Hydrogen diffusion in the trip 800 steel with higher phosphorus content - Effect of deformation

Hydrogen diffusion characteristics were studied in C-Mn-Si TRIP 800 steel containing 0.05 mass % of phosphorus. The steel was tested in as-received state and also in states corresponding to 5 and 10 % tensile deformation. Steel microstructure was analyzed by light microscopy, scanning electron microscopy and X-ray diffraction. Hydrogen diffusion characteristics were studied by means of electrochemical permeation method. Two build up transients corresponding to lower and higher charging current densities as well as a decay transient were recorded during permeation experiments. In the as-received state, steel microstructure consisted of ferrite, bainite and approx. 14 % of retained austenite. The tensile deformation resulted in a decrease of retained austenite content and in a considerable increase of yield and tensile strength. Hydrogen diffusion coefficients of the studied TRIP steel varied from 1.6 x 10-7 cm2-1 to 1.3 x 10-6 cm2-1. The lowest values corresponded to the as-received state - without deformation; the highest ones corresponded to 5 % tensile deformation. For one state, the lowest values of hydrogen diffusion coefficient were observed during the first build up transient. During the second build up transient corresponding to the higher charging current density, hydrogen diffusion coefficients increased markedly reflecting thus the fact that hydrogen trapping was less pronounced. For decay transients hydrogen diffusion coefficients were situated between values obtained for the 1st and 2nd build up transients. Very high sub-surface hydrogen concentrations (up to 35 ppm) were obtained from the permeation results.