Hydrogen diffusion characteristics in the TRIP 800 C-Mn-Si steel

The presented paper is devoted to the study of hydrogen diffusion characteristics in the CMn- Si TRIP 800 steel. The steel was tested in three different states: in as-received state after hot and cold rolling and subsequent heat treatment; and furthermore after 5% and 10% tensile deformation. 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 experiment. The lowest values of hydrogen diffusion coefficient (from 1 to 3.5.10^-7 cm².s^-1) were observed during the first build up transient; the value of 3.5.10^-7 cm2.s^-1 corresponded to 10% tensile deformation. During the 2^nd 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 1^st and 2^nd build up transients. In all studied states, a rather high sub-surface hydrogen concentration was observed during the 1^st build up transient rising to 12.6 ppm of hydrogen in as-received state. The obtained results are explained taking into account steel microstructure and hydrogen trapping.