Effect of Thermomechanical Processing on Fatigue Crack Propagation in INCONEL Alloy 783

Abstract

Recently developed INCONELe alloy 783 (nominal composition of Ni-34Co-26Fe-5.4A1-3Nb-3Cr) is precipitation strengthened by N&Al-type y’ and NiAl-type S phases. Due to its low coefficient of thermal expansion, high strength, and good oxidation resistance, alloy 783 is used for casings and bolting applications in gas turbines. During thermomechanical processing (TMP), the high Al content in alloy 783 results in the formation of S phase in an austenite matrix. The size and distribution of S phase governs the microstructure of the alloy. Therefore, optimization of TMP is critical for microstructure and mechanical properties. This study presents the effect of TMP on fatigue crack growth in alloy 783, especially in time-dependent condition. Initially, the material was hot rolled in the mill at 1095°C. This was followed by rolling in the laboratory to 50% reduction at temperatures of 870°C 1010°C and 1150°C. Materials rolled in the mill and laboratory were direct aged without solution annealing. The aged materials were subjected to fatigue crack growth testing at room temperature, 300 “C, 450 “C and 600 “C and constant stress intensity mode. The results exhibit that timedependent fatigue crack propagation resistance of alloy 783 can be dramatically improved by TMP. Comprehensive microstructure characterization and fractographic analysis suggests that the enhancement of fatigue crack growth in time-dependent condition is associated with the change of fracture mode. The SAGBO (Stress Accelerated Grain Boundary Oxidation) mechanism and damage zone model were applied to explain these phenomena.