Surface engineering of IN-718 by low-temperature carburisation: properties and thermal stability

Abstract

IN-718 was surface-engineered by low-temperature gas-phase carburisation, generating a highly concentrated solid solution of interstitial carbon with an average depth of 10 μm and a carbon fraction up to 0.18, exceeding equilibrium solubility by orders of magnitude. Here, we investigate the stability of this highly supersaturated material against heat exposure and proton irradiation. We found: (i) Short-term exposure to high temperature causes decarburisation. Long-term exposure to moderate (reactor operating) temperature does not. (ii) Exposure to MeV proton irradiation at elevated temperatureas well as high-temperature onlycauses partial de-carburisation and precipitation of nanometre-sized Cr-rich carbides. This reduces Cr loss by evaporation. Physical models are proposed to explain the observed phenomena in terms of microscopic mechanisms. In conclusion, low-temperature gas-phase carburisation can significantly improve the properties of IN-718 for technical applications, including nuclear reactors.

Microstructure characteristics and surface oxidation behaviour of SLM WC/IN718 composite

“…Ni-based superalloys, widely used as industrial gas turbine, guide vanes, and turbine disks, exhibit excellent corrosion and oxidation resistance, as well as appropriate strength at high temperature. Among them, Inconel (IN) 718, as a typical Ni-Cr-Fe austenite (γ) superalloy, exhibits superior creep performance and fatigue strength at 253–650°C, as well as excellent oxidation resistance and favourable weldability [1,2,3,4,5]. This makes them one of the basic commercially used materials used as the core element of hot-end components in energy and aerospace industry such as turbine blades and combustion chambers [6,7].…”

Phase instability of the topmost surface layer of Inconel-718 superalloy in low-temperature gas carburization atmosphere, revealed by means of (HR)(S)TEM

“…It is missing the visualization of the new microstructure by means of (HR)(S)TEM and analysis. The effect of LT gas carburization is currently explained by a model of surface grains lattice dilatation due to the so called colossal carbon supersaturation (LTCSS) [2] that is not conform to the classical phase diagrams and is claimed to be valid in all alloys including IN-718 [4]. Our work reports new findings that reveal the up to now unknown nanostructure of the topmost carburized (at 570ºC) layer ~5 µm thick of IN-718 and cast doubts on the general validity of the LTCSS.…”