Substructural changes during hot deformation of an Fe-26Cr ferritic stainless steel
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I. INTRODUCTION
DYNAMIC recovery and recrystallization can occur during hot deformation of metals. In general, alloys with high stacking fault energies such as Al alloys and ferritic Fe alloys soften mainly by dynamic recovery; those with low stacking fault energies such as Ni, Cu, and austenitic Fe alloys can undergo both dynamic recovery and recrystallization.[1–4] However, exceptions can be found. For example, dynamic recrystallization in a Mg alloy with a high stacking fault energy was reported.[5] For ferritic Fe alloys, it has been suggested that a limiting value of the Zener–Hollomon parameter, Z (Section III-A for its definition), exists below which dynamic recrystallization operates although dynamic recovery would otherwise be dominating.[6] Dynamic recrystallization was observed in a high-purity a-Fe,[7] a Fe-16Cr ferritic steel,[8] and a Fe-26Cr ferritic steel[9] at higher deformation temperatures and lower strain rates. Limited dynamic recrystallization also occurred at a lower temperature in a Fe-25Cr ferritic steel.[10] Some dynamic recrystallization took place in a Fe-17Cr ferritic steel at very high strain rate and high temperature.[11] Dynamic recrystallization in a Fe11Cr ferritic steel was not observed at lower temperatures, although some evidence of geometric dynamic recrystallization was shown at higher temperatures.[12] In a study on the ferrite softening in a duplex Fe-21Cr-10Ni-3Mo steel,[13] the misorientations between subgrains in the ferrite phase increased to as much as 20 deg, although the process was termed “extended dynamic recovery” and it was argued that dynamic recrystallization did not occur. In the present study, some special etching techniques were used to reveal the substructures of a hot-deformed Fe-26Cr ferritic steel with a view to providing more experimental evidence about the mechanism of substructure evolution in FEI GAO, Lecturer, and BAOYUN SONG, Professor, are with Dalian Railway University, Dalian, Liaoning 116028, People’s Republic of China. YOURONG XU, Professor, is with Shanghai University, Jiading, Shanghai 201800, People’s Republic of China. KENONG XIA, Senior Lecturer, is with the Department of Mechanical and Manufacturing Engineering, University of Melbourne, Parkville, Victoria 3052, Australia. Manuscript submitted February 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
such an alloy. A schematic model was established to describe the substructural changes.
II. EXPERIMENTAL MATERIAL AND PROCEDURES The chemical composition (weight percent) of the steel used in this investigation is as follows: Fe-25.7Cr-0.84Si0.35Ti-0.29Mn-0.07C. The material was forged at 850 8C to 1000 8C into bars of 15 mm in diameter and then annealed at 850 8C for 1 hour. These forged and annealed bars were machined into specimens of 8 mm in diameter and 12 mm in length, with shallow grooves of 0.05 mm in depth at each end. A glass powder was put in the grooves as lubricant to ensure homogeneous deformation. Hot deformation was performed in compression at temperatures between 900 8C
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