship between the tangential force coefficient and the relative slip range. The relative slip range is represented by the average value after 104 cycles of fretting. The general trend that the tangential force coefficient increases with the increase in the relative slip range is consistent with other fretting fatigue studies [22,23]. The tangential force coefficient corresponding to a certain value of the relative slip range is higher in the hydrogen than in air. In Fig. 5, the tangential force coefficient is shown as a function of the stress amplitude. Since the increase in the stress amplitude increases the relative slip range, the trend in Fig. 8 is similar to that in Fig. 5. Fig. 9 show the section of the specimen fretted in hydrogen cut along the axial direction. Small cracks, which are approximately 10 μm in length, were observed at the contact edge. Table 3 shows the result of the identification of the small cracks. For the same given loading conditions (tangential force coefficient and contact pressure), the small cracks emanated in hydrogen, whereas there were no cracks in air. This indicates that hydrogen assists with the crack nucleation during fretting. Fig. 10 shows the distributions of the maximum shear stress near the contact edge on the specimen surface in a particular relative slip range for each environment at the contact pressure of 50 MPa with the tangential force coefficient of 0.5 obtained by FE analysis. As the result of the calculation, the friction coefficient to realize the given conditions is higher for the hydrogen environment than for air. The
displacement applied to the specimen was lower in the hydrogen environment than in the air. These results correspond to the fact that adhesion occurs in the hydrogen. As shown in the graph, the distributions of the maximum shear stress corresponding to each environment agreed with each other. This indicates that mechanical stress at the contact part is similar between in hydrogen and in air if the same pc and ϕ are applied to the fretting test in both environments, whereas crack nucleation occurred in the hydrogen but did not occur in air in the fretting test under these conditions. Therefore, this may suggest that hydrogen participates in the facilitation of the crack nucleation. As shown in Fig. 10, a very high stress is generated at the contact edge. Since the FE analysis was conducted under elastic conditions, there is segregation from the real situation that relaxation of the stress occurs by local plastic deformation [24] and fretting wear [25,26]. Even so, the fact still remains that a high stress is generated at the contact edge. This may allow for the fact that the crack nucleation during the fretting test is regarded as a low cycle fatigue phenomenon. During the low cycle fatigue of the same kind of material, hydrogen assists with the crack nucleation [27,28]. Considering the hydrogen diffusion coefficient in an austenitic material, which is very low [29], and the low pressure of the hydrogen in this study, it may raise doubt about hydrogen and the material interaction. However, the transport of hydrogen by mobile dislocations [30,31], the attraction of hydrogen to a region with a higher strain [32], the microstructure transformation from austenite to martensite due to fretting [33] (the diffusion coefficient of hydrogen in martensite is very high compared to that in austenite [29]), the vulnerability of martensite to hydrogen, and removal of the surface oxide due to fretting (the oxide layer prevents hydrogen permeation [34]) are possible causes that hydrogen assisted crack nucleation during fretting. 3.3. Adhesion mimic fatigue test It was described that hydrogen participates in the crack nucleation during the fretting through the fretting test and FE
R. Komoda et al. / Tribology International 76 (2014) 92–99
Tangential force coefficient , φ
Table 3 Result of identification of small crack (N ¼105 cycles).
0.8
Tangential force coefficient, ϕ
0.6 SUS304 Fretting f = 5 Hz In 0.13MPa H2 In H2 In air
0.4 0.2 0
0
2 4 6 Relative slip range , ΔS (μm)
8
100
(a) In air 0.3 0.5 0.7
Not found Not found Not found
Not found Not found Not found
Not found Not found Not found
(b) In 0.12 MPa H2 0.3 0.5 0.7
Not found Not found Found
Not found Not found Found
Found Found Found
600
0.6 SUS304 Fretting f = 5 Hz In 0.13MPa H2 In H2 In air
0.4 0.2
0
2 4 6 Relative slip range , ΔS (μm)
8
0.6
In air ( d = 2.68 μm , μ = 0.8 , C = 0.032 mm/s ) In H2 ( d = 1.83 μm , μ = 1.2 , C = 0.045 mm/s )
400 300
Contact pad
200 Specimen
100
0
x
5 10 Distance from edge of contact surface , x (μm)
15
Fig. 10. Distribution of maximum shear stress in the vicinity of the contact edge of the specimen.
