Fatigue crack growth behaviour in austenitic stainless steels subjected to superficial and entire hydrogenation

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Abstract

Hydrogen effects on fatigue of type 304 and 316L austenitic stainless steels were studied using superficially and entirely hydrogenated specimens, which had a gradation and uniformity of the hydrogen concentration, respectively. Two contradictory mechanisms exist in the fatigue crack growth associated to hydrogen. Hydrogen reduces the strain at the crack tip, while the plastic deformation concentrates there through the hydrogen-enhanced slip planarity. When the former effect is dominant, e.g., in the case of the entirely hydrogenated type 316L, the fatigue crack growth resistance is remarkably improved. The presence of hydrogen does not change the mode of deformation-induced martensitic transformation, whereas it enhances the planarity of slip, particularly in the type 304 having low austenite stability. The fatigue crack growth rate is increased through the restriction of crack blunting by the superficial hydrogenation of the type 304. This is because the hydrogen-enhanced slip planarity prevents transfer of slips generated from the crack tip.

Highlights

► We studied fatigue crack growth in hydrogenated austenitic stainless steels. ► Superficial and entire hydrogenation provided different hydrogen distributions. ► Two contradictory mechanisms exist in fatigue crack growth associated to hydrogen. ► Superficial hydrogenation enhances crack growth via restriction of crack blunting. ► Crack-tip strain reduced by hydrogen is emphasized in entirely hydrogenated specimen.

Introduction

Embrittlement of martensite formed during deformation has been of major concern in hydrogen embrittlement (HE) of metastable austenitic stainless steels [1], [2], [3], [4]. On the other hand, Eliezer et al. [5] reported that the formation of martensite is not prerequisite for HE. Indeed, loss of ductility owing to hydrogen in stable austenitic steels was observed by tensile testing of entirely hydrogenated specimens [6], [7], [8], [9], [10], [11]. Austenitic stainless steels have been selected for applications in the various industrial sectors related to hydrogen because of their superior anti-permeation of hydrogen as well as little susceptibility to HE. These alloys when undergoing exposure to a hydrogen environment at near room temperature are anticipated to have a gradation of hydrogen concentration from the surface to the interior owing to low diffusivity of hydrogen [12], [13]. Tensile testing of superficially hydrogenated specimens [14] revealed two competitive mechanisms: microvoid nucleation and linkage in the interior region, which was non-hydrogenated; shear-type crack initiation and growth from the hydrogenated surface. Therefore, even when the superficially hydrogenated layer is thin, hydrogen can facilitate the shear-type cracking from the surface, resulting in significant loss of ductility. Meanwhile, using entirely hydrogenated specimens, it was reported that the susceptibility to HE depended on the alloy composition (austenite stability), the hydrogen content, the strain rate, and so on [6], [7], [8], [9], [10], [11]. In general, occurrence of ductility loss is interpreted by premature plastic instability, in which the following relationship is satisfied:σdσdε,where σ is the true stress and / is the strain-hardening rate. The contribution of hydrogen to the HE in the relatively stable austenitic steels was characterized by two mechanisms: the increased stress for plastic instability and the reduced strain-hardening ability caused by the enhanced slip planarity, both of which led to the premature failure [11].

Regarding the effect of hydrogen on fatigue behaviour, the crack growth rates in the superficially hydrogenated specimens of type 304, 316, 316L and 310S were reported to be higher than or nearly identical to those in the uncharged counterparts [15], [16]. In contrast, it was recently reported that entire hydrogenation remarkably prolonged the life of fatigue crack growth in austenitic stainless steels [17]. The current study addresses two roles of hydrogen, i.e., detrimental and desirable effects, in fatigue characteristics of austenitic stainless steels.

Section snippets

Materials and experimental methods

The materials used in this study was type 304 (JIS-SUS304) and type 316L (JIS-SUS316L) austenitic stainless steels, which were received in a form of rods with a diameter of 22 and 20 mm, respectively, after solution treatment. Table 1 contains the chemical compositions of these materials, where the hydrogen content was measured by thermal desorption spectrometry (TDS). The Vickers hardness, HV, (with an applied force of 9.8 N for duration of 30 s) was 176 for the type 304 and 157 for the type 316L.

Fatigue crack growth and crack opening behaviours

Fig. 3a and b shows the fatigue crack length plotted against the number of cycles and the crack opening behaviour at a crack length of approximately 1000 μm for the type 304. The fatigue crack growth rate of the superficially hydrogenated specimen was approximately 1.6 times higher than that of the uncharged specimen (Fig. 3a). The entirely hydrogenated specimen with the lower hydrogen content (CH, calc = ∼25 mass ppm) exhibited intermediate resistance of the fatigue crack growth between the

Conclusions

This study was conducted using superficially and entirely hydrogenated specimens to elucidate two contradictory roles of hydrogen in hydrogen embrittlement of fatigue of austenitic stainless steels. The conclusions can be summarized as follows:

  • (1)

    In the type 304, the fatigue crack growth was increased by both the superficial hydrogenation and the entire hydrogenation at the level of ∼25 mass ppm. These hydrogenated specimens exhibited low crack opening displacement (COD) values as compared to those

Acknowledgments

This work was supported in part by the NEDO, Fundamental Research Project on Advanced Hydrogen Science (2006–2012).

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