Measurement and prediction of hydrogen embrittlement in high strength carbon steel

Abstract

Hydrogen embrittlement tests were performed on 0·254 mm diameter BS 5216 M4 high strength carbon steel wire using constant loads to give initial tensile stresses in the range 48–91% ultimate tensile stress. The wires were electrolytically charged with hydrogen in 4%H2SO4 at current densities of 75 and 150 mA cm–2. The failure times at each applied stress and charging rate were displayed on Weibull statistical plots and shown to correlate with a diffusion model of hydrogen transport. At high stresses, crack initiation occurred rapidly and the failure time was controlled by the rate of inward hydrogen diffusion to maintain a threshold concentration for crack propagation. At low applied stresses, crack initiation required a higher hydrogen concentration and occurred more slowly. In this case, the failure time was controlled by the size and location of the significant microstructural flaw at which crack initiation occurred. The model enabled failure times to be predicted in specimens with differing dimensions.