Visualization of hydrogen diffusion promoted by stress gradient and plastic deformation in steels

Delayed fracture in steel has been correlated with concentration of hydroge n to the point of crack initiation. In the present study, effects of plasti c deformation and stress gradient on hydrogen diffusion in carbon steels we re studied with hydrogen microprint technique (HMT), which can visualize po ints of hydrogen emission as silver particles superposed on the microstruct ure. Three kinds of carbon steels, 0.002% C steel, 0.45% C steel, and 0.85% C steel were prepared and behavior of hydrogen transport was studied durin g tensile deformation and under bending stress. Hydrogen was transported to the surface with gliding dislocations during tensile plastic deformation i n ferrite. In pearlite, however, behavior of hydrogen, which was transporte d along carbide or carbide-ferrite interfaces, was irrelevant to tensile pl astic deformation. Behavior of hydrogen transport under various stress grad ient was also studied in 0.002% C steel. Stress was applied by using a four -point bending tool, and hydrogen emission was visualized in the area appli ed with maximal tensile stress. It was demonstrated that even elastic stres s gradient promoted hydrogen transport and the amount of hydrogen transport was increased with an increase in stress gradient. Hydrogen transport in t he plastically bent specimen is expected to be based on the mixed mechanism , including hydrogen transport by moving dislocation, hydrogen diffusion by stress gradient, and hydrogen diffusion along dislocation core. These fact ors were separately visualized by applying HMT to the specimens prepared in appropriate testing conditions.