The effect of a very high overheating on the microstructural degradation of superalloy 718

Abstract

Under the harsh conditions imposed by the processing or due to an “overheating event”, the superalloy 718 may be exposed to very high temperatures, which can result in degradation of the microstructure. The time under such high temperatures can be short, as during the welding process, or long, as during reheating or homogenization of billets. Operational events or accidents that induce very high temperatures can also be a source of microstructural degradation. The present work aims to study the effect of a very high temperature on the degradation of the microstructure of the nickel base superalloy 718. Samples were heated up to 1300 °C under air at different time intervals from 1 to 480 min. The microstructural changes were evaluated by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction and compared with thermodynamic calculations. It was evidenced that even for a very short time as 1 min, Nb segregation at grain boundaries induces constitutional liquation and oxidation at those regions, as well as the formation of Laves phase and Nb-rich MC carbides with “Chinese script” morphology. With the continuous grain growth, a more intense liquation of the grain boundaries is induced, up to circa 60 min. After this threshold value, the intergranular liquation is “channeled out” via the interconnected grain boundary network, resulting in the disruption of grain boundaries and accelerated structural collapse of the material. Keywords: Superalloy 718, Grain boundary liquation, Oxidation