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

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important scientific and engineering problem. It is well known that hydrogen may demonstrates various kinds of influence on the mechanical properties of structural metals and alloys but, up to now, there is no complete understanding of the entire spectrum of physical mechanisms of this influence, specific features of their realization, and conditions of coexistence. Therefore, the concepts and approaches to the solution of the most important problems of hydrogen materials science are quite contradictory: Somerday et al. (2009, 2014). Furthermore, prevail numbers of investigations in this field have a substantial disadvantage: the true level of hydrogen concentration in the metal is unknown and, hence, the degree of its hydrogenation is taken into account indirectly. However, just the volume concentration of hydrogen in the metal and its local concentration in the zones of high mechanical stresses in the material are determining for the realization of various mechanisms of its influence on the characteristics of strength, plasticity, and crack resistance of structures. In past years, together with colleagues from other scientific institutions, we performed a complex of investigations of low-alloy steels, which are used the pipeline systems of transportation of hydrogen-containing media. Here, the comparative analysis of the specific features of hydrogenation of promising pipe steels with different levels of ultimate strength was carried out by Capelle et al. (2010, 2013), the influence of hydrogen present in the metal on the fracture processes in these steels near stress concentrators was analyzed by Capelle et al. (2008, 2011), and the relationship between the fatigue crack growth rate and local hydrogen concentration near the crack tip was established by Dmytrakh et al. (2010, 2013). In the present work, we generalize the results of our recent studies that devoted to the analysis of the influence of hydrogen concentration on the specific features of deformation and fracture of low-alloyed steels in hydrogen containing media: Dmytrakh et al. (2014, 2015 and 2017) and Syrotyuk and Dmytrakh (2015). The tests were carried out for known values of the volume concentration of hydrogen in the metal. The behaviour of the mechanical characteristics of the material and its fracture strength under the influence of hydrogen was determined as a function of hydrogen concentration. The object of study was low alloyed pipeline steel (σ y = 260 MPa and σ u = 440 MPa) with nominal chemical composition (in weight %): C = 0.17 – 0.24; Si = 0.17 – 0.37; Mn = 0.35 – 0.65; S<0.04; remainder Fe. This material consists of grains of ferrite-pearlite, typical for all steel of this class. The specimens were manufactured from real pipes. The hydrogenation of specimens was made by electrochemical method under cathodic polarisation at the certain constant potential E cath = const. With the aim to simulate the hydrogen entry at real operating conditions of the buried pipeline, the following procedure has been applied. The special deoxygenated, near-neutral pH NS4 solution, which is the model of underground water, was chosen as the electrolyte for hydrogen charging of steel. The chemical composition of the NS4 solution is given in Table 1. Taking into account the situation of freely corroding system that exists for the real pipeline, the potential of polarisation E cath was slightly more negative than the free corrosion potential E corr for given steel. 2. Objects, materials and experimental procedure

Table 1. Chemical composition of NS4 solution (gram/litre). NaHCO 3 KCl CaCl 2 MgCl 2 . H 2 O 0.483 0.120 0.137 0.131

Hydrogen concentration in a bulk of steel has been determined on the base of hydrogen discharging process under anodic polarisation with using of modified hydrogen electrochemical oxidation method proposed by Capelle et al. (2008) and Dmytrakh et al. (2014). For realisation of experimental studies on the hydrogen charging of specimens and determination of the hydrogen concentration in a bulk of steel the special testing stand was developed by Dmytrakh et al. (2014). This facility is based on the dynamic electrochemical laboratory VoltaLab40 (Radiometer Analytical). As preliminary stage of study the experimental dependence “hydrogen concentration C H in specimen – time of exposure  ” was received. These experimental data were described by power relation: