Austenitic Reversion of Cryo-rolled Ti-Stabilized Austenitic Stainless Steel: High-Resolution EBSD Investigation
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JMEPEG (2018) 27:889–904 https://doi.org/10.1007/s11665-018-3180-6
Austenitic Reversion of Cryo-rolled Ti-Stabilized Austenitic Stainless Steel: High-Resolution EBSD Investigation A.A. Tiamiyu, A.G. Odeshi, and J.A. Szpunar (Submitted July 24, 2017; in revised form October 26, 2017; published online January 23, 2018) In this study, AISI 321 austenitic stainless steel (ASS) was cryo-rolled and subsequently annealed at 650 and 800 °C to reverse BCC a¢-martensite to FCC c-austenite. The texture evolution associated with the reversion at the selected temperatures was investigated using high-resolution EBSD. After the reversion, TiC precipitates were observed to be more stable in 650 °C-annealed specimens than those reversed at 800 °C. {110} texture was mainly developed in specimens subjected to both annealing temperatures. However, specimens reversed at 650 °C have stronger texture than those annealed at 800 °C, even at the higher annealing time. The strong intensity of {110} texture component is attributed to the ability of AISI 321 ASS to memorize the crystallographic orientation of the deformed austenite, a phenomenon termed texture memory. The development of weaker texture in 800 °C-annealed specimens is attributed to the residual strain relief in grains, dissolution of grain boundary precipitates, and an increase in atomic migration along the grain boundaries. Based on the observed features of the reversed austenite grains and estimation from an existing model, it is suspected that the austenite reversion at 650 and 800 °C undergone diffusional and martensitic shear reversion, respectively. Keywords
carbide precipitation, EBSD, stainless steel, straininduced phase transformation, texture memory, thermo-mechanical processing
1. Introduction Austenitic stainless steels (ASS) have a face-centered cubic (FCC) crystal structure and low stacking fault energy (SFE) (Ref 1). They possess low yield strength, which restricts their structural applications in many areas where their excellent corrosion resistance is desired (Ref 2). The thermodynamically metastable austenitic microstructure in ASS undergoes partial transformation to a martensitic microstructure during deformation below the Md temperature (Ref 2, 3). Md is the temperature below which martensite will evolve during deformation, while the martensite produced by this process is called strain-induced martensite (SIM or a¢-martensite). This phenomenon is therefore harnessed to improve yield strength of ASS via the development of ultrafine grain (UFG) structure. A thermomechanical process involving conventional cold [optimal temperature, 84 C (Ref 4)] or cryogenic [temperature of the order of 150 C (Ref 4)] rolling followed by annealing (Ref 5, 6) is used to fabricate ultrafine-grained steel and is considered a very suitable process for industrial application (Ref 6, 7). Other techniques such as high-pressure torsion, equal channel angular pressing, and cyclic extrusion compression (Ref 5, 8, 9) have been developed for obtaining ultrafine A.A. Tiamiyu, A.G. Odes
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