Optimization of hot workability in stainless
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I.
INTRODUCTION
THE hot working characteristics of austenitic stainless steels have been studied extensively using torsion, tension, and compression techniques, and the influence of hot deformation parameters on the hot workability and the development of microstructure has been rep o r t e d . [I-4] The hot ductility is higher in the regime of dynamic recrystallization (DRX), which occurs in the temperature range of 1100 ~ to 1200 ~ and torsional strain-rate range of 0.1 to 5 s -~. The apparent activation energy for hot deformation was estimated to be in the range of 347 to 508 kJ/mol. Although these studies have led to the understanding of the mechanisms of hot deformation, they cannot be directly used for the optimization of hot workability. The aim of the present investigation is to develop a processing map for hot working of 304L stainless steel and use it for the optimization of hot workability and controlling the microstructure. The constitutive behavior of 304L stainless steel has been studied in detail by Semiatin and Holbrook tS] using hot torsion and hot compression. The material undergoes flow localization at higher strain rates and lower temperatures, while DRX occurs above 1000 ~ The influence of forging speed on the microstructure of 304L has been investigated by Mataya et al.,[61 and processingproperty maps have been generated. Dynamic recovery occurred at lower forging temperature, while softening due to recrystallization was seen at higher strain rates and temperatures. The processing maps are developed on the basis of the principles of the Dynamic Materials Model, tT~ which is reviewed by Gegel et al.tS] and Alexander.t9] In this model, the workpiece under hot working conditions is considered to be a dissipator of power. At any instant, the power
dissipation occurs through a temperature rise (G content) and a microstructural change (J co-content), and the power partitioning between these two is decided by the strainrate sensitivity (m) of flow stress (o'). At a given temperature and strain, the J co-content is given by tTl m
J = ~ m+l
METALLURGICAL TRANSACTIONS A
[1]
where g is the strain rate. The J co-content of the workpiece, which is a nonlinear dissipator, is normalized with that of an ideal linear dissipator (m = 1) to obtain a dimensionless parameter called efficiency of power dissipation:
.
J . . Jm=
2m . m + 1
[2]
The variation of ~/with temperature and strain rate constitutes a processing map. The various domains in the map may be correlated with specific microstructural processes and applied for microstructural control. The Dynamic Materials Model has its basis in the extremum principles of irreversible thermodynamics as applied to large plastic flow described by Ziegler. ~176 Kumar tm and Prasad ml developed a continuum criterion, combining these principles with those of separability of power dissipation, and have shown that flow instability will occur during hot deformation if
~(g) = S. VENUGOPAL, Scientific Officer, and S.L. MANNAN, Head, are with the Materials Developme
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