On the development of instability criteria during hotworking with reference to IN 718

A simple instability condition based on the Ziegler’s continuum principles as applied to large plastic flow, is developed for delineating the regions of unstable metal flow during hot deformation. It can be used for any type of the flow stress versus strain rate curve. This criterion has been validated using the flow stress data of IN 718 with microstructural observations. The optimum hot working conditions for the superalloy IN 718 are suggested based on the instability map.