Blisters and internal cracks below free surface of a prestrained ASE 4340 steel were ob served after hydrogen was cathodically charged in the steel. Excess large amount of hydrogen was charged in the electrolyte, 4% $H_2SO_4$ solution, to which arsenic oxide was added as the amount of 20 mg per liter of the solution.
It is hypothesized that blistering which is a result of the local plastic deformation is taking place due to a modified triaxial stress state around a void in which hydrogen molecules generate gas pressure. Triaxial stress state can be modified by the external or residual stress to generate the shear component which induces the plastic deformation.
It was observed that the maximum depth of cracks was increased with the amount of cold working, such as tensile, rolling, and bending deformation. To see the possible effect of the microstructural change by the cold works on the crack depth, cold worked specimens were stress-relief annealed and then hydrogen was charged. In these cases, it was observed that the crack depths did not increase as much as those of cold worked specimens with residual stress developed due to cold work. These results give a confirmation of the proposed hypothesis.
It is concluded that blisters and cracks are resulted from hydrogen gas pressure in voids near free surface.
