Microstructural Evolutions During Reversion Annealing of Cold-Rolled AISI 316 Austenitic Stainless Steel
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TION
OWING to their excellent corrosion resistance, good toughness, and acceptable weldability, austenitic Stainless steels (ASSs) are considered as viable engineering materials. However, their relatively low strength hinders many potential applications of these materials. As a result, various grain refining techniques,[1–8] solid solution strengthening,[9,10] and cold working accompanied by strain-induced martensitic transformation[11–13] have been practiced so far to strengthen these alloys. A thermomechanical treatment based on the formation and reversion of strain-induced martensite in metastable ASSs has been developed for grain refinement.[14–22] Regarding the formation of martensite, the stability of austenite (characterized by its chemical composition and the initial grain size) and the cold-working variables are important factors. Generally,
MEYSAM NAGHIZADEH and HAMED MIRZADEH are with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran. Contact e-mail: [email protected]. Manuscript submitted September 30, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
with increasing the amount of alloying elements, austenite stability with respect to strain-induced martensitic transformation increases,[17,23,24] and hence, the AISI 316 stainless steel[25–31] is more resistant to strain-induced martensitic transformation compared with the AISI 304 and AISI 301 stainless steels.[32–35] While the reversion process has received considerable attentions for enhancement of mechanical properties, the present authors[6] showed that more investigations are required. During annealing of a cold-worked 304 ASS, three stages of martensite reversion, recrystallization of the retained austenite, and grain growth can be identified.[6] At the same time, other researchers have also discussed some of these effects.[36, 37] These stages should be controlled carefully to enable microstructure control during thermomechanical treatment. In another front, the present authors [38] quantitatively showed that the presence of ~ 2 wt pct Mo in AISI 316 stainless steel might severely retard the grain growth phenomenon (and probably the recrystallization process) and this effect is highly temperature sensitive. Therefore, the microstructural evolutions during annealing of the cold-rolled AISI 316 stainless steel might be different from those observed for AISI 304 stainless steel and the annealing stages should be carefully characterized, which is important from the scientific and technological standpoints. The present work aims to deal with this subject.
II.
EXPERIMENTAL MATERIALS AND PROCEDURE
A commercial AISI 316 austenitic stainless steel with the chemical composition (wt pct) of 0.028C-17Cr-10.4Ni1.55Mn-2Mo-0.55Si-0.14Cu-0.02Nb was used in this study. The average grain size of the as-received material was found to be ~ 14 lm. After immersing the sheets in a mixture of water and ice, multi-pass cold rolling with different thickness reductions of 5, 30, 50, and 70 pct was performed to achi
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