SUS304 Fretting f = 5 Hz In 0.13MPa H2 In H2 In air
0.4 0.2 0
pc = 50 MPa , φ = 0.5
500
0
0.8
0
2 4 6 Relative slip range , ΔS (μm)
8
Fig. 8. Relationship between tangential force coefficient and relative slip range in hydrogen and in air. (a) Contact pressure, pc ¼ 25 MPa, (b) Contact pressure, pc ¼ 50 MPa and (c) Contact pressure, pc ¼100 MPa.
Edge of contact surface Contact surface Contact surface
10μm
50
0.8
0
Contact pressure, pc (MPa) 25
Maximum shear stress , τmax (MPa)
Tangential force coefficient , φ
Tangential force coefficient , φ
96
Crack
Cutting Observation
Fig. 9. An example of fretting crack (In H2, pc ¼50 MPa, ϕ ¼0.7, N¼ 105).
analysis. However, the oxide particles were not considered into the FE analysis. Furthermore, the adhesion was not correctly modeled. These were imposed on the friction coefficient. Therefore, in order to more clearly consider the role of the hydrogen in the crack nucleation during the fretting in hydrogen, an additional test was carried out. A spot-welded nugget was attempted to emulate the adhered spot during the fretting in hydrogen for an evaluation. Thus, this
test is called the adhesion mimic fatigue test. The test method is basically the same as that used in the fretting test except for welded specimen and contact pad. Fig. 11(a) shows the preparation of the specimen. After assembling the specimen and contact pads, they were welded using a spot welding machine. Since a 0.2 mm contact length used in the fretting test was too short to obtain the same quality weld, the contact length was increased to 1 mm. The spot welding was carried out at a welding current of 5500 A for 2.1 ms at a contact pressure of 40 MPa. A compressive load, which corresponds to the contact load during the fretting test, was applied in order to produce similar stress conditions in the fretting test in hydrogen. The adhesion mimic fatigue test was carried out in three environments, which are air, 0.13 MPa hydrogen and 0.13 MPa oxygen–hydrogen mixture (100 vol. ppm O2), with the stress ratio, R, of 1 at the loading frequency, f, of 5 Hz. The fatigue test was interrupted at 105 cycles. Identification of a small crack was then carried out. As shown in Fig. 11(b), complete welding between the specimen and pad was achieved. The change in the microstructure by the spot welding may cause some difference in the crack nucleation behavior compared to that of unwelded specimen. Fig. 12 shows the microstructure of the welded part after adhesion mimic fatigue test. Since a spot welding was used to weld specimen and pad, the microstructure change is not significant. The fatigue cracks emanated from at the corner of the welded part. The photograph clearly indicates that the crack was created by the stress concentration at the corner rather than the effect of the microstructure. Therefore, it can be considered that the purpose of this experiment which is to understand the effect of hydrogen on the crack initiation under severe stress concentration is achieved.
Contact ressure 40MPa Direct current 5500A , 2.1msec Electrode Contact pad Specimen Contact pad Weld
Fatigue load amplitude, Fa (N)
R. Komoda et al. / Tribology International 76 (2014) 92–99
0.5 mm
Specimen
Fig. 11. Preparation of welded specimen and contact pad for adhesion mimic fatigue test. (a) Welding method and (b) Section of weld part.
100μm
Fatigue load amplitude, Fa (N)
Pad
Fatigue load amplitude, Fa (N)
Welding spot
97
pc Crack (MPa) initiated No crack 25 50 100
100
50
0
0
50 100 Contact pressure , pc (MPa)
pc Crack (MPa) initiated No crack 25 50 100
100
50
0
0
50 100 Contact pressure , pc (MPa)
pc Crack (MPa) initiated No crack 25 50 100
100
50
0
0
50 100 Contact pressure , pc (MPa)
Fig. 13. Result of the adhesion mimic fatigue test. (a) In air, (b) In 0.13 MPa hydrogen and (c) In 0.13 MPa oxygen hydrogen mixture (100 vol. ppm O2).
Crack 50μm Fig. 12. Microstructure of the welded part and fatigue crack (In H2, pc ¼ 50 MPa, F¼ 32N, N ¼ 105).
The crack nucleation limit corresponding to each contact pressure in the adhesion mimic fatigue test is shown in Fig. 13. For the same contact pressure, cracks emanated at a lower fatigue load in the hydrogen than in the air. This result clearly indicates that hydrogen plays a major role in the crack nucleation. The crack nucleation limit in the oxygen–hydrogen mixture is higher than that in the hydrogen. This indicated that the addition of oxygen mitigated the hydrogen-assisted crack nucleation. The mechanism that addition of oxygen increased crack initiation limit in the adhesion mimic fatigue test remains to be clarified. For one of the useful reference, Somerday et al. reported the effect of
oxygen addition on the fatigue crack growth behavior [35]. They determined the role of oxygen such that it prevented the hydrogen-assisted crack propagation. An elastic–plastic finite element analysis was performed in order to more clearly understand the result. The analysis of the adhesion mimic fatigue test is relatively more simple than that of the fretting test, because there is no contact and no wear. Therefore, the analysis was carried out under elastic–plastic plane strain condition. The minimum mesh size was 0.5 μm. The fatigue loading, which is divided into 40 steps, was applied for 16 cycles. Fig. 14 shows the crack nucleation in relation to the shear stress range in the vicinity of the crack nucleation site. The shear stress range was obtained in the critical plane where the range of the shear stress takes its greatest value during one cycle of fatigue loading. The result of the adhesion mimic fatigue test was evaluated as a straight line by the maximum range of shear stress regardless of the contact pressure, although the results of experiment shown by fatigue load varied depending on contact pressure as shown in Fig. 13. Therefore, it is presumed that the maximum
R. Komoda et al. / Tribology International 76 (2014) 92–99
Maximum range of shear stress , Δτmax (MPa)
Maximum range of shear stress , Δτmax (MPa)
Maximum range of shear stress , Δτmax (MPa)
98
addition were studied from the view point of the crack nucleation by fretting test and adhesion mimic test. The results are as follows:
1000 pc Crack (MPa) initiated No crack 25 50 100
800 600 400 200 0
0
20 40 60 80 Fatigue load amplitude, Fa (N)
100
1000 pc Crack (MPa) initiated No crack 25 50 100
800 600
(1) The fretting fatigue strength in hydrogen was increased by oxygen addition to the hydrogen. (2) It can be presumed that one of the causes of the increase in the fretting fatigue strength in the oxygen–hydrogen mixture was the mitigation of the adhesion by the generation of oxidized fretting wear particles. (3) Hydrogen participates in the crack nucleation during the fretting test. The reduction in the crack nucleation limit is another cause of the reduced fretting fatigue strength in hydrogen. (4) The addition of oxygen to the hydrogen increased the crack nucleation limit in the adhesion mimic fatigue test. This is one of the possible reasons for the increase in the fretting fatigue strength in the oxygen–hydrogen mixture.
400
Acknowledgments
200
This study was carried out with the support of AIR LIQUIDE, France, and AIR LIQUIDE JAPAN in the framework of the Air Liquide Industrial Chair on Hydrogen Structure Materials and Fracture at the Department of Mechanical Engineering of Kyushu University. This work was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan. The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.
0
0
20 40 60 80 Fatigue load amplitude, Fa (N)
100
1000 pc Crack (MPa) initiated No crack 25 50 100
800 600
References 400 200 0
0
20 40 60 80 Fatigue load amplitude, Fa (N)
100
Fig. 14. Evaluation of the result of the adhesion mimic fatigue test based on the maximum range of shear stress. (a) In air, (b) In 0.13 MPa hydrogen and (c) In 0.13 MPa oxygen hydrogen mixture (100 vol. ppm O2).
range of share stress is the dominant stress component in the crack initiation in the adhesion mimic fatigue test. The critical stress to crack nucleation is significantly lower in hydrogen than in air. It was confirmed that hydrogen assisted the crack nucleation in this experiment. This is one of the mechanisms other than stress concentration due to local adhesion that causes a reduction in the fretting fatigue strength in hydrogen. The critical stress to crack nucleation is higher in the oxygen–hydrogen mixture than in the hydrogen. Although challenges remain in achieving a quantitative understanding, this can be one of the causes of the increase in the fretting fatigue strength in hydrogen by the addition of oxygen and in air.
4. Conclusions The effect of oxygen addition as impurity in hydrogen on fretting fatigue strength of SUS304 austenitic stainless steel in hydrogen was characterized. Also the mechanisms of the reduced fretting fatigue strength in hydrogen and the effect of oxygen
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