An investigation of the effect of aged structures on diffusivity and solubility of hydrogen in a commercial 18Ni maraging steel has been carried out at room temperature by the electrochemical permeation technique.
The diffusion coefficient and solubility of hydrogen vary with the aged structures of the steel. When the steel is aged at 400 to 450°C for 1 to 3 h, the former shows a minimum, while the latter exhibits a maximum. These variations in diffusivity and solubility of hydrogen can be interpreted in terms of the difference between hydrogen trapping effects of various lattice imperfections, such as dislocations and their stress fields, together with the interfaces between the precipitates and the matrix of martensites introduced by the formation of zones and/or by the precipitation of metastable phases during aging. The solubility of hydrogen is high and the diffusion coefficient is low in the structures aged at higher temperatures for long times. This corresponds to the overaging structure caused by the combination of usual overaging and the formation of nickel rich-austenites in the structure. Thus, the diffusion coefficient in the austenitic structure formed by the reversion of martensites during aging seems to be lower than that in the martensitic structure caused by solution treatment.