The recrystallisation behaviour of IN 718 has been analysed using a Gleeble 1500 thermomechanical simulator. The use of a rapid water quenching system immediately after the compression test has resulted in the ability to separate dynamic behaviour from meta-dynamic. The rate of dynamic recrystallisation has been found to be proportional to temperature and strain, but inversely proportional to strain rate. The nucleation rate of recrystallised grains during deformation has been found to be directly linked to strain rate and temperature. The rate of meta-dynamic recrystallisation appears to be directly proportional to temperature and strain rate, but no new nucleation events were observed after the end of deformation MethodsMaterial was supplied for the experiments by Firth Rixson Forgings Ltd in the form of a forged and rolled billet of IN 718. Cylindrical samples of height 12 mm and diameter 8 mm were spark eroded from the billet and deformed using a Gleeble 1500 thermomechanical simulator.Initially, the effects of temperature and strain rate on dynamic recrystallisation were investigated. Samples were deformed at a temperature of either 980 • C or 1040 • C to predefined strains of 0·09, 0·18, 0·29, 0·41, 0·53, 0·67, and 0·82 at a set die velocity resulting in average strain rates of 0·01, 0·1, 0·45, and 1·1 s −1 followed by an immediate water quench. To investigate meta-dynamic recrystallisation, samples deformed to a strain of 0·82 were also held at the test temperature for times ranging from 1 second to 10 minutes before undergoing a water quench. Each experiment begins with a linear heat-up to test temperature over 2 minutes and then a hold for 1 minute. The tests were followed by a water quench designed to reduce the temperature of the sample as quickly as possible. Figure 1 shows the temperature profile of a typical test, showing the heat-up, hold, and quench. Thermal data is plotted for both the centre of the sample, and either end, measured using thermocouples welded to the sample. It can be seen that, although a large thermal gradient exists during the heating stage, after a 1 minute hold period, the sample is at a uniform temperature. Referring to Figure 1 (b), it can be seen that the water quench stage lowers the temperature of the specimen extremely rapidly. It takes only 1·1 seconds for the sample to reach 200 • C, and less than 0·2 seconds for the centre of the sample to fall below 800 • C (a temperature below which the rate of recrystallisation is almost zero).

A dislocation density based state variable model has been developed to describe the characteristic flow stress behaviour during hot deformation of polycrystalline superalloy Inconel 718. Model equations have been formulated to describe the role of the evolving microstructures on the macroscopic flow stress response to deformation. Following a peak in the flow stress associated with strain hardening, the model utilises mechanisms associated with dynamic recovery and recrystallisation to explain the gradual decrease in flow stress with continued deformation. Incorporation of these microstructure based state variables also enables prediction of microstructures associated with a range of hot deformation conditions. Model flow stress predictions have been validated against isothermal uniaxial compression tests conducted over a range of temperatures and strain rates relevant to industrial forging conditions.MST/6228

A new Inconel 718 microstructural evolution model that works in tandem with finite element software is described, and is then used to predict the microstructure during the production of a typical two-stage forging route. The model works by grouping similar grains into grainsets and predicting the dislocation density within these grainsets during thermomechanical processing. The model is shown to accurately predict both the grain size and presence of δ precipitates during the processing route, and displays results both numerically and graphically.

Forged parts, specifically designed for the validation of microstructural models, have been manufactured in Inconel alloy 718 using a wide range of thermo-mechanical histories. The microstructural evolution observed in the forged samples has been compared with predictions from two models for dynamic recrystallisation and grain growth.

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