Criteria for determining hydrogen compatibility and the mechanisms for hydrogen-assisted, surface crack growth in austenitic stainless steels
Introduction
Fuel cell vehicles (FCVs) have just been commercialized in Japan, and constructions of hydrogen fueling stations have also been promoted. For safety use of such systems, it is necessary to properly control the strength degradation of mechanical components used in hydrogen environment and to perform strength design of the components in consideration for the detrimental effect of hydrogen [1], [2].
In the material selection for metallic components used in high-pressure hydrogen gas, relative reduction in area (RRA) has often been used as a criterion for characterizing the hydrogen embrittlement (HE) [3], [4], [5], [6], [7], [8], [9], [10]. The RRA is obtained via a slow strain rate tensile (SSRT) test, which defines the ratio of a reduction in area (RA) in hydrogen gas, φH, to a RA in inert gas, φ, i.e., RRA = φH/φ. Yamada and Kobayashi [8] proposed the following relational expression for qualifying 300-series austenitic stainless steels (e.g., Types 316 and 316L) for their use in hydrogen gas:
In most of the materials used in the analysis by Yamada and Kobayashi, the RA values under air exceeded 75% [3]. On the other hand, according to Japanese Industrial Standards (JIS G 4303), the minimum requirement for RA is 60% for 300-series stainless steels [8]. Based on the two preceding facts, Yamada and Kobayashi required the 300-series stainless steels to satisfy RRA ⩾ 0.8 (=60/75), i.e., minor hydrogen embrittlement was tolerated for the materials.
Moreover, using existing literature, Yamada and Kobayashi [8] analysed a series of RRA data for Types 304, 316 and 316L, showing that a relationship between the RRA and the nickel equivalent content (Nieq) in 70-MPa hydrogen gas at −40 °C was successfully fitted by the following equation:where A = 60, B = 40, C = 1.339 and D = 27.76. The Nieq [mass%] is calculated by [11]:where the unit of the elements is mass%. From Eq. (2), it is calculated that RRA ⩾ 0.8 is equivalent to Nieq ⩾ 28.5 mass%.
On the basis of the analyses described by Eqs. (1), (2), (3), Yamada and Kobayashi [8] suggested that 300-series austenitic stainless steels, satisfying both φ ⩾ 75% in inert gas and Nieq ⩾ 28.5 mass%, are eligible for use in hydrogen gas up to 70 MPa, at temperatures ranging from −40 °C to 85 °C. For the infinite fatigue life design of components, it is noted that fatigue limit should not be degraded in hydrogen gas, in addition to the above requirement based on the RRA.
In order to improve the economic efficiency of the 70-MPa hydrogen station, lower-cost, austenitic stainless steels with less austenitic stability, as well as low-alloy and carbon steels, are greatly sought for use by industry. However, it has been reported that these steels do not satisfy RRA ⩾ 0.8 [3], [10]. Consequently, Matsunaga et al. [12] proposed the concept of no degradation in tensile strength (TS) by hydrogen as another criterion for determining the hydrogen compatibility of steels for design by rule (safety factor (SF) = 4). No TS degradation means that a specimen will fail during the necking process after reaching TS. In the strength design of high-pressure gas components, the SF is defined as follows:where σB is the TS of the material and σallowable is the allowable design stress. As a further requirement for determining hydrogen compatibility for the infinite fatigue life design of hydrogen components, Matsunaga et al. [12] also advanced the theory of no degradation in the fatigue limit under the maximum design pressure of hydrogen gas. Their proposal was essentially identical to that introduced by Yamada and Kobayashi [8].
To validate these criteria, Matsunaga et al. [12] performed SSRT tests on the low-alloy steel, JIS-SCM435, with a TS of 823 MPa, as well as on the carbon steel, JIS-SM490B, with a TS of 537 MPa, in 115-MPa hydrogen gas at −40 °C, room temperature (RT) and 120 °C. Results of the SSRT testing of JIS-SCM435 indicated an absence of degradation in TS in hydrogen gas at each test temperature. A considerable loss of ductility was, however, noted, with RRA values of 0.51, 0.64 and 0.65, at −40 °C, RT and 120 °C, respectively. Accordingly, the material did not satisfy the criterion, RRA ⩾ 0.8. On the other hand, in JIS-SM490B, TS degradation occurred in 115-MPa hydrogen gas at −40 °C, but no TS degradation was observed at either RT or at 120 °C. The RRA values were 0.30, 0.40 and 0.42, at −40 °C, RT and 120 °C, respectively. In addition to SSRT tests, Matsunaga et al. [12] conducted fatigue life tests on JIS-SCM435 and JIS-SM490B in 115-MPa hydrogen at RT. In both the steels, the results revealed an absence of hydrogen-induced, fatigue strength degradation in the long-life regime, where the number of cycles exceeded ≈105. On the basis of these results, having exhibited no degradation in either TS or fatigue limit, JIS-SCM435 was deemed eligible for use in the infinite fatigue life design of components according to design by rule, as well as in hydrogen gas at pressures of ⩽115 MPa and at temperatures ranging from −40 °C to 120 °C [12].
Furthermore, Yamabe et al. [13] proposed that hydrogen components could be designed following the finite fatigue life design based on design by analysis (SF = 2.4) [14], [15], [16], depending on the existence of an upper bound in fatigue crack growth (FCG) acceleration under the maximum design pressure of hydrogen gas.
It has been reported that, during the SSRT testing in hydrogen gas, the degradation in the RRA was caused by hydrogen-assisted surface crack growth (HASCG) [7], [10]. In the tests performed in inert gas (e.g., air and nitrogen gas) on low- or medium-strength steels (e.g., austenitic stainless, low-alloy and carbon steels), the specimens exhibited ductile, cup-and-cone fractures and the fracture surfaces were covered with dimples [7], [9], [12]. Even in hydrogen gas, austenitic stainless steels with high austenitic stability also exhibited ductile, cup-and-cone fractures and, as a consequence, the RRA ≈ 1 [7], [10]. By contrast, the steels with low austenitic stability, as well as low-alloy and carbon steels, did not exhibit clear, ductile, cup-and-cone fractures and as a result, the RRA was smaller than 1. The fracture surfaces were covered with a mixture of quasi-cleavages and dimples, while a number of cracks remained on the specimen surfaces. Owing to the fracture morphologies of these steels in hydrogen gas, it is presumed that the HASCG during the SSRT process caused the quasi-cleavages, resulting in a small RRA value [7], [10], [12]. However, the HASCG mechanism remains to be elucidated.
In this study, SSRT, elasto-plastic fracture toughness (JIC), fatigue crack growth (FCG) and fatigue life tests were carried out on Types 304, 316, 316L and 316 (over 12 mass% nickel) steels, in hydrogen gas at pressures ranging from 78 to 115 MPa and temperatures spanning −40 °C to RT. Steel Type 316 (over 12 mass% nickel) is hereinafter referred to as Type 316 (hi-Ni). Firstly, the results of the SSRT test were investigated from the viewpoint of no TS degradation in high-pressure hydrogen gas [12], later proposed as the first criterion for authorizing the hydrogen compatibility of Types 316L and 316 (hi-Ni) steels with RRA < 0.8, at pressures of ⩽115 MPa and at temperatures from −40 °C to RT. It was subsequently revealed that, in hydrogen gas, the surface cracks during the SSRT process grew by the same mechanism as the through cracks in the JIC and FCG tests. A second criterion was then introduced for the determination of the hydrogen compatibility of steels with lower austenitic stability, i.e., Types 304 and 316, and their fitness for use in hydrogen gas at pressures lower than 115 MPa and at temperatures ranging from −40 °C to RT.
Section snippets
Nominal stress–strain curve and fracture morphology
Table 1, Table 2 document the chemical composition and mechanical properties of Types 304, 316, 316L and 316 (hi-Ni) steels. The Nieq values calculated by Eq. (3) are also listed in Table 1.
As presented in Fig. 1, a smooth, round-bar specimen with a diameter of 6 mm and a gauge length of 30 mm was used for the SSRT test, performed in hydrogen gas at pressures ranging from 78 MPa to 115 MPa and at temperatures from −40 °C to RT, in accordance with ASTM G142 [17]. The cross-head speed was 0.002 mm/s
Criterion for ensuring no TS degradation in hydrogen gas: φH ⩾ 57% or RRA ⩾ 0.68
As mentioned previously, the absence of TS degradation in hydrogen gas can be a foundation for prescribing the hydrogen compatibility of steels used under design by rule. Based on the results of the SSRT test, φH ⩾ 57% or RRA ⩾ 0.68 has been proposed as the criterion for ensuring no TS degradation in hydrogen gas. According to this criterion, with appropriate design considerations, Types 316L and 316 (hi-Ni) steels may be used in hydrogen gas at pressures below 115 MPa for temperatures ranging from
Hydrogen-enhanced successive fatigue crack-growth (HESFCG) model
Matsumoto et al. [19] performed a JIC test on a CT specimen with a fatigue pre-crack (JIS-SM490B) in 0.7-MPa hydrogen gas. These JIC test results were then compared with those obtained from the FCG test in 0.7-MPa hydrogen gas [20], [21], revealing that the crack growth during the JIC test (JIC crack growth) was equivalent to that of the FCG per one cycle in hydrogen gas. The investigation conducted by Matsumoto et al. is based on the hydrogen-enhanced successive fatigue crack-growth (HESFCG)
Fatigue life testing of a smooth, round-bar specimen
To obtain the evidence that the SSRT crack also grows via the same mechanism (HESFCG) as the JIC and FCG cracks, fatigue life tests of a smooth, round-bar specimen (cf. Fig. 26) were performed at R = −1 and f = 1 Hz in air and in 115-MPa hydrogen gas at RT. The S–N data are presented in Fig. 27. In the tests in 115-MPa hydrogen gas, all of the specimens failed at fatigue life, Nf ≈ 104. It is well-known that, in such a low-cycle regime, fatigue cracks initiate immediately after the fatigue test starts
Criterion for determining HE, based on the mechanism for SSRT surface crack growth: δH ⩾ 10% or φH ⩾ 10%
From results obtained in the preceding sections, it can be concluded that the SSRT surface crack grows via the same mechanism as growth during the JIC and FCG tests, i.e., all crack growth can be uniformly explained on the basis of the HESFCG model. This model considers slip localization by hydrogen near the crack tip, leading to minor crack blunting and successive crack growth. The localization of slip deformations, the key factor in the HESFCG regime, is related to the HELP model [27], which
Conclusions
In order to investigate the criteria for determining the HE of austenitic stainless steels (Types 304, 316, 316L and 316 (hi-Ni)) and to elucidate the mechanism for hydrogen-assisted, surface crack growth (HASCG), SSRT, JIC, FCG and fatigue life tests of the respective steels were performed in hydrogen gas, at pressures spanning 78 MPa to 115 MPa and at temperatures ranging from −40 °C to RT. The conclusions can be summarized as follows:
- (1)
In cases where strength design with a safety factor
Acknowledgements
This work was partially supported by the New Energy and Industrial Technology Development Organisation (NEDO), Hydrogen Utilisation Technology (2013–2018).
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