Carbon plays an important role in controlling the mechanical properties of steels. In this study, the effect of different states of carbon―solute carbon, fine transgranular cementite, or coarse intergranular cementite―on the fatigue properties of Fe-0.017mass%C alloy was investigated. Transmission electron microscopy was used to examine the dislocation structure. Rigorous X-ray line profile analysis was conducted to evaluate the change in dislocation density, dislocation arrangement, and dislocation character as a function of the state of carbon. The fatigue limit ratio (fatigue strength/ultimate tensile strength) under the constant stress amplitude conditions was the highest for the solute carbon specimen, followed by the transgranular cementite and intergranular cementite specimens. The solute carbon specimen exhibited a prominent vein dislocation structure, whereas the transgranular cementite and intergranular cementite specimens showed tangled and cellular dislocation structures, respectively. X-ray line profile analysis revealed that the vein structure presumably consisted of a bunch of edge dislocations, whereas the cell structure mostly consisted of screw dislocations. The fractured surface of the solute carbon specimen showed large asperity and no clear striations, whereas the transgranular and intergranular cementite specimens revealed clear striations. Moreover, large cracks were observed along the striations, presumably owing to the existence of cementite.
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