INTERNAL HYDROGEN EMBRITTLEMENT OF A FERRITIC STAINLESS-STEEL

AL 29-4-2 ferritic stainless steel was cathodically precharged with hydrogen at 230 degrees C in a molten salt electrolyte. Constant load crack growth tests were performed in air and in hydrogen gas at 108 kPa pressure at room temperature on both uncharged specimens and specimens containing 2 wt ppm hydrogen. The DC potential drop method was calibrated with optical measurements to continuously monitor the crack position, so that the crack velocity vs stress intensity relation could be calculated. From the test results, it was found that dissolved hydrogen does allow subcritical crack growth for tests in air, but its effect is masked by the external hydrogen for tests in hydrogen gas. The distribution of slip bands on the side surface indicates that the dissolved hydrogen causes localization of plasticity, with the strain concentrated in coarse slip bands. Comparing the embrittling effect of internal hydrogen with that of external hydrogen, it can be concluded that environmental hydrogen is far more damaging than internal hydrogen, at least in specimens containing 2 wt ppm hydrogen.