ADVANCED MATERIALS & PROCESSES | JULY 2026 25 at cryogenic temperatures was enhanced by applying a super solvus anneal. These two heat treatments were promoted by Inco in its product literature. Research into the effects of hydrogen in 718 initially supported by NASA and later by the nuclear and oil and gas industries revealed the critical role of microstructure on toughness and crack propagation. While the actual mechanism of hydrogen embrittlement is still controversial, a major finding was the detrimental effect of inter- granular delta phase. This relationship was later extended to stress corrosion cracking in high purity water and chloride environ- ments[14]. The super solvus heat treatment, when coupled with niobium at the lower end of the specification range and water quenching, generates a microstructure free of delta phase, which proved desirable for oilfield and nuclear power applications. A low cobalt version of alloy 718 was developed for use in the complex fuel rod drive and containment assem- blies of pressurized water reactors. These assemblies are constructed from cold rolled sheet and small diameter rod with spring characteristics that hold the flexible rods in place in the turbulent environment of the reactor core. Composition and heat-treatment modifications to produce delta-free microstructure were also developed for oilfield applications as discussed next. OIL AND GAS As alloy 718 became established in the aerospace industries, Inco began to explore potential applications that combined the need for high strength, notch toughness, and corrosion resistance in aqueous environments. Oil and gas exploration and production presented an opportunity. The first use in the oil patch is not documented but an early Inco data bulletin states: “It (alloy 718) also is useful in combatting chloride ion stress- corrosion cracking”[15]. Initial oilