PSI - Issue 13

Galina Maier et al. / Procedia Structural Integrity 13 (2018) 1053–1058 Galina G. Maier et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 2. SEM micrographs of lateral surfaces of H-free (a, d) and H-charged specimens (b, c, e, f): (a-c) – 0V-HNS, (d-e) – 1.5V-HNS. Figures (c) and (f) provide higher magnification of the areas marked on images (b), (e) respectively.

Hydrogen-saturation causes an appearance of brittle surface layers on both lateral surfaces of the specimens. The central parts of H-saturated specimens fracture by viscous transgranular mode similar to one in H-free steels. For H charged 0V-HNS, the lateral surfaces of the specimens underwent mixed transgranular and intergranular surface cracking under tension (Fig. 2 b, c). Even though hydrogen does not cause a notable decrease in elongation in 1.5V HNS, numerous macroscopic cracks are also observed on the lateral surfaces of the specimens. The lateral surfaces show predominantly intergranular cracking (Fig. 2 e, f). The depth of the H-assisted diffusion zone, K H , (thickness of a brittle surface layer) can be estimated using SEM images of fracture surfaces obtained for the steel specimens after tensile deformation. The depth of the H-induced brittle surface layers and their fracture micromechanisms are compositional-dependent characteristics. Surface layers with the mean thickness of 84 µ m in 0V-HNS is much wider than that of 30 µ m in 1.5V-HNS. In 0V-HNS, the surface layers are characterized by predominantly transgranular brittle fracture mechanism (Fig. 3b, c), which is similar to quasi-cleavage fracture in high-carbon or high-nitrogen steels (Berns et al., 2013; Tomota et al., 1998; Astafurova et al., 2009). For 1.5V-HNS, fracture occurs predominantly by brittle intergranular mechanism in H-assisted layers (Fig. 3 e, f). Rather large (0.4 µ m) and deep hollows are visible on the brittle fracture elements (Fig. 3f). The size of such hollows indicates that they form as a result of the "crumbling" of spherical particles (V,Cr)(N,C). The change in the fracture behavior of the surface brittle layer correlates with the dependence of the HEI coefficient on the composition of the steels. That is, the formation of transgranular brittle cracks (quasi-cleavage) on the surface of the samples and deeper H-assisted layer contribute to greater effects of embrittlement in 0V-HNS compared to intergranular fracture of the surface and shallower brittle layer in H-charged 1.5V-HNS. Similar effect was observed for H-charged Fe-17Cr-10Mn-7Ni-1V-0.1C-0.7N steel (Astafurova et al., 2018), where the mechanism of the destruction in tension and HEI -value varied with duration of H-saturation. Brittle mixed transgranular and intergranular fracture causes higher HEI -value (up to 26%) than that for fully intergranular fracture ( HEI =17%) (Astafurova et al., 2018).