Issue 51

A. A. Lakhdari et alii, Frattura ed Integrità Strutturale, 51 (2020) 236-253; DOI: 10.3221/IGF-ESIS.51.19 238 The development of deformation models and the evaluation of the longevity of structural elements subjected to hydrogen embrittlement have been the subject of several studies [11], and in this work the finite difference method was generally used to calculate the structures. In the work [17], the process of hydrogenation and deformation of a hollow cylinder was studied, taking into account the effect of the stress state on the hydrogen saturation kinetics of the structure. In this work, we examine finite element modeling of the behavior of a long hollow cylinder under hydrogenation conditions, taking into account the effect and concentration of hydrogen as well as the rigidity of the flow diagram state of stress on the kinetics of penetration of hydrogen into the cylinder. L OW TEMPERATURE HYDROGEN EMBRITTLEMENT t is observed at temperatures below 200°C (-20 to 200°C), and in this case, the source of hydrogen is either hydrogen itself, or hydrogen is a by-product in some technological processes, so hydrogen enters the metal simply under pressure. The effect of hydrogen at low temperature is distinguished by the penetration of hydrogen by diffusion in the elements of metal structures under constraints or not. In addition, the hydrogen penetrates intensively in the zones under traction, and much less in the zones under compression. Thus, it accumulates and after reaching a certain concentration, it causes a change in the mechanical properties of the material of the structure, depending on its concentration. For a low concentration of hydrogen, there is practically no change in the mechanical properties. But by reaching a critical concentration level, the hydrogen causes an intense degradation of the properties, and for a maximum concentration (saturation limit), the change of the mechanical properties is slowed down, although the hydrogen saturation of the material continues [17, 18]. In the structures under stress and subjected to a hydrogenation at low temperature, there is a more intense variation of the mechanical properties of the material in the zones under traction than in the other zones under compression. The non-uniform variation of the mechanical properties causes a redistribution of the stress field, which in turn influences the distribution of the hydrogen concentration. The redistribution of the stresses and the hydrogen field in the mass of the structural element will remain unstable as long as the state of the structure is not stabilized. During the low temperature hydrogenation, a physico-chemical interaction of the steel with the hydrogen takes place, leading to an irreversible change of the mechanical properties. This is mainly due to the destruction of the carbide component. This physico-chemical phenomenon is called hydrogen corrosion of steel. Hydrogen corrosion develops in carbon steels after long operation at high temperature and pressure in an environment containing hydrogen. At the base of the mechanism of hydrogen corrosion is the interaction of hydrogen with carbon with the formation of methane. This reaction begins with the decarburization of the surface and the formation of microcracks, which progressively propagate in the metal, reducing its strength and plasticity. Hydrogen embrittlement of metal structures is closely related to their microstructure, and in particular to the processes of segregation and diffusion occurring at interfaces and defects. F ORMULATION OF THE PROBLEM he behavioral model of a long hollow cylinder under low temperature hydrogen embrittlement conditions is examined, in which hydrogen, without chemical interaction with the metal, penetrates into the volume of the structural element and accumulates there. It is accepted that the penetration of hydrogen takes place by the diffusion mechanism. In this case, as the experimental data show, the hydrogen does not penetrate equally in the different zones of the structural element in the zones where the compressive stresses predominate, the hydrogen penetrates more slowly, in the zones where tensile stresses are predominant, hydrogen penetrates faster. Thus, we have a medium whose diffusion characteristics depend on the state of stress [17, 18]. Also according to experimental data, the diffusion characteristics of the cylinder material depend on the hydrogen concentration. Therefore, it is further assumed that the diffusion coefficient of hydrogen in structure D ( C , S ) depends on the rigidity of the state of stress S and the hydrogen concentration C:     0 , 1 D C S D C S       (1) I T