Flow localization during plane strain punch stretching of a ferrite-austenite steel
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INTRODUCTION
THIS paper reports a study of plastic flow localization during sheet forming of a relatively new duplex stainless steel. DuPlex steels contain a mixture of ferrite and austenite grains. In contrast to fully austenitic or fully ferritic stainless steels, duplex steels are more highly alloyed, and have higher strength and greater resistance to certain types of corrosive environments. ~-5 Accompanying the higher strength is reduced sheet formability, particularly in situations requiring material to thin under in-plane loading. As an example, the material of this study, FERRALIUM | alloy 255, has a | is the registered trademark of Bonar Langley Alloys, United Kingdom.
yield strength which is approximately three times that of 304 stainless steel, but a tensile elongation which is only half as large. While the increased strength is certainly a desirable property, it can be expected that the decreased elongation of the alloy may limit stretchability. Alloy 255 contains ferrite and austenite grains in approximately equal volume fraction. This proportion is achieved by a balance of alloying additions and by control of conditions during processing. A microstructure of delta ferrite forms during solidification. 6 Partial transformation to austenite occurs during cooling to produce pools of austenite in a ferrite matrix. During hot rolling and annealing the microstructure remains within the two-phase region of the phase diagram. 7 A fraction of the austenite and ferrite grains can dissolve or precipitate as temperature changes during processing, but it is not possible to replace fully the structure left from prior operations. Instead, one must use mechanical J.E. BIRD is Assistant Professor, School of Materials Engineering, Purdue University, West Lafayette, IN 47907. T. POLLOCK, formerly Student in the School of Materials Engineering, Purdue University, is Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. S.K. SRIVASTAVA is Research Associate, Technology Department, Cabot Corporation, 1020 West Park Avenue, Kokomo, IN 46901. Manuscript submitted December 17, 1984. METALLURGICALTRANSACTIONS A
working in conjunction with thermal treatments to control the distribution of austenite and ferrite. Microstructures produced by mill processing contain multigrain islands of finegrained austenite dispersed in a ferrite matrix. Thus, while the microstructure is composed of fine grains of two phases, it is not microduplex in the sense that austenite and ferrite grains are intimately mixed. Figure 1 shows a microstructure similar to that developed during mill processing. Austenite grains (7) are distinguishable by their smaller size and elongated shape. Ferrite grains (a) appear larger and more blocky. Austenite islands lie in the rolling plane. One purpose of this investigation is to determine whether a more uniform distribution of austenite and ferrite might improve the forming properties of the alloy. Laboratory thermomechanical processing wa
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