MEYSAM NAGHIZADEH and HAMED MIRZADEHMicrostructural evolutions during annealing of a plastically deformed AISI 304 stainless steel were investigated. Three distinct stages were identified for the reversion of strain-induced martensite to austenite, which were followed by the recrystallization of the retained austenite phase and overall grain growth. It was shown that the primary recrystallization of the retained austenite postpones the formation of an equiaxed microstructure, which coincides with the coarsening of the very fine reversed grains. The latter can effectively impair the usefulness of this thermomechanical treatment for grain refinement at both high and low annealing temperatures. The final grain growth stage, however, was found to be significant at high annealing temperatures, which makes it difficult to control the reversion annealing process for enhancement of mechanical properties. Conclusively, this work unravels the important microstructural evolution stages during reversion annealing and can shed light on the requirements and limitations of this efficient grain refining approach.

Grain size effects on the properties of AISI 304 austenitic stainless steel are studied. Yield stress (YS) and ultimate tensile strength (UTS) increased with decreasing grain size (Hall–Petch law) while the difference between YS and UTS decreased. Three distinct stages for work‐hardening rate are identified: I) initial rapid fall until reaching a minimum value, II) subsequent rise to a maximum due to the transformation‐induced plasticity (TRIP) effect, which is found to enhance by increasing grain size, and III) final fall until the onset of necking. More in‐depth analysis on the mechanical stability of austenite reveals that for below‐average grain size of ≈50 μm, by decreasing grain size, the TRIP effect diminishes; whereas for grain size larger than ≈50 μm, by increasing grain size, the TRIP effect becomes less pronounced. Due to the strong TRIP effect, high incremental work‐hardening exponents (n‐values of higher than 0.8) and low‐yield ratios (smaller than 0.5) are observed. It is also found that when the average grain size increases, the tensile toughness and the size and depth of dimples increase. The latter is responsible for the tardiness of the microcavity coalescence, which is indicative of high ductility of this austenitic alloy.

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Recent progress in the understanding of the deformation-induced martensitic transformation, the transformation-induced plasticity (TRIP) effect, and the reversion annealing in the metastable austenitic stainless steels are reviewed in the present work. For this purpose, the introduced methods for the measurement of martensite content are summarized. Moreover, the austenite stability as the key factor for controlling the austenite to martensite transformation is critically discussed. This is realized by analyzing the effects of chemical composition, initial grain size, applied strain, deformation temperature, strain rate, and deformation mode (stress state). For instance, the effect of initial grain size is found to be complicated, especially in the ultrafine grained (UFG) regime. Furthermore, it seems that there is a critical grain size for changing the trend of α′-martensite formation. Decreasing the deformation temperature motivates the formation of α′-martensite, but there is a critical temperature for achieving the maximum tensile ductility. Afterwards, the modeling techniques for the transformation kinetics and the contribution of deformation-induced martensitic transformation to the strengthening of material and also strength-ductility trade-off are critically surveyed. The processing of UFG microstructure during reversion annealing, the effects of the recrystallization of the retained austenite, the martensitic shear and diffusional reversion mechanisms, and the annealing-induced martensitic transformation are also summarized. Accordingly, this overview presents the opportunities that the strain-induced martensitic transformation can offer for controlling the microstructure and mechanical properties of metastable austenitic stainless steels.

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