Superalloy 718: Evolution of the Alloy from High to Low Temperature Application

Alloy 718 (UNS N07718) was the culmination of a research project started in the mid-1950s to develop a stronger pipe alloy for coal-fired power plants. It was never used for that application, but it was quickly adopted for aircraft turbine engines because of its very high strength, thermal stability, formability and weldability compared to the gamma'strengthened alloys available in the 1960s. Alloy 718 derives its unique combination of strength and fabricability from a coherent ordered tetragonal phase. gamma" and the slow diffusion rate of its main constituent, niobium. Very early in its commercial life, alloy 718 was recognized as having attributes for both ambient and cryogenic temperature uses as well. Impact toughness, aqueous corrosion resistance, and non-ferromagnetic properties were important attributes. Alloy 718 replaced established age-hardened iron/nickel-base alloys such as A-286, K-500 and X-750 as well as martensitic steels in a wide range of components in the space launch, oil and gas, marine, nuclear and superconducting magnet industries. As the applications became more specialized, so did the heat treatments and microstructures to accentuate specific properties. A common thread through these low temperature applications has been hydrogen embrittlement and a substantial body of literature documents our increasing awareness of its role in service performance. In recent decades, new alloys based on alloy 718 and the gamma" strengthening phase have been introduced, especially for oil and gas production equipment. This paper describes the early development of alloy 718 for these important applications, along with alloy, microstructure and heat-treatment evolution and current status.