Aerospace gas turbine disks operate in an environment of relatively high stresses caused by centrifugal forces and elevated temperatures. These severe conditions necessitate the need for materials with high temperature strength and good low cycle fatigue resistance. One class of alloys used for this task is the nickel base superalloys, out of which, IN 718 is the most widely used in the aerospace industry (1). The properties of IN 718 are attributed to the combined effects of the chemistry, heat treatment, and microstructure. The chemistry is tailored not only for solid solution strengthening but also for precipitation hardening developed during heat treatment, which combined with a fine grained microstructure lead to excellent mechanical properties such as low cycle fatigue resistance and elevated temperature strength. The properties of a gas turbine disk are sensitive to the microstructure, in particular the grain size, which is dependent on the processing history. The ability to precisely control the microstructural development during forging is dependent on controlling the process so that the workpiece is deformed within a “safe” region where no microstructural damage or flow instabilities occur. The microstructural mechanisms during deformation may themselves vary within the “safe” region and it is desirable to determine them within the range of parameters that are commonly used in industrial processing. The objective of this work is to establish a relationship between the grain size and the process control parameters i.e., temperature and strain rate, in the hot working of IN 178.