Investigation of Non-Metallic Inclusions in Inconel 718 by Using Conventional Cross-Sectioned (CS) Method and Experimental Extraction Metal-Surfaced (MS) Method

Abstract: Inconel 718 is a nickel-based alloy, predominantly used in the oil and gas industry, in aircraft engines as well as in gas turbine engines. It provides exceptional properties like high strength, creep resistance, corrosion resistance and long fatigue life at high temperatures as well as in cryogenic environments. However, presence of non-metallic inclusions in the metal matrix affects these properties and reduces the quality of alloy. Therefore, the determination of inclusion characteristics like morphology, chemical composition, number and size is very important to control the product quality. The purpose of this study was to find an effective method for investigating non-metallic inclusions characteristics in a short period of time. Two investigation methods were used: i) Conventional CS method in which the sample is grinded and polished for 2D cross-sectional metallographic examinations under a scanning electron microscope (SEM), ii) New experimental MS method in which the sample is electrolyzed in a non-aqueous solution for a short period of time, followed by 3D metallographic examinations under a scanning electron microscope (SEM). Four types of non-metallic inclusions were observed in tested samples of Inconel718 alloy named as; carbides, sulphides/carbosulphides, carbonitrides/nitrides and intermetallic carbides. It was also found that medium and large inclusions appeared more on MS compared to CS and provides good precision of morphology and size. However, it is not useful in finding the total number of inclusions, including small-sized inclusions. Both methods showed very little or no influence of metal matrix on inclusions >6mm. From the findings, it is suggested to use MS method for analysing medium and large-sized inclusions. But to achieve more accurate information, results from both methods should be combined.