PSI - Issue 16

Ihor Dmytrakh et al. / Procedia Structural Integrity 16 (2019) 113–120 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2019) 000 – 000

116

4

C H * . It is worth noting that the value of C H

* for given steel is sufficiently low, i.e., low volume concentrations of

hydrogen facilitate the plastic deformation of metal. We generalized the results of investigations in the form of a 3D-diagram (Fig. 2), where one can clearly see a specific domain corresponding to C H = C H * , which reproduces the changes in the mechanism of action of hydrogen on the deformation of steel. The characteristic value of concentration C H * can be regarded as an important engineering parameter in the evaluation of the characteristics of strength and fracture of materials and structural elements in hydrogen- containing media and also in the development of the technologies of hydrogen treatment of structural materials aimed at the optimization of their operating characteristics. 4. Influence of hydrogen concentration on the characteristics of cyclic crack-growth resistance of low alloy steels The above-described results of investigations were obtained for the hydrogenation of steel from smooth strained surfaces without defects, i.e., notches and cracks. For deeper understanding of the effect of hydrogen on the durability of structural elements of hydrogen energy infrastructure under the actual operating conditions, it is necessary to know the regularities of propagation of crack-like defects under its influence. Therefore, we studied the specific features of fatigue crack growth in given steel in the course of its hydrogenation. The rectangular cross-section beam specimens were manufactured with real pipes, which were supplied from two different manufacturers (test series A and test series B). The longitudinal cracks were studied and cut off of specimens from pipe was corresponded to this case. The initial edge crack of length a . mm 2 5 0  in the specimen was nucleated in air conditions. Before test, the specimens were hydrogen charged to assigned level of C H . After that, the specimens were subjected by cyclic pure bending with frequency f = 1 Hz under stress ratio R = 0 in NS4 solution.

Fig. 3. Diagrams of fatigue crack-growth rate for the specimens with different in bulk hydrogen concentration: (а) test series A: ( ♦ ) C H = 0.001 ppm, ( ▲ ) C H = 0.209, ( ● ) C H = 0.456 ppm; (b) test series B: ( ♦ ) C H = 0.001 ppm, ( ▲ ) C H = 0.209 ppm, ( ○ ) C H = 0.514 ppm, ( ● ) C H = 1.231 ppm. The results of the tests were first presented as a collection of experimental data on the fatigue crack-growth rate da/dN depending on the range of the stress intensity factor (SIF) Δ K near the crack tip (Fig. 3). An insignificant difference was visually revealed between the results of investigations of specimens from batch A (Fig. 3а) and B (Fig. 3b), which can be explained by the natural spread of data typical of these tests. However, if we superimpose these two plots, then the results of both test series for C H = 0.209 ppm (triangles) completely coincides. This reveals an ambiguous dependence of the crack-growth rate da/dN on the concentration C H , which is confirmed by the obtained results. For their comparison and analysis, we used the relation proposed by Paris et al. (1963):