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Dislocations in austenitic and ferritic stainless steels (SSs) under cyclic loading were quantitatively evaluated via X-ray diffraction line-profile analysis to determine the relationship between the dislocation density and low-cycle fatigue (LCF) life in both SSs. The dislocation density of the austenitic and ferritic SSs varied linearly with respect to the LCF life in a double-logarithmic graph, with different slopes of the line. The dislocation density normalized by the maximum work hardening for both SSs exhibited a log-log linear relationship with the LCF life. The fraction of screw dislocations in the ferritic SS decreased with decreasing LCF life owing to the easy cross-slip of dislocations. Because of the difficulty of the cross-slip of dislocations in the austenitic SS, the fraction of screw dislocations remained almost constant throughout the LCF life. Analysis of the crystallite size and the dislocation arrangement with respect to the dislocation density under tensile and cyclic loading revealed that the dislocation arrangement for cyclic loading was smaller than that for tensile loading. Thus, the dislocation arrangement was related to the cyclic loading. In the plot of the dislocation evolution versus the number of cycles, two stages were observed in the variation of the dislocation characteristics for both SSs. In the first stage, the dislocation density increased, and the crystallite size decreased. The dislocation arrangement parameter of the ferritic and austenitic SS decreased and remained the same, respectively, in the first stage. In the second stage, the dislocation density, dislocation arrangement parameter, and crystallite size remained constant.

Dislocation characteristics and deformation‐induced martensite (DIM) transformation in an asymmetrically cyclic‐loaded metastable type 304 stainless steel (SS) are investigated compared with a stable type 316SS using EBSD and X‐ray diffraction line‐profile analysis. In low‐cycle fatigue (LCF) regime, differences between dislocation densities of type 304SS and type 316SS are increased with decreasing fatigue life, and contribution of α′‐martensite to work hardening of cyclically loaded type 304SS increased with decreasing fatigue life. However, the contribution of austenite slightly changed. Higher work hardening via DIM transformation improves the fatigue life of type 304SS than that of type 316SS in LCF regime in the same fatigue stress amplitude. Thus, the contribution of α′‐martensite to work hardening is a determinant parameter, affecting the fatigue behavior of type 304SS. In HCF regime when the α′‐martensite had a small contribution to the work hardening, the fatigue life of type 304SS is slightly higher than that of type 316SS.

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