The mechanical behavior of lnconel 718 processed using the Delta Process (DP) conversion technique was studied. The DP conversion sequence uses a intentional delta phase precipitation cycle and subsequent thermomechanical processing to produce uniform fine grain billet and bar stock. The process was developed to improve the mechanical properties through the elimination of occasional coarse grains which are common in conventionally processed materials. The net effect resulted in a decreased standard deviation in the mechanical properties and an increase in the minimum properties. The microstructure, tensile, low cycle fatigue, and cyclic crack growth behaviors of DP718 impeller, disk forgings and bar stock were investigated. Forged DP718 properties were compared to conventionally processed lnconel 718 and another high strength material. The DP718 bar stock properties were compared to conventionally processed lnconel 718.
The aerospace materials community has demonstrated considerable interest in achieving improved properties in Alloy 718 by finer grain microstructures and better cleanliness. Its composition has been investigated in the past, in combination with standard and novel fine grain processing techniques, to improve low cycle fatigue capabilities. Lower carbon content is one compositional change that has been evaluated and debated for a number of years. Investigators are in agreement that tensile strength is not effected by low carbon, and that improved carbide distribution provides an opportunity for better fatigue properties. However, the effect on creep properties remains unresolved.Production ingots of standard (0.025 wt% C) and low carbon (0.010 wt% C) Allvac 718 were processed to 5-inch diameter forged bar. Processing and heat treatment was aimed at producing fine grain microstructure and maximizing tensile properties. Two process techniques were used; 1) a tine grain process employing super and sub 6-solvus temperature forging, 2) a mini-grain process which involves an intentional &phase precipitation and sub 8-soivus forging. All material was direct age heat treated. Preliminary creep properties showed no difference between low carbon Allvac 718 and standard carbon Allvac 718. Supporting previous studies, the material displayed high tensile strength at room and elevated temperatures.Creep and low cycle fatigue testing were not completed by the deadline for the written presentation.
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