To study the interaction of hydrogen and surface structure of iron, two types of tensile test are carried out under hydrogen gas environment and cathodic hydrogen charging condition. According to the tensile tests, hydrogen induces reduction of flow stress (softening) for the specimens without deformed surface, but increase of flow stress (hardening) for the one with deformed surface. The results of jump tests signify that hydrogen enhances the dislocation mobility by reducing the thermal activation volume for overcoming barriers, and because of this, in the samples with smooth surface, homogeneously distributed hydrogen leads to the softening effect. On the other hand, the deformed layer just under the surface has larger solubility of H due to trap sites provided by dislocation cell structures. As a result of hydrogen shielding effect, the strong interaction between dislocations in surface layer and multiplication of new defects causes the hardening effect.

Spheroidal graphite cast iron is a material with a wide range of uses such as in automobile parts. By applying active screen plasma nitriding (ASPN) treatment, the use of spheroidal graphite cast iron is expanded, and it can be expected to be used under special load conditions. In this study, we evaluated the effect of ASPN treatment on the mechanical properties of spheroidal graphite cast iron. With ASPN treatment, a nitride layer was formed on the sample surface and a diffusion layer was formed further inside the nitride layer. The thickness of nitride layer increased as the treatment temperature increased. The hardness was improved by ASPN treatment. The abrasion resistance was improved by ASPN treatment, and longer treatment time resulted in higher abrasion resistance. The fatigue strength was improved by ASPN treatment, and longer treatment time resulted in higher fatigue strength. ASPN treatment also improved the corrosion resistance.