Optimization of cold and warm workability in 304 stainless steel using instability maps
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I.
INTRODUCTION
C O L D and warm working techniques offer great potential in the area of near-net-shape and net-shape manufacturing technology.[~] The intrinsic workability of the materials is inherently lower in the cold and warm working regions than in the hot working regime. Besides, some materials may exhibit processes like dynamic strain aging (DSA), formation of new phases, adiabatic shear deformation, flow localization, and void generation, all of which decrease the ductility of the materials. The DSA phenomenon can give rise to highly localized deformation bands, tz] The adiabatic shear produces shear zone and internal cracks, which become the sites for eventual failure of the components during forming as well as in service.t3] The previous phenomena may also introduce inhomogeneous deformationt4,5,6] and produce components possessing microstructural defects like nonuniform microstructuretTl and shear bands, ts,91 Inhomogeneous deformation may also be caused by friction and tool geometry, and these may be termed as geometrical. These inhomogeneities are detrimental to the mechanical properties of the product, in particular, the ductility, fracture toughness, and fatigue crack growth rates. It is therefore necessary to avoid the processing regimes where flow instabilities are likely to occur so as to achieve a large deformation ratio and to obtain defectfree components with desired dimensional accuracy. For identifying the instability regimes, two criteria are developed. These include (1) phenomenological criterion based on the strain hardening and strain-rate sensitivity suggested by Semiatin and co-workers t8,9,~~ and (2) continuum criteriat11,12,131based on the principle of maximum entropy production proposed by Ziegler. t~4] The advantage of the
S. VENUGOPAL, Scientific Officer, on leave from the Materials Development Division, Indira Gandhi Centre for Atomic Research, is NRC-USAF Resident Research Associate, Materials Process Design, Materials Directorate, Wright Laboratory, Wright Patterson Air Force Base, OH 45433-7746. S.L. MANNAN, Head, is with the Materials Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Y.V.R.K. PRASAD, Chairman, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted December 15, 1994. METALLURGICALAND MATERIALS TRANSACTIONS A
continuum criteria is that the approach is fundamental and the criteria are derived based on the principles of irreversible thermodynamics. The criterion developed by Prasadtt2] and Kumafl~3~ is based on the irreversible thermodynamics of large plastic flow proposed by ZieglerY 41 According to the principle of maximum rate of entropy production, a system undergoing large plastic deformation will be unstable if the rate change in the dissipation function (D(k)) with strain rate (k) satisfies the inequality
dD
D(k) < --U
[1]
The dissipation function is the characteristics of the constitutive behavior of the workpiece. It is well known tll,ls
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