Development of a stable high-aluminum austenitic stainless steel for hydrogen applications

https://doi.org/10.1016/j.ijhydene.2013.02.127Get rights and content

Highlights

  • Novel austenitic steel with high resistance to hydrogen environment embrittlement.

  • Slow strain rate tensile testing in 40 MPa hydrogen gas and air (0.1 MPa) at −50 °C.

  • Yield strength, tensile strength and elongation to rupture not affected by hydrogen.

  • Extremely high stability against strain-induced α′ martensite formation.

  • Cost reduction due to the absence of Mo and the reduction in Ni content.

Abstract

A novel high-aluminum austenitic stainless steel has been produced in the laboratory with the aim of developing a lean-alloyed material with a high resistance to hydrogen environment embrittlement. The susceptibility to hydrogen environment embrittlement was evaluated by means of tensile tests at a slow strain rate in pure hydrogen gas at a pressure of 40 MPa and a temperature of −50 °C. Under these conditions, the yield strength, tensile strength and elongation to rupture are not affected by hydrogen in comparison to companion tests carried out in air. Moreover, a very high ductility in hydrogen is evidenced by a reduction of area of 70% in the high-pressure and low-temperature hydrogen environment. The lean degree of alloying is reflected in the molybdenum-free character of the material and a nickel content of 8.0 wt.%. With regard to the alloy concept, a combination of high-carbon, high-manganese, and high-aluminum contents confer an extremely high stability against the formation of strain-induced martensite. This aspect was investigated by means of in-situ magnetic measurements and ex-situ X-ray diffraction. The overall performance of the novel alloy was compared with two reference materials, 304L and 316L austenitic stainless steels, both industrially produced. Its capability of maintaining a fully austenitic structure during tensile testing has been identified as a key aspect to avoid hydrogen environment embrittlement.

Introduction

The use of hydrogen for power generation in mobile and stationary applications is generally considered to be a very promising alternative among renewable and carbon-free energy sources. Nevertheless, the development of hydrogen energy is limited by the high costs associated with the few materials that guarantee safe handling of hydrogen. In this regard, most metallic materials suffer deterioration of their mechanical properties and ductility on coming into contact with any hydrogen source. This phenomenon is known as hydrogen embrittlement [1]. Therefore, current hydrogen applications make use of high-alloyed austenitic stainless steels, such as AISI type 316 and 310, which show a high resistance to hydrogen embrittlement [2], [3], [4], [5], [6], [7]. However, these alloys represent an expensive solution due to their high nickel and molybdenum contents. As a consequence, new steels with equivalent properties but lower associated costs are needed. Such materials could support the sustainable development of hydrogen energy on a global scale, which will demand a huge infrastructure for hydrogen production, storage, distribution, and end-use applications.

For decades, the use of austenitic stainless steels in hydrogen applications has gained major attention due to their higher performance compared to other metallic materials [8], [9], [10]. More specifically, if the susceptibility to hydrogen environment embrittlement (HEE) is evaluated, both former and present literature agree on the fact that stable austenitic stainless steels show higher resistance to HEE [2], [3], [5]. In this context, the term “stable” refers to the property of avoiding the formation of α′ and ɛ-martensite under the applied strain at a given temperature. Whereas ɛ-martensite is assumed to play a minor role in HEE [11], [12], the formation of α′-martensite is always accompanied by detrimental effects. In particular, higher ductility losses are encountered for higher tendencies to undergo strain-induced α′-martensite transformation [2], [3], [13], [4], [5], [14], [15]. The latter is frequently associated with the higher diffusivity of hydrogen in the bcc lattice and its enhanced transport through the fast diffusion paths represented by the transformed structure [16], [17]. Several investigations have focused on increasing the stability of austenitic stainless steels by modifying the content of interstitial and substitutional elements [18], [19], [6], [20]. These studies have revealed that increasing the austenite stability has a beneficial impact on the ductility response of the material in a hydrogen environment. This can be interpreted not only on the direction of mitigating the formation of strain-induced α′-martensite, but also on the relationship between austenite stability and the corresponding stacking fault energy (SFE) of the material. Specifically, an increasing SFE will favor more homogenous deformation by means of cross-slip instead of the planar slip mechanism that is promoted by low SFE values [21], [22], [23], [19].

The need to combine cost reduction with a high resistance to HEE has motivated the identification of a minimum-required nickel content, specially, in modified AISI type 316 steel. The minimum value has been found between 11.5 and 13 wt.%, depending on the testing conditions [19], [24], [25], [20], which is still too high concerning cost-efficiency. Beyond this strategy of minimizing the nickel content in AISI type 316 austenitic stainless steel, not many contributions are encountered in the literature concerning lean-alloyed and HEE-resistant steels. One of the earliest contributions in this context corresponds to the work published by Louthan and Caskey in 1976 [8]. They proposed a 21Cr-6Ni-9Mn stainless steel, commercially known as Nitronic 40, as a possible candidate for hydrogen applications. This material represents a very interesting case of a low-nickel austenitic stainless steel that does not form strain-induced α′-martensite at room temperature [26]. As shown in Ref. [8], the tensile properties of the 21-6-9 steel are equivalent after testing in 69 MPa hydrogen and helium gas at room temperature. However, the same work reports a reduction in ductility response of about 50% after tensile tests of thermally precharged specimens in air at room temperature. A more detailed characterization of this alloy was published by West and Louthan in 1982 [27], in which nineteen different fabrication routes were investigated in the thermally precharged and uncharged condition by means of tensile testing at a strain rate of 5.4·10−4 s−1 at room temperature. Tensile tests of the uncharged specimens in the annealed condition in 120 MPa hydrogen led to a loss of ductility of around 30% according to values of the reduction of area. The authors concluded “this austenitic stainless steel is susceptible to hydrogen-induced cracking at grain boundaries, slip bands, and other interfaces” [27]. A more recent investigation on elastic–plastic fracture mechanics of 21-6-9 performed by Nibur et al. shows a significant reduction in the fracture initiation toughness and crack-growth resistance of thermally precharged specimens [28]. As discussed in detail by the authors, a high concentration of hydrogen can modify the fracture mechanisms in the 21-6-9 stainless steels. Moreover, taking into account the absence of the strain-induced martensitic transformation, the hydrogen-assisted fracture can be interpreted by means of deformation mechanisms that promote localization of deformation [29], [28].

Another candidate alloy for use in hydrogen applications is the 22Cr-13Ni-5Mn steel, also introduced by Louthan and Caskey in Ref. [8]. This material exhibits very attractive properties because it combines a high stability against strain-induced martensite formation, a high strength due to nitrogen addition, and a high fracture toughness in the hydrogen precharged condition [30]. The only drawback of this alloy for hydrogen applications could be cost restrictions driven by the relatively high nickel content. Continuing with the idea of combining a stable austenitic matrix, high strength, and high ductility, Cr–Mn–N austenitic steels have also been evaluated with regard to their susceptibility to HEE. In this case, the replacement of nickel by manganese and nitrogen addition proved to be unsuccessful in slow strain rate tensile tests carried out in a 10 MPa hydrogen atmosphere at −50 °C [31]. Specifically, the ductility response of the material was severely reduced, despite the negligible fraction of material that is transformed into strain-induced α′-martensite. The brittle behavior was mainly attributed to the role of nitrogen in promoting short-range ordering and therefore a higher degree of planar slip during deformation [31], [32], [33], [34].

The goal of this study was to design a lean-alloyed and HEE-resistant austenitic stainless steel as a potential candidate for high-pressure hydrogen applications at both room temperature and subzero temperatures. The novel alloy was empirically developed and qualified by means of slow strain rate tensile tests in high-pressure hydrogen gas. Being aware of the dependency of HEE-susceptibility on the temperature and strain rate [13], [35], [36], [5], [37], [38], [25], [39], the developed material and the reference alloys (304L, 316L) were tested at −50 °C, 5.5·10−5 s−1 and 40 MPa of pure hydrogen gas, which represents a condition of maximum susceptibility to HEE.

Section snippets

Alloying concept

The first decision made concerning the novel material was to take the nominal composition of a lean-alloyed austenitic steel, e.g. AISI type 304, as the basis for developing the alloy. This decision relies on the general trend that austenitic stainless steels show a higher resistance to hydrogen embrittlement than ferritic steels [8], [9], [10], [4], [6]. Three different aspects were considered as milestones for the alloying concept: a) sufficient thermodynamic stability to ensure a fully

Thermodynamic stability

The measured chemical composition of alloy 10-8-2.5, presented in Table 1, was used to calculate the corresponding phase diagram with the Calphad software ThermoCalc S [61] in conjunction with the thermodynamic database TCFe6.2 [62]. The resulting diagram is shown in Fig. 1, in which the dashed line depicts the corresponding carbon isopleth. As can be seen, alloy 10-8-2.5 presents a primary ferritic solidification followed by a two-phase field (bcc + fcc) that becomes a fully austenitic field

Discussion

Many years of investigation on HEE of austenitic stainless steels have revealed a correlation between the tendency of the material to undergo strain-induced α′-martensitic transformation and the ductility response in a hydrogen atmosphere. Namely, the ductility of the material in hydrogen decreases with increasing volume fraction of strain-induced α′ martensite. Specifically, stable austenitic stainless steels show a high resistance to HEE, whereas metastable steels present a poorer resistance

Summary

A novel lean-alloyed austenitic steel with a high resistance to hydrogen environment embrittlement (HEE) was developed in the laboratory by means of an empirical approach. In particular, the combination of high-carbon, high-manganese, and high-aluminum contents is the basis of a molybdenum-free and 8.0 wt. % nickel-containing material. The susceptibility of the alloy to HEE was evaluated by means of slow strain rate tensile testing in a 40 MPa pure hydrogen gas atmosphere at −50 °C, and

Acknowledgment

The authors gratefully acknowledge the financial support of the Bundesministerium für Wirtschaft und Technologie (BMWi) under contract number 0327802D. Tensile tests in hydrogen were performed at “The Welding Institute” (TWI, Cambridge, UK).

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