Analysis of microstructural properties of in 718 after high speed forging

The precipitation of the γ-″ is the key to the excellent mechanical properties of IN 718. These can be improved by forging the material on a screw press (high strain rate forming) instead of a hydraulic press (low strain rate forming) [1]. The increase in hardness cannot be explained by grain size effects but is due to finely dispersed γ-″ -particles found in the as-forged material. The time-temperature-transformation-diagram of IN 718 [2] predicts no precipitation of the γ-″-phase during cooling from the forging temperature. Therefore, it seems that the common TTT-diagram for IN 718 as reported in [2] is not applicable to the material exposed to high strain rates as the precipitates develop much earlier than predicted. In this work investigations on the effects of high strain rate forging on the precipitation behavior of IN 718 are presented. A modified TTT-diagram is suggested for IN 718 exposed to high strain rates.