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Effects of carbon on the low-cycle fatigue behavior of 20Cr-15Ni and 15Cr-15Ni stainless steels have been investigated at room temperature. Thin foils sliced from the fatigue tested specimens were observed with electron microscopy. Single crystals produced by remelting some of the steels were deformed by compression, and sliced in the predetermined directions for electron microscopy of dislocation structure. The cooperative effects of carbon and silicon addition on proof strength, cyclic deformation behavior and dislocation configurations were also studied. It was shown that carbon induced fatigue softening as reported previously in a 0.18C-25Cr-20Ni steel. The softening was observed even at the small number of cycles, and was enhanced with an increase in carbon content and a decrease in strain amplitude. Carbon also inhibited the dislocations to form cellular configurations. It was revealed that such effects of carbon were enhanced by silicon addition and that silicon increased the solid solution hardenability of carbon. All these results were qualitatively consistent with the concept that solid solution hardening by some complexes between carbon and chromium atoms existed in the as-solution treated condition and broke down during cyclic deformation resulting in fatigue softening.

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