13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 3. Influence of Temperature on Hydrogen Embrittlement of the Alloys. The strength of the alloy 05Cr19Ni55 significantly decreases in helium at temperatures higher than 873 K. This process is accompanied by a decrease in plasticity parameters for the alloy 05Cr19Ni55 at temperature 873 K and a significant increase in the relative elongation and reduction of area for specimens made of this alloy at temperatures 973…1073 K (Fig.5, curves 5, 6). At temperatures range 973…1073 K we observed a decrease in plasticity parameters for the alloy 04Cr16Ni56 (Fig.5, curves 7, 8). 400 600 800 1000 0 20 40 60 80 600 800 1000 1200 2 4 σu, MPa ψ, % T, K 1 3 5 6 8 7 Figure 5. Temperature dependences of ultimate strength σu (1-4) and reduction of area ψ (5-8) of 04Cr16Ni56 (1, 2, 7, 8) and 05Cr19Ni55 (3, 4, 5, 6) alloys at helium (1, 3, 5, 7) and hydrogen (2, 4, 6, 8) under the pressure 30 MPa. The drop of plasticity of the dispersion-hardening materials within the temperature range of intense phase transformations is caused by the localization of strains on the grain boundaries due to the intense redistribution of Ni, Ti, and Al in the boundary regions. Moreover, the increase in plasticity observed at higher temperatures is caused both by partial coagulation of hardening phases and possible dissolution of small amounts of finely divided precipitations [8]. In alloy 04Cr16Ni56 that contain higher amount of refractory elements (Table 1), the diffusion processes are decelerated and, therefore, the drop of strength is small and the temperature of plasticization is probably higher than 1073 K (Fig. 5, curves 1, 7). The ultimate strength of alloy 04Cr16Ni56 is sensitive to hydrogen action (Fig. 5, curves 1, 2). At the same time σu of specimens made of alloy 05Cr19Ni55 decreases in hydrogen insignifically (Fig. 5, curves 3, 4). The hydrogen effect on plasticity parameters δ and ψ (Fig. 6), low-cycle durability N (Fig. 6) and plane-stress fracture toughness Kc (Fig. 7) of both alloys is essential in the temperature range 293…773 K and appreciable at all other temperatures. As temperature increases from 773 to 873 K the hydrogen effect on reduction of area of the alloy 05Cr19Ni55 specimens initially decrease and again increase at 973 and 1073 K (Fig. 5, curves 5, 6). That suggests that at 1073 K the hydrogen-induced processes of localization of strains and fracture (mainly intergranular) occur in this alloy so intensely that the coagulation of hardening phases is insufficient for the high-temperature plasticization observed in helium. This phenomenon we established earlier on dispersion-hardening austenitic steels [13]. The additional effect of preliminary hydrogenation on parameter of static growth resistance Kc for the alloy 05Cr19Ni55 is appreciable only at room temperature (Fig. 6, curves 3, 4, 6, 7). At 373 K the value of Kc for hydrogenated and non-hydrogenated specimens are almost equal, i.e., this temperature is sufficient for hydrogenation of dispersion-hardening nickel base alloys from the hydrogen atmosphere. For all loading modes, the degree and temperature interval of hydrogen degradation for alloy 04Cr16Ni56 is much larger than for alloy 05Cr19Ni55 (figs. 5-7). The plane-strain condition required for the evaluation of KІс were fulfilled on compact tension specimens made of alloy 04Cr16Ni56 with a thickness of 20 mm at hydrogen pressure above 10 MPa in the temperature range 293…473 K. Fracture toughness Kc for alloy 05Cr19Ni55 was decreased at 293 K from 116 MPa·m1/2 in helium to 78 MPa·m1/2 in hydrogen under the pressure
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