PSI - Issue 13

Motomichi Koyama et al. / Procedia Structural Integrity 13 (2018) 292–297 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction: designing hydrogen-resistant steels Finding a breakthrough in the development of hydrogen-resistant steels is necessary to realize a hydrogen-energy based society. Here, the phrase "hydrogen resistance" refers to the mechanical resistance of a material when exposed to hydrogen, including its tensile ductility/strength and fatigue life/strength. The study of hydrogen resistance is divided into problems: crack initiation and crack propagation. Monotonic-tension-related and fatigue-related cracks are easily initiated under hydrogen environments, and, thus, crack propagation is key to overcoming the strength-ductility trade off that occurs when hydrogen worsens the mechanical performance of steels. In this regard, microstructural design is an important route for enhancing crack resistance. In general, hydrogen resistant steels have been designed by increasing the stability of austenite (Takaki et al., 2016); this is because the martensitic transformation from fcc (γ) to bcc (α´) increases hydrogen diffusivity, increasing hydrogen embrittlement susceptibility. However, here we must note the other martensitic transformation: the γ to hcp (ε) martensitic transformation (Tsuzaki et al., 2016). This transformation, to the close-packed structure, does not increase hydrogen diffusivity. Moreover, according to ab-initio calculations, the hydrogen diffusivity of ε -iron is lower than that of γ -iron (He et al., 2017). Hence, the decrease in stability on transformation from γ to the ε phase does not increase the hydrogen embrittlement susceptibility, opening a new chemical and microstructural design strategy. In this paper, the instability of austenite to ε - martensitic transformation is referred to as ε -metastability. In addition, the ε -metastability concept is compatible with an area of recent research attention: increasing the configurational entropy of alloys (Zhang et al., 2014). The maximization of entropy realizes extraordinary crack resistance (Gludovatz et al., 2014), and the crack resistance can be further enhanced by decreasing the stability of the steel phase, from austenite to the ε phase (Li et al., 2016). Accordingly, we investigate two alloy-design concepts for hydrogen-resistant steels: the effect of ε -metastability and the effect of configurational entropy. It is well-known that the α´ -martensitic transformation reduces hydrogen embrittlement resistance. Figure 1(a) shows an example of hydrogen-induced mechanical degradation associated with the α´ -martensitic transformation. The Fe 19Cr-8Ni-0.05C (wt.%) steel shows the α´ -martensitic transformation immediately after yielding (Ogawa et al., 2017). When hydrogen is introduced to the Fe-19Cr-8Ni-0.05C steel, the grain boundaries covered with the α´ -martensite act as preferential crack initiation sites, as seen in Figures 1(b) and (c); this causes early crack formation and associated distinct mechanical degradation. Stabilization of austenite by adding carbon delays the fracture, but the mechanical degradation is still remarkable. The reason for this degradation is as follows: Even when intergranular crack initiation is prevented, transgranular crack initiation occurs in the α´ -martensite. Once a crack is initiated, crack opening causes deformation-induced martensitic transformation at its tip, as shown in Figures 1(c) and (d), resulting in further propagation. As a result, hydrogen-assisted failure occurs. Hence, as long as the α´ -martensitic transformation occurs preferentially, achieving a drastic improvement in hydrogen-embrittlement resistance is challenging. 2. Effects of metastability 2.1 α´ - martensitic transformation

Fig. 1. (a) Hydrogen embrittlement in Fe-19Cr-8Ni-0.05 and Fe-19Cr-8Ni-0.14C (wt.%) steels with α´ - martensitic transformations. (b) Rolling direction inverse pole figure (RD-IPF) and (c) phase maps showing intergranular cracking in the Fe-19Cr-8Ni-0.05C steel. (d) RD-IPF and (e) phase maps showing transgranular cracking in the Fe-19Cr-8Ni-0.15C steel. The details of this experiment are given elsewhere (Koyama et al., 2017). Reproduced with permission from Int. J. Hydrogen Energy , 42 , 26423 (2017), copyright 2017, Elsevier.