Effect of thermomechanical working on the microstructure and mechanical properties of hot isostatically pressed superalloy Inconel 718

Due to the high oxygen content (275 ppm) of prealloyed powder, the hot isostatically pressed (HIPed) superalloy 718 consisted of prior particle boundary (PPB) networks affecting the tensile ductility and stress rupture properties adversely. To improve the mechanical properties, the HIPed alloy was hot rolled at two different temperatures, viz. 1150degreesC and 950degreesC with a reduction of 40% in each case. The material rolled at 1150degreesC was heat treated as per the conventional schedule consisting of solution treatment at 995 degreesC/1 h, followed by water quenching (WQ) and ageing treatment at 720degreesC/8 h/furnace cooling (FC) to 620degreesC and holding at 620degreesC/8 h/air cooling (AC) to room temperature, while the alloy rolled at 950degreesC was directly aged without solution treatment. Optical and transmission electron microscopy (TEM) studies revealed that while these treatments eliminated the PPB networks, the fine-scale precipitation of gamma" and gamma' phases was similar to that of the HIPed and heat-treated alloy. Hot rolling at 1150degreesC followed by conventional heat treatment offered a very good combination of yield strength (YS; 938 MPa), ultimate tensile strength (UTS; 1033 MPa), ductility (18% elongation (EL) and 36% reduction in area (RA)) and excellent stress rupture properties at 650degreesC. On the other hand, rolling at 950degreesC + direct ageing treatment showed tremendous improvement in YS (1185 MPa) and UTS (1277 MPa) with acceptable level of ductility and stress rupture properties required for critical components. (C) 2004 Elsevier B.V. All rights reserved.