Special Diagram for Hydrogen Effect Evaluation on Mechanical Characterizations of Pipeline Steel

The derived diagram defines three main ranges of hydrogen concentration in the metal, which have their own specific mechanisms of hydrogen influence on the characteristics of strength and ductility of the pipeline steel. It has been demonstrated that the nature of the hydrogen effect mechanism depends on the ratio of diffusible C-H(dif) and residual (trapped) hydrogen C-H(res) in the steel. A specific effect has been found, namely: at the hydrogen concentration C-H congruent to 0.01-0.2 ppm where all hydrogen is practically diffusible, the plastic deformation of the steel is facilitated, i.e., the deforming of the material takes place more easily. At the concentration range C-H congruent to 0.8-10 ppm, where the trapped hydrogen prevails, the mechanism of hydrogen embrittlement of the steel is completely dominated. Therefore, the hydrogen concentration range beginning from C-H congruent to 0.8 ppm can be considered as critical. The range of values of hydrogen concentration in the metal C-H congruent to 0.2-0.8 ppm is a transition zone where the simultaneous coexistence of these two mechanisms is possible. The constructed diagram can be applied for the interpretation of the inspection results of long-term operated pipelines, and also for the choice of materials for the construction of new networks for hydrogen transportation.