Microstructural Characterization of Cast 718

Abstract

A detailed investigation of cast IN 718 was conducted on phases formed during solidification, phases precipitated on cooling and phase stability. Clearly cast 718 is a composite structure. Extensive as-cast segregation is evident which shows a low Cb content, about 2%, in the dendritic matrix, and a high Cb content, about 10% in the interdendritic region. Homogenization studies show that Laves solutioning does not uniformly redistribute the Cb. This work also indicates that the standard GE homogenization heat treatment of 2OOO'F/l hour dissolves most of the delta and a portion of the Laves. Depending on the Cb content in the alloy, total Laves solutioning requires at least 3 hours at 21OO"F, but a more uniform Cb distribution requires 100 hours or more beyond the standard heating cycle. The results of this investigation may serve as a guide in microstructural interpretation and can offer a viable explanation for the variability of mechanical properties on bars extracted from castings.

Additively Manufactured Inconel 718 : Microstructures and Mechanical Properties

“…Though IN718 is being used in the wrought form as turbine disk material as mentioned, cast IN718 has still gained applications in aircraft engines for compressor and turbine frames, combustor cases, fuel nozzle rings and other hot engine structures [39]. Severe interdendritic segregation and presence of Laves phases, with/without cast porosity, make the cast microstructure considerably different from that of wrought form.…”

“…Severe interdendritic segregation and presence of Laves phases, with/without cast porosity, make the cast microstructure considerably different from that of wrought form. Cast porosity can be closed by applying a HIP cycle; minimizing segregation can be achieved partly by the HIP cycle or by a separate homogenization heat treatment [5,29,39]. The homogenization temperature and duration are largely dependent on size of Laves phase: the bigger Laves phases, the higher homogenization temperature and longer homogenization duration [40].…”

Aging Effects on the γ' and γ'' Precipitates of Inconel 718 Superalloy

“…For wrought Inconel 718, the solution temperature was increased to 1032 ºC to improve the high temperature crack propagation resistance [5]. For cast Inconel 718, the standard homogenization heat treatment of 1095 ºC/1 h dissolves most of the δ-phase and a portion of the Laves [6]. Since niobium is the major element for the precipitation strengthening through γ′ and γ′′, a higher solution temperature will increase the niobium content and thus increase the precipitation strengthening.…”

Development of Prior Particle Boundary Free Hot-Isostatic-Pressing Process for Inconel 718 Powder

“…Owing to the slow cooling rate during solidification, cast components have coarse grain size, heavy dendritic segregation and relatively poor mechanical properties. Solidification defects, such as shrinkage cavities and porosity, also form in the castings [5]. Homogenization treatment and HIPing are always required to eliminate the segregation, to close the internal pores and to improve the quality of the castings.…”

“…Among these applications, the cast and wrought (C&W) version is frequently used in disks and shafts [4], while the cast version is always applied in turbine frames, cases and other structures with large size and complex geometry [2,5]. As with other alloys processed using C&W route, a large portion of material will be machined to obtain the final geometry, leading to considerable material loss and increase in processing cost.…”