Strengthening by annealing of partially deformation-induced transformed microstructure in type 304 stainless steel

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<p>To control the strength-ductility balance of type 304 metastable austenitic stainless steel, it is effective to perform annealing after partially deformation-induced transformation to change the properties of deformation-induced martensite and retained austenite. However, the changes in each microstructure and partitioning of alloying elements after annealing are not necessarily clear. In this study, the dual-phase microstructures of deformation-induced martensite and retained austenite in partially transformed type 304 stainless steel were annealed under different conditions, and the mechanical properties of each phase were investigated. The partially transformed specimen, which was subjected to 30% cold rolling after solution treatment at 1373 K-1.8 ks, has a dual-phase microstructure with approximately 70% deformation-induced martensite and 30% retained austenite. When annealing at 673 and 773 K, the austenite fraction did not change significantly; however, when annealing at 873 K, a reverse transformation occurred, and the austenite fraction increased to approximately 80%. Tensile tests revealed that the cold-rolled specimen exhibited a high yield stress of 1200 MPa and excellent ductility, with a total elongation of 15% owing to the TRIP effect. The yield stress increased at 673 K annealing but decreased at higher annealing temperatures. However, as a result of the nanoindentation tests, the hardness of both martensite and austenite hardly increased, even in the 673 K-annealed specimen, although its yield strength was increased. Therefore, it is presumed that the strength of the interface increased, and the interface became a strong obstacle to dislocation motion.</p>

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