Abstract
Delayed fracture in steel has been correlated with concentration of hydrogen to the point of crack initiation. In the present study, effects of plastic deformation and stress gradient on hydrogen diffusion in carbon steels were studied with hydrogen microprint technique (HMT), which can visualize points of hydrogen emission as silver particles superposed on the microstructure. 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 during tensile deformation and under bending stress. Hydrogen was transported to the surface with gliding dislocations during tensile plastic deformation in ferrite. In pearlite, however, behavior of hydrogen, which was transported along carbide or carbide-ferrite interfaces, was irrelevant to tensile plastic deformation. Behavior of hydrogen transport under various stress gradient was also studied in 0.002% C steel. Stress was applied by using a fourpoint bending tool, and hydrogen emission was visualized in the area applied with maximal tensile stress. It was demonstrated that even elastic stress gradient promoted hydrogen transport and the amount of hydrogen transport was increased with an increase in stress gradient. Hydrogen transport in the 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 factors were separately visualized by applying HMT to the specimens prepared in appropriate testing conditions.
