Effect of Stress Concentrations on the Mechanical Properties of Fe-Cr-Ni Steels and Alloys in Gaseous Hydrogen at High Pressures and Temperatures

The relation between the ultimate strength sigma b and contraction psi vs stress concentration factor Kt of smooth and notched cylindrical specimens from cast and aging austenitic-martensitic steels, solution-hardened austenitic steel, and powder and deformable dispersion-hardened nickel alloys was studied at pressures of gaseous hydrogen up to 35 MPa and temperatures up to 1073 K. At room temperature, the effect of hydrogen on sigma b and psi of all materials was revealed at 1 MPa; it is enhanced with the pressure up to 20 MPa and Kt from 1 to 4.602 in V-notch specimens with maximum and minimum diameters of 7 and 5 mm, respectively, and radii at the notch root of 0.1 and 0.2 mm, a further increase in the hydrogen pressure to 35 MPa does not bring about additional deterioration of material properties. The hydrogen embrittlement of an EK-62 heat-resistant nickel alloy does not almost depend on the stress concentrations. The effect of hydrogen decreases with temperature but remains essential in the range of 293-723 K (VNL-6 steel), 293-973 K (EP-901 alloy), and 293-1073 K (EK-62 alloy). The most sensitive to hydrogen are VNL-6 cast austenitic coarse-acicular martensitic steel and an EK-62 alloy with a great amount of intermetallics wherein, unlike other materials, the embrittlement appears not only as a noticeable decrease in ductility but also in ultimate strength of both smooth and notched specimens. The strengths of hardened specimens from ChS-37 steel, EP-666 alloy, and VZhL-14P aged alloy are not affected in the hydrogen atmosphere, and contraction is relatively small. A VZhL-14 powder metallurgical fine-dispersed alloy with almost the same ultimate strength is the least sensitive to hydrogen embrittlement among heat-resistant dispersion-hardened nickel-base alloys.