Abstract The effects of atomic hydrogen and flaking on mechanical properties of a wheel steel have been investigated. The results show the critical diffusible hydrogen concentration for forming flaking is C*0=1.3×10-6（or total hydrogen concentration C*T=3.7×10-6）. If the amount of the flacking exceeds a critical value,e.g., C0≥5.6×10-6, it causes the plasticity and strength to decrease sharply. For a wheel, the amount of flaking is below the critical value because of C0<3.9×10-6,thus there is no effect of flaking on tensile properties and fracture toughness KIC. Atomic hydrogen doesn’t influence fracture toughness, but causes plasticity to decrease evidently during slow strain rate tension. Atomic hydrogen can also causes hydrogen –induced delayed cracking under constant deflection loading and the threshold stress intensity factor of hydrogen –induced delayed cracking, KIH, decreases linearly with increasing diffusible hydrogen concentration i.e., KIH＝57.8-3.9 C0. However there is no effect of the flaking on hydrogen-induced delayed cracking
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