Dependence of the hydrogen permeation in stainless steel on carbon content, heat treatment and cold work

The hydrogen permeation behaviour in austenitic stainless steel (1.4301, AI SI 304) was studied between 25 to 85 degrees C using the electrochemical do uble cell permeation technique. Influences of parameters like carbon conten t, grain size and cold work were investigated. Using the current transient plots from the permeation experiments, apparent diffusion coefficient, D-ap p values were calculated. Values ranging from 5.8.10(-12) to 2.7.10(-10) cm (2)/s were obtained for various conditions of the material. The increase in carbon mass content from 0.045 to 0.085 % resulted in a decrease in D-app by a factor of 8. This was attributed to the increased blocking of intersti tial sites by higher carbon in the latter steel. The annealing treatment, r esulting in a slight grain growth caused a decrease in D-app to about its h alf, in contrast to the reduced trapping effect of the grain boundaries. Th is was explained to be due to a longer hydrogen transport path required thr ough the grain and less contribution of fast diffusion paths in the larger grain size material. A slight cold work (5% reduction in thickness) decreas ed the D-app whereas higher cold working (30 %) increased the D-app. The co ld working results in an increase in dislocation density and the dislocatio ns act as traps for hydrogen transport. This results in a decrease in D-app values for 5 % cold work material. Strain induced martensite (alpha', bcc phase) formation in the matrix occurred in the higher cold worked material (10 and 30%). The presence of this bcc phase overcame the trapping effect o f increased dislocation density resulting in an enhancement of hydrogen tra nsport. From the activation energy calculated, it was concluded that the al pha' phase present did not provide a continuous medium for hydrogen transpo rt, but added to the overall increase in the hydrogen transport process.