The effect of hydrogen on the tensile mechanical properties was examined fo
r 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 nit
rided at 773 K during 16 h generating a white layer 5 mu m thick and a diff
usion zone measuring about 550 mu m. Electrochemical tests were performed t
o obtain hydrogen permeation curves at 323 K, using a 0.1 N NaOH solution a
s 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 stage
s. 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 a
geing 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, al
l deformation stages were affected by hydrogen with stages 2 and 3 more str
ongly diminished in extent than stage 1. This demonstrated that the entire
plastic domain was affected by hydrogen. (C) 2000 Elsevier Science S.A. All
rights reserved.