Microstructural control in hot working of IN-718 superalloy using processing map
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I.
INTRODUCTION
THE nickel-base superalloy IN-718"
is used for sev-
*IN-718 is a trademark of Huntington Alloys, Huntington, WV.
oral critical gas-turbine components, many of which are hot-forged. For obtaining required low-cycle fatigue and fracture properties, it is essential that the microstructure is controlled at the processing stage. For example, in the turbine disc application, a fine-grained structure is preferred for which the technology of forging and thermomechanical processing are described. [L2] The hot ductility of IN-718 tested under tension in the temperature range of 1000 0(2 to 1050 ~ reaches its peak value at a strain rate of about 2.5 s-l. t3] Recently, Chaudhury et al. [4] have developed a processing map for hot deformation of IN-718 in the temperature range of 975 ~ to 1150 ~ and in the strain-rate range of 0.01 to 25 s -~ and interpreted the map in terms of dynamic recrystallization (DRX) mechanism and phase changes occurring in the material. As the commercial forging practice involves initial forging at high temperatures (>1150 ~ and finish forging below 980 ~ to obtain fine-grained structures, it will be beneficial if data are obtained in wider temperature and strain-rate ranges. The purpose of the present investigation is to evaluate the hot deformation behavior in wide temperature and strain-rate ranges and generate a processing map for hot working of IN-718 with a view to optimizing its workability and microstructure during processing. Processing maps are developed on the basis of the dynamic materials model, tS] which is reviewed by Gegel et al. I6] and Alexander. t71The model considers the N. SRINIVASAN, Graduate Student, and Y.V.R.K. PRASAD, Professor and Chairman, are with the Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India. Manuscript submitted October 26, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
workpiece as a dissipator of power, and the instantaneous power dissipated at a given strain rate (~) consists of two complementary parts: G content and J cocontent representing the temperature rise and microstructural dissipation, respectively. The factor that partitions power between J and G is the strain-rate sensitivity (m) of flow stress (Or). The J cocontent is given by tS]
where ~ is strain rate. For an ideal linear dissipator, J = Jn~x = Or. g / 2 and the efficiency of power dissi-
pation of a nonlinear dissipator may be expressed in terms of a dimensionless parameter: 7/
J J~
2m (m + 1)
[2]
The variation of ,/with temperature and strain rate constitutes the power dissipation map, the domains of which may be interpreted in terms of specific microstructural processes. The extremum principles of irreversible thermodynamics as applied to large plastic flowtsl axe applicable to the dynamic materials model. Kuma~9] and Prasad ~176 combined these principles with those of separability of power dissipation and obtained a continuum criterion for obtaining flow instability during hot deformation, and it is given by ~(~) =
0 In ( m / m + 1) +
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