Influence of hydrogen contamination on the tensile behavior of a plasma ion nitrided steel

The effect of hydrogen on the tensile mechanical properties was examined for samples taken from a nitrided layer (white layer plus diffusion zone) as well as from the substrate steel. An API X-65 type steel was plasma ion nitrided at 773 K during 16 h generating a white layer 5 μm thick and a diffusion zone measuring about 550 μm. Electrochemical tests were performed to obtain hydrogen permeation curves at 323 K, using a 0.1 N NaOH solution as electrolyte. This enabled hydrogen permeability, apparent solubility and diffusivity of the substrate, the nitrided layer and the diffusion zone to be determined. The nitriding treatment led to a strong decrease of hydrogen permeability compared with that of the as received steel. Three different phenomenological equations describing plasticity (Hollomon’s, Ludwig’s and Swift’s) were used to study the effect of hydrogen on the deformation stages. Ludwig’s equation was found to be most sensitive to the hydrogen effects. The main effects of hydrogen in the as received steel were to induce an ageing type yield drop and to reduce ductility by about 7%. For the nitrided material a ductility loss of about 70% was observed. In both materials, all deformation stages were affected by hydrogen with stages 2 and 3 more strongly diminished in extent than stage 1. This demonstrated that the entire plastic domain was affected by hydrogen.