Investigations of Large Size Non-Metallic Inclusions in the Nickel-based alloy Inconel 718

Abstract: Inclusions of different types are present in all steels. How dangerous these are for the properties of the material depends on their chemical composition and the manufacturing process. The following work examined large non-metallic inclusions in the nickel-based alloy Inconel 718 according to their composition, size, location, and number, this to find out their effect on the corrosion resistance of the material. Furthermore, a literature study was conducted where different methods were studied and experiments where the MS method (an examination of inclusions on the metal surface after electrolytic extraction) was performed for extractions at 20 and 500 Coulomb (C). The extraction depths were analyzed and proved to be suitable for different situations. Inclusions of type TiNb-S were noted to be the most dangerous for the corrosion resistance, followed by Ti(Nb)-N(C), NbTi-C, and Ti-N+Nb-C. Moreover, it was observed that the 20 C extraction had a more heterogeneous distribution of numbers of inclusions compared to 500 C. No difference in the effect on the corrosion resistance between inclusions located on grains compared to those along grain boundaries could be observed after 20 C. For 500 C, more studies need to be conducted regarding the location for a conclusion to be drawn.