Effect of carbon content on hydrogen occlusivity and embrittlement of ferrite–pearlite steels

Experiments on a series of pure Fe–C alloys consisting of ferrite and pearlite only have shown that the ferrite/pearlite and pearlite/pearlite interfaces are effective hydrogen trapping sites. The ferrite/cementite interfaces within the pearlite colonies, however, have little effect on the hydrogen occlusivity. With an increase in carbon content, more ferrite/pearlite interfaces are created and these increase the hydrogen occlusivity. Although the ferrite/cementite lamella interface has little effect on the hydrogen occlusivity, it does appear that the lamellae interfere with the hydrogen diffusion path across the pearlite colonies. Thus, the higher-carbon alloys in the pearlitic condition have a lower apparent hydrogen diffusivity. Hydrogen has little effect on the tensile strength, but significantly reduces the ductility. After hydrogen charging, high-carbon alloys suffer a lower ductility loss. However, in terms of absolute values, the low-carbon specimens are always more ductile than the high-carbon alloys when saturated with hydrogen.