13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- influence of hydrogen on the plasticity, low-cycle fatigue life, and static and cyclic crack resistance of martensitic steels and high nickel alloys [3, 7, 9]. In short-term tension, austenitic dispersion-hardened steels are substantially embrittled by hydrogen only after preliminary hydrogenation at elevated temperatures when its content becomes greater than 12 wppm [3, 7]. This is why we held a part of the specimens for 10 h in a hydrogen atmosphere under 623 K and 35 MPa. These regimes provide the hydrogenation of specimens to hydrogen contents of 19 wppm (05Cr19Ni55 alloy) and 20 wppm (04Cr16Ni56 alloy). Hydrogenated and non-hydrogenated specimens were tested in helium and hydrogen under different pressures. Hydrogen concentration (CH) was determined with a LECO TCH 600 instrument [11] with precise 0.1 ppm (3 samples for point). 2. Hydrogen Pressure Influence on the Mechanical Properties. It is known from the literature for almost materials and testing methods the influence of hydrogen increases proportionally to the square root of pressure [1, 4, 6, 12]. The pressure range for which this dependence is valid is studied insufficiently. In the process of short-term static tension, the properties of carbon steels deteriorate only in the range 0-10 MPa. Embrittlement of specimens made of Inconel-718 alloy begins only when the pressure reaches 10 MPa and increases up to 70 MPa [12]. Hydrogen influence on plasticity of steels and many other materials increases in the entire pressure range (0-70 MPa) [1, 3, 6, 12]. In the case of 04Cr16Ni56 alloy the dependence of low-cycle durability (N), the plasticity parameters (δ and ψ) (Fig.1a) and plane-stress fracture toughness (Kc) (Fig.2, curves 3, 4) on the hydrogen pressure consists of two regions. In the first region (low pressures), parameters N, δ, ψ and Kc drops abruptly, and in the second, the negative influence of hydrogen is practically independent of pressure (figs. 1, 2). This means that there exists a pressure which cause the limiting degradation of materials properties. An additional effect of preliminary dissolved hydrogen (CH = 20 wppm) on the properties of 04Cr16Ni56 alloy was manifested at hydrogen environment pressure less than 10 MPa. The parameters of loading and the modes of hydrogen action for which the mechanical characteristics of the investigated alloys are minimum at 293 K can be formulated as follows: –the strain rate Vdef = 0,1 mm/min (6.7·10 -5 s-1) at short-term static tension and static crack propagation; – the frequency and amplitude of bending under the conditions of low-cycle fatigue are ν = 0, 5 Hz and ε = 1.6% respectively, and the pressure of hydrogen must be higher than 10 MPa. 0 5 10152025 0 10 20 30 40 1000 2000 3000 N, cycles δ,ψ,% Р, MPa 1 2 3 4 5 a 6 0 5 10152025 10 20 30 1000 2000 N, cycles δ,ψ,% Р, MPa 1 2 3 4 5 b 6 Figure 1. The relative elongation δ (1, 2), reduction of area ψ (3, 4) (V = 0,1 mm/min.) and number cycles to failure N (5, 6) (ε = 1,6%) specimens of 04Cr16Ni56 (a) and 05Cr19Ni55 (b) alloys versus hydrogen pressure P at 293 K: 1, 3, 5 – non-hydrogenated specimens; 2, 4, 6 – hydrogenated specimens. In the case of 05Cr19Ni55 alloy the low-cycle durability N, plasticity parameters δ and ψ (Fig.2b, curves 1, 3, 5) and plane-stress fracture toughness Kc (Fig.3, curve 1) decrease in whole hydrogen pressure range. Preliminary dissolved hydrogen (CH = 19 wppm) leads to a considerable additional decrease in the properties of this alloy (Fig.2b, curves 2, 4, 6; Fig.3, curve 2). Maximum hydrogen effect achieved on hydrogenated specimens at hydrogen pressure above 10 MPa.
RkJQdWJsaXNoZXIy MjM0NDE=