Effect of cyclc loading on hydrogen diffuson in low alloy steels

The experimental work is aimed to the study of the hydrogen embrittlement mechanism in carbon and low-alloy steels. In particular, the theme of hydrogen diffusion within the metal in matrices subjected to cyclic loading is addressed. Literature models have been applied to fit experimental hydrogen permeation curves obtained in the presence and in the absence of reversible and irreversible traps (Figure 1). Relevant shift to the left of the permeation curve has been noticed mainly due to the application of load at values exceeding yield stress. At 110%TYS-i.e. In presence of plastic strain-the shape of the curve also modifies due to the presence of irreversible traps that subtract diffusible hydrogen. However, their effect is slight at the highest maximum load considered in this work. Significant reduction in apparent diffusion coefficient (Figure 1) and considerable increase of the number of traps (Figure 2)-mainly reversible ones-has been noticed as maximum load exceeded the yield strength. Cyclic loading at tensile stress slightly higher than the yield strength of the material increases hydrogen entrapment phenomena. The stress causes a marked and instant reduction of the concentration of mobile hydrogen within the metal lattice since 55% of the yield strength and increases significantly in the plastic field.