Formation of Nanocrystalline Structure in 301 Stainless Steel Produced by Martensite Treatment
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TENITIC stainless steels are more commonly characterized by favorable ductility and excellent overall corrosion resistance. However, they possess relatively low yield strength (250 to 350 MPa),[1] which limits their structural applications. There has been a growing interest in developing nano-/ultra-fine-grained stainless steels in order to achieve high strength and good ductility.[2–6] Austenitic stainless steels contain thermodynamically metastable austenite at room temperature and are easily transformed into martensite (strain-induced martensite (SIM)) with deformation below the Md temperature. The volume fraction of SIM increases with increasing strain. At a specific strain, called saturating strain (es), martensite formation becomes saturated. The fragmentation of martensite occurs during deformation and increases lattice defects inside SIM. Finally, the martensite is reverted to austenite during subsequent annealing, leading to a noticeable grain refinement. This thermomechanical process is called martensite treatment. The volume fractions of martensite and es play important roles in achieving the nano-/ultra-fine-grained structure M. ESKANDARI, Master of Science of Materials Engineering, A. KERMANPUR, Associate Professor, and A. NAJAFIZADEH, Professor, are with the Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. Contact e-mail: [email protected] Manuscript submitted January 5, 2009. Article published online July 23, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
in this treatment.[7] These two parameters also depend on other process variables including deformation conditions (strain, strain rate, strain path, and rolling temperature), austenite stability (composition and initial grain size), and primary dislocation density.[8–12] The volume fraction of the resulting austenite and the extent of its refinement are also dependent upon the volume fraction of SIM and the value of es. Decreasing es facilitates the process of grain refinement due to the greater amount of martensite fragmentation provided. The martensite fragmentation produces large numbers of crystal defects in the structure and provides nucleation sites for austenite during reverse transformation. A number of researchers have tried to produce 301 stainless steels with nano-/ultra-fine-grained sizes ranging from 0.3 to 5 lm using cold rolling followed by annealing.[4,5,13] In all these works, commercial hotrolled plates have been used as the starting materials, while no explicit report is provided on the effects of initial grain refinement. To the best of our knowledge, no work has been so far reported on producing nanocrystalline 301 stainless steels using martensite treatment. The present work deals with a combination of austenite recrystallization during hot rolling for primary grain refinement of the as-cast structure and cold rolling with subsequent annealing for secondary grain refinement. Effects of process parameters including rolling temperature, strain, strain path, and initial grain size are inves
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