Effect of internal hydrogen on the tensile properties of different CrMo(V) steel grades: Influence of vanadium addition on hydrogen trapping and diffusion

The influence of hydrogen on the mechanical behaviour of different quenched and tempered CrMo steels with or without vanadium were investigated by means of tensile tests. Smooth and circunferentially-notched round-bar specimens pre-charged with gaseous hydrogen in a high pressure hydrogen reactor were tested. The degradation of the tensile properties was correlated with the interaction between hydrogen atoms and microstructure, which was analysed by means of thermal desorption analysis (TDA) and permeation tests. A LECO DH603 hydrogen analyzer was used to study the activation energies of the different microstructural traps and also to study the hydrogen eggresion kinetics at room temperature. Moreover, electrochemical hydrogen permeation tests were also used to determine the apparent hydrogen diffusion coefficients and the density of traps present in the different steel grades. Hydrogen embrittlement measured in notched specimens was much greater than that observed in the smooth samples, being this effect more notable in the steel grades with higher yield strengths, tempered at the lowest temperatures, where a change in the fracture micromechanism from ductile in the absence of hydrogen to intermediate and brittle in the presence of internal hydrogen was clearly observed, especially in tests performed at the lowest displacement rates. Results were discussed through FEM simulations of local stresses acting on the process zone. On the other hand, the V-added steel grades were less sensitive to hydrogen embrittlement due to the effect of the submicrometric VC precipitated during the tempering treatment, which might be considered non-diffusible hydrogen-trapping sites, in view of their strong hydrogen-trapping capability (35-41 kJ/mol). In these steels (V-added grades), diffusible hydrogen and hydrogen accumulation in the process zone decrease, improving hydrogen embrittlement resistance. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.