Room and elevated temperature ductility, strength, fracture toughness and fatigue (low and high cycle) properties are strongly related to the grain size of a fully processed superalloy product. Variations in grain size are normally observed in a heavy section forging weighing one half ton to several tons. These variations in grain size may lead to considerable scatter in the mechanical properties. Very large IN-706 disc forgings weighing up to 30,000 lb are currently fabricated for General Electric Power Systems Division large turbine engines. In order to control the variations in grain size and to create fine grained products (grain size ASTM-S or finer), thermomechanical processes (‘IMP) are being optimized using inputs from statistically designed experimental results and analysis. A large number of variables play a significant role in the development of the final grain size in a forged product. It is extremely difficult to identify the individual effect of these 25 or more processing, as well as material variables. The synergistic and interactive effects of these variables are, however, very important in controlling the final grain size. To quantitatively determine the effects of these variables and their interaction, statistical experimental designs were adopted using a 2@ fractional factorial design. Higher orders of fractional factorial design are not statistically suitable for a more precise analysis. Several fractional factorial designs were overlapped to cover the effect of all significant processing variables. Results indicate that an ASTM-5 or finer grain size without mixed fine and coarse grains can be developed in large IN-706 disc forgings through TMP optimization; whereas, ASTM-2 or finer grain size is developed through the current conventional processing conditions. Results also indicate that thermal treatments and grain size exhibit significant influence on some of the mechanical properties.