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

1055

3

austenite solid solution of the steels. Using the approaches of Lee et al. (2005) and Srinivas et al. (1997), the interstitial concentration, corresponded to such lattice parameters, reaches ≈0.7 wt.%. Additionally, to austenite, a small portion of  -phase ( ≈ 5%) was found is solution-treated 0V-HNS. According to X-ray diffraction analysis and TEM researches, initial structure of 1.5V-HNS was composed by austenite and spherical particles (V,Cr)(N,C) with dimensions of (0.3-0.5) μ m. These particles are uniformly distributed in the steel structure (both in grain bodies and grain boundaries). The average grain size of 0V-HNS is d=50 μ m as measured using LM. Alloying with vanadium promotes finer grain size formation (about 10 μ m), which relates to precipitate-assisted suppression of grain growth in 1.5V-HNS during heat treatment. Figure 1 shows the tensile “ engineering stress-engineering strain ” curves for H-free and H-charged specimens. The deformation of the steels develops with the high strain-hardening both for H-free and H-charged specimens (Fig.1 b). Steels are characterized by different mechanical properties: 1.5V-HNS possesses higher values of the yield stress ( YS ), the ultimate tensile strength ( UTS ) and strain-hardening coefficient in comparison with 0V-HNS. This difference arises due to particle-strengthening and smaller grain size in 1.5V-HNS as compared with 0V-HNS (Tabls. 1 and 2).

Fig.1. The effect of H-charging on tensile “ engineering stress – engineering strain ” curves (a) and work hardening rate (d  /d  ) versus true strain (b) in 0V-HNS and 1.5V-HNS Table 2. Mechanical properties of H-free and H-charged steels Steel Treatment YS (MPa) UTS (MPa)  un (%) HEI (%) 0V-HNS H-free 570 1230 44 60 H-charged 660 970 18 1.5V-HNS H-free 740 1530 35 5 H-charged 820 1580 33 Independently of the chemical composition, H-charging provides solid-solution hardening of the steels. The YS values increase as a result of surface hydrogen saturation: by 16 % in 0V-HNS and by 11 % in 1.5V-HNS (Tabl.2). Despite similar hardening effect, hydrogen-charging causes substantially different embrittlement in V-free- and V containing steels. The HEI is strongly dependent on steel phase (chemical) composition (Tabl.2). Namely, 1.5V-HNS possesses higher resistivity to hydrogen embrittlement compared to 0V-HNS. The work-hardening rate (d σ /d ε ) slightly increases after H-saturation in 1.5V-HNS, but the flow of plastic deformation does not change substantially (Fig. 1). For 0V-HNS, contrarily, H-charging drastically decreases strain-hardening coefficient (Fig. 1). The micrographs of the lateral surfaces and fracture surfaces of specimens in H-free and H-charged specimens subjected to tensile deformation to fracture are presented in Figures 2 and 3 respectively. A wide diffuse neck and ductile rupture are the main fracture characteristics in H-free steels. For both steels, a slip-band morphology, typical for ductile materials, is visible on lateral surfaces of H-free specimens (Fig. 2 a, d). The neck is more pronounced in 0V-HNS compared to 1.5V-HNS (Fig. 2 a), which is correlated with data on mechanical properties (Fig. 1 a). A microscopic fracture mechanism in H-free 0V-HNS and 1.5V-HNS is transgranular dimple fracture. Dimple sizes are lower in 1.5V-HNS with lower grain size than that in 0V-HNS with higher grain size (Fig. 3 a, d), which is in line with researches of Ganesh Sundara Raman and Padmanabhan (1994).