Characterization of carburized layer on low-alloy steel fabricated by hydrogen-free carburizing process using carbon ions

Hydrogen-free carburized layer was fabricated by irradiating the carbon ions onto a heated low-alloy steel JIS SCM420 substrate using a coaxial arc plasma gun and controlling the diffusion of carbon atoms. The maximum hardness of carburized layer was 733 HV, and the hardness decreased reflecting the carbon concentration dis-tribution in the depth direction of the substrate. The carburized layer was formed to depth of 200 mu m. A martensitic structure was observed in the cross-sectional structure observation. Surface elemental analysis by SEM-EDS did not show segregation of chromium or oxygen to grain boundaries. Hydrogen concentration analysis by thermal desorption spectroscopy revealed that the concentration of diffusible hydrogen, which contributes to hydrogen embrittlement, was 0.0064 wt. ppm. Susceptibility to hydrogen embrittlement test was conducted using the three-point bending slow strain rate tensile test. The hydrogen-free carburized layer exhibited about 2.7 times the strength and about 2.3 times the elongation to the hydrogen charged layer after hydrogen-free carburizing. These results led us to conclude that the hydrogen-free carburizing method is notably effective in preventing hydrogen embrittlement.