Effect of Hydrogen Charging on Low Cycle Fatigue Life and its Dependence on Cementite Morphology

Abstract

Hydrogen deteriorates the strength of steel, and it is well known as hydrogen embrittlement. The susceptibility to this embrittlement is generally increased more with higher strength steels under the usual static loading. Meanwhile fatigue strength is more important in structural use; it is desired to clarify the effect of hydrogen on the properties under cyclic loading. Present paper deals with the effects of hydrogen entry on the strain controlled low cycle fatigue properties of low carbon steel (JIS S10C), which is the most basic kind of steels. With the information about hydrogen in steels, the extent of hydrogen damage in fatigue life and its dependence to cementite morphology were discussed. The total hydrogen content of 0.5–1.5 mass-ppm severely degrades the fatigue life of normalized S10C, which is not so strong and almost insusceptible to hydrogen embrittlement under static loading. The fracture surface is accompanied with fish-eye fracture surface. This is originated at inclusion and propagating with quasi-cleavage and vague striation. The hydrogen gas atmosphere formed around inclusion during cyclic loading is claimed to be the major reason for the degradation. Spheroidizing heat treatment makes the steel free from the degradation. The hydrogen claimed for the degradation of fatigue life is what has been trapped by pearlite or cementite/ferrite interface, because this interface is reduced by the spheroidizing treatment. It is not the hydrogen that was trapped by non-metallic inclusion before fatigue test.