Development of Ultra-Fine-Grained Structure in AISI 321 Austenitic Stainless Steel
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NTRODUCTION
AISI 321 is a titanium (Ti)-stabilized austenitic stainless steel (ASS) that is used widely in chemical process plants[1] and fabrication of pressure vessels[2] where excellent corrosion resistance and good mechanical strength are important considerations in materials selection. Ti in the steel reacts with carbon to form titanium carbide (TiC) precipitates thereby bypassing the precipitation of chromium carbides along the grain boundaries (sensitization) at elevated temperatures.[3] Despite its attractive corrosion resistance, AISI 321 steel is characterized by low yield strength and low hardness, which leads to very poor tribological properties.[1,4] This has, therefore, led to the interest in the development of nano-/ultra-fine-grained (UFG) structure in stainless
A.A. TIAMIYU, J.A. SZPUNAR, A.G. ODESHI, and I. OGUOCHA are with the Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, Canada. Contact e-mail: [email protected] M. ESKANDARI is with the Department of Materials Science & Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran. Manuscript submitted April 6, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
steels in order to obtain high strength and good ductility.[4] Several severe plastic deformation (SPD) techniques such as high-pressure torsion (HPT), equal channel angular pressing (ECAP), and cyclic extrusion compression[5–7] are used to develop ultra-fine-grained structure in metallic alloys. However, repetitive application of conventional thermo-mechanical processing is reported to be very suitable for developing UFG structure in steels.[8] This process involves conventional cold or cryogenic rolling (cryo-rolling) followed by annealing at appropriate temperature.[5,8] The relatively low accumulated strain required in this process (about 1 to 3.6 in comparison to 3 to 6 required in SPD) is an advantage over other SPD techniques. This suggests that the thermo-mechanical process of developing UFG structure is amenable to large-scale production and the process parameters can easily be optimized, whereby, combined benefits of phase transformation and controlled cooling are exploited.[8] The a¢-martensite start temperature (Ms) of AISI 300-series austenitic stainless steels depends on the alloy content as expressed in Eq. [1]. However, plastic deformation can introduce the necessary energy for martensitic transformation, which increases the
martensite formation temperature to Md. Md is the temperature, below which martensite will evolve during deformation. The martensite produced via plastic deformation is called strain-induced martensite (SIM). Angel[9] studied the effect of chemical composition on the stability of austenite in steels and established an expression for Md30/50, which is the temperature (in C) at which 50 pct of austenite will transform to a¢-martensite at 30 pct true strain. The expression was subsequently modified by Nohara et al.[10] to incorporate the effect of austenite grain size as stated in Eq. [2]. Ms ð
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