The effects of hydrogen on Charpy impact properties of JIS-SGP carbon steel pipe for hydrogen pipelines were investigated. The brittle-ductile transition temperature was not changed by hydrogen charge. The Charpy impact values above the brittle-ductile transition temperature were decreased 20〜40J/cm^2 by the effect of hydrogen. The brittle fracture area of hydrogen-charged specimens was smaller than that of uncharged specimens. The elongated dimple area near the notch of hydrogen-charged specimens was larger than that of uncharged specimens. It is presumed that, the basic mechanism of so-called hydrogen embrittlement was produced by the enhancement of localized plasticity rather than lattice decohesion.
