13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- with niobium, vanadium, titanium, aluminum and boron leads to the formation of carbide TiC, borides Me3B2, and intermetallics Ni3(Al,Ti,Nb) in the amounts of to 8 % for 05Cr19Ni55 alloy and up to 15…17 % for 04Cr16Ni56 alloy, which substantially increases their high-temperature strength [8] and significantly influences the sensitivity to the action of hydrogen [3, 9]. Heat treatment regimes and the mechanical properties of alloys (Table 1) in helium and in hydrogen under a pressure of 35 MPa after preliminary hydrogenation to hydrogen content of 19 wppm (05Cr19Ni55 alloy) and 20 wppm (04Cr16Ni56 alloy). After heat treatment, the grain size was 40…60 µ, and the particle size of the intermetallic γ΄-phase was 320…500 Ǻ. Table 1. Chemical Composition, Modes of Thermal Treatment and Mechanical Properties of Alloys in Helium/Hydrogen (35 MPa) at Room Temperature Thermal Treatment Mechanical Properties Alloy Solution treatment Mode of aging σu, MPa σy MPa δ % ψ % Number of cycles to fracture, bending strain 1.6 % Kc MPa· m1/2 04Cr16Ni56 (C-0.04; Si-0.12; Cr-16.4; Mo-5.24; V-0.35; Nb-5.19; Ti-0.58; Al-1.0; Fe-15.14; Cu-0.49; Ni-bal) 1373К, 1 h 1023 К, 16 h + 923 К, 10 h 1320 880 840 750 34 4 48 12 3205 54 134 52 05Cr19Ni55 (C-0.05; Si-0.23; Cr-19.0; Mo-8.87; Nb-1.73; l-1.49; Fe-12.0; Cu-0.02; Ni-bal) 1323К, 1 h 1000 К, 15 h + 923 К, 10 h 1080 970 650 660 35 7 38 21 2560 199 116 68 Static tensile tests with recorded “stress-strain” curves were carried out on standard five-fold cylindrical specimens using displacement rate V = 0.1 mm/min. During the test the specimens were positioned into the chamber specially designed for high-temperature tests at 293...1093 K temperature range under 0.1...30 MPa hydrogen pressure. Specimens were tested in temperature range 293...1073 K under 30 MPa of hydrogen pressure and, as a comparison, in helium. The low-cycle durability for pure strain-controlled sign-preserving bending was investigated under pressures of 30 MPa for the strain amplitudes ε = 1.6% and loading frequency of 0.5 Hz on polished plane specimens with a working part size 3 x 6 x 20 mm. Stress intensity factor under static loading Kc was computed either for the maximum force Fc in the "F-V" linear diagram or for the force FQ determined by using the 5% secant for nonlinear diagrams. Rectangular compact specimens 50 × 60 × 20 mm in size were tested for eccentric tension in a high-pressure chamber under pressures of 0.4-30 MPa at strain rate of 0.1 mm/min. The values of Kc can be calculated from the Srawley-Gross formula [10]. To determine the indicated mechanical characteristics in hydrogen, the working chambers were preliminarily evacuated, blown-out with hydrogen, again evacuated, and filled up with hydrogen to a given pressure. At high temperatures, the specimens were held under the testing conditions for 30 min until the attainment of thermal equilibrium. Earlier we established that, at some region of hydrogen pressure and strain rate exists a maximum
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