Microstructural Evolutions During Annealing of Plastically Deformed AISI 304 Austenitic Stainless Steel: Martensite Reve
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AUSTENITIC stainless steels (ASSs) have attracted a considerable attention due to their versatile features such as excellent corrosion resistance, good toughness, and acceptable weldability. However, the strength of these steels is relatively low in the annealed state, which is a drawback for many potential applications. As a result, various methods such as grain refinement,[1–4] solid solution strengthening,[5] and work hardening[6] have been practiced so far. Among these mechanisms, grain refinement can be exploited to improve strength as well as toughness. Since these steels do not have any notable phase transformations during cooling, the static and dynamic recrystallization processes were conventionally used to refine their grain size.[3,7] However, these processes are not very effective compared with severe plastic deformation techniques.[8] As a result, a heuristic thermomechanical treatment has been developed, which is based on the formation and subsequent reversion of strain-induced martensite (SIM) in metastable ASSs.[9–15] Austenite stability (chemical composition and the initial grain size)
MEYSAM NAGHIZADEH, Master Student, and HAMED MIRZADEH, Assistant Professor, are with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran. Contact e-mail: hmirzadeh@ ut.ac.ir Manuscript submitted March 31, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
and cold working variables (temperature, strain, strain rate, and stress state) play critical roles on the formation of martensite and its amount.[1,9,11] The martensite phase is not stable at elevated temperatures and its reversion to fine-grained austenite can occur by annealing at elevated temperatures.[1,16] While the reversion process has received a considerable attention in recent years for enhancement of mechanical properties,[1,17–22] much more works are required on this subject to identify the different stages of microstructural evolution to enable its control during thermomechanical treatment. Moreover, it has been reported that for obtaining a marked grain refinement,[1,11] the availability of great amounts of martensite before reversion might be required. However, this point needs further research. The present work aims to deal with these subjects.
II.
EXPERIMENTAL DETAILS
A. Processing A commercial AISI 304 stainless steel with the chemical composition shown in Table I was used in this study. The average grain size of the as-received material was found to be ~23 lm. After immersing the sheets in a mixture of water and ice, multi-pass cold rolling with different reduction in thicknesses was performed to achieve various amounts of strain-induced martensite. Subsequently, the 70 pct cold-rolled sheets were annealed at temperatures of 1023 K and 1273 K (750 C and 1000 C) for different soaking times to investigate the evolution of microstructure.
Table I. Element Weight Percent
Chemical Composition of the As-received Material
C
Si
Mn
Cr
Ni
Mo
Cu
Nb
Fe
0.044
0.479
1.65
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