The hot deformation behavior of Alloy 718 was characterized on the basis of the dynamic materials model and compression data in the temperature range of 900 to 1177°C and strain rate range of 0.005 to 5s'. The flow curves at all temperatures and strain rates showed little dependence on strains larger than 0.3. Constitutive equations were used to characterize the dependence of flow stress on strain, strain rate and temperature. A threedimensional distribution of strain rate sensitivity with strain rate and temperature revealed a decrease in sensitivity with an increase in strain rate and a decrease in temperature. A processing (power dissipation) map constructed from the dynamics materials model and the corresponding isoefficient contours at a strain of 0.5 exposed a domain of peak efficiency of 35 % at temperatures of 1132 to 1177°C and the strain rates of 0.05 to 0.5 set", which would be optimum parameters for hot working. These results are in good agreement with previous recrystallization-temperature-time maps reported in the literature. The activation energy for plastic flow in the 900 to 1177°C range was about 483 KJ/mole, and the constitutive relationship between flow stress and temperature compensated strain rate (Zener-Hollomon parameter) was found to be valid in the temperature range of 900 to 1177°C.

With today's requirements for increased productivity and tighter mechanical property and microstructural specifications imposed by customers, a good understanding of microstructural development and control is required. At Into Alloys International Inc. (IAII), a pilot mill and a production mill were used concurrently to process INCONEL alloys 625 and 718. Finite element software was used for increased understanding of thermomechanical processing (TMP) during flat rolling. As-rolled grain size and volume fraction of recrystallization were analyzed. It was determined that finishing temperature plays an important part in recrystallized grain size and that volume fraction of recrystallized grains is dependant on many factors, especially strain rate and temperature.

Technology demand for improved tensile and fatigue properties in the final components requires Alloy 718 producers to develop a fine grain practice for the cogged billet derived from the ingot. The effect of thermomechanical processing on the microstructural refinement of Alloy 718 was studied. The results of this study revealed that under certain thermomechanical processing conditions, substantial microstructural refinement can be obtained. Large portion of the softening associated with the recrystallization process did not occur by the formation of new, strain free grains via the motion of high angle boundaries, but rather, by the formation of dislocation free annealing twins that grow directly into the deformed matrix.

The purpose of this study was to evaluate a 2D finite element forge model. The code incorporates a microstructural model for prediction of recrystallized volume fraction and recrystallized grain size. Because the model assumes plane strain deformation, flat rolling provides a potential method for evaluating the microstructural model. Pilot mill rolling experiments were conducted with deformation temperatures ranging from 925°C (1700°F) to 1150°C (2 1 OO'F) and a maximum draft of -75%. The analysis of results focused on the onset of recrystallization and predictions for partial recrystallization in INCONEL@ alloy 718. The model predicts the onset of recrystallization reasonably well for most temperatures. An accelerated recrystallization rate to a fully recrystallized structure is predicted compared to experimental results.