Improvement in Mechanical Properties of Ni-Base Superalloy Alloy 247 Using Hot Forging

To improve the low-cycle fatigueFatigue (LCF) strength of Alloy 247Alloy 247, refinement of the alloy microstructureMicrostructure through the forgingForging process has been trialed, and the mechanical propertiesMechanical properties and microstructureMicrostructure of the forged alloy were compared with that of the conventional cast alloy. By utilizing the MH (Mitsubishi Hitachi) process, an ingot of Alloy 247Alloy 247 with high volume fraction of γ′ phase was successfully forged without crackingCracking. In contrast with cast Alloy 247, forged Alloy 247Alloy 247 exhibited uniform microstructureMicrostructure with γ′ precipitatesPrecipitates, the grain sizeGrain size was significantly reduced from 10 mm in the cast alloy to 200 μm in the forged alloy, and the eutectic structure was eliminated completely. Results obtained from high-temperatureHigh temperature tensile testing showed remarkable improvement in elongation, which led to higher ultimate tensile strength (UTS) values. Owing to the fine microstructureMicrostructure, forged Alloy 247Alloy 247 showed 15 times longer LCF life than that of the cast alloy although creepCreep strength slightly decreased in low stress regions. Significant improvement in LCF life of Alloy 247Alloy 247, which has intrinsically high creepCreep strength, allows for tuning of the mechanical propertiesMechanical properties based on requirements and broadens the application of the alloy to hot section parts.