Microstructure and mechanical properties of HIP PM 718

Abstract

This paper explores advantages of powder metallurgical (P/M) processing of Alloy 718 using gas atomization and hot isostatic pressing (HIP). To evaluate the effect of different processing parameters on HIP P/M 718, powders with low and standard carbon levels were produced by nitrogen and argon atomization. A small amount of Mg was added to one heat to investigate the possibility of improving elevated temperature properties. The Mg-treatment is a necessity to obtain good creep properties. The consolidated materials are fully dense and have very fine and homogeneous microstructures typical for fully dense P/M alloys. Yield strength levels range from 140 to 170 ksi at room temperature, and from 115 to 145 ksi at 1200°F, while tensile ductilities range form 19 to 15 and 9 to 24%, respectively. Different heat treatments were evaluated, since the standard heat treatment for cast and wrought 718 led to heavy decoration of prior powder particle and grain boundaries, which resulted in low ductility, especially at 1200°F. Creep rupture properties were evaluated at 1200°F and 100ksi. Depending on heat treatment, time to rupture was as high as 170 hrs. Higher solution temperatures resulted in microstructures with less grain boundary decoration, better ductilities and improved creep properties with little loss in strength. During solutioning at 1800°F, no grain growth was observed, and very limited grain growth was observed during heat treatments at temperatures up to 2200°F. Taking into account the relatively low levels of precipitate forming elements, the strength levels are still fairly good.