THE ROLE OF PHOSPHORUS AND SULFUR IN INCONEL 718

Phosphorus and sulfur are generally regarded as the most common impurities and detrimental elements in nickelbase superalloys. For further understanding the role of P and S in Inconel 718 nine experimental heats were melted on the base of conventional Inconel 718 chemical composition with variation of P (from 10 to 130 ppm) and S (from 15 to 175 ppm) respectively. Phosphorus and sulfur both have almost no influence on strengths and ductilities at room temperature tensile test, Phosphorus also has no effect on the yield and ultimate strengths and elongation at 650°C tensile test, However, sulfur has an obviously decreasing effect on 650°C tensile elongation but no effect on yield and ultimate strengths. Sulfur has a remarkable detrimental effect on stress rupture life and especially on ductility loss at 650°C. However, phosphorus is in total difference to sulfur effect, generally can increase stress rupture life and ductility both. Microstructure observation on grain structure, S-phase and strengthening phase y” and y’ can not reveal the effect of P and S on the morphology and amount of precipitates. Fractography analyses show different patterns of stress rupture specimens with the variation of P and S contents. Experimental results lead us to consider the segregation behaviors of P and S at grain boundaries, however the effect of P and S should be different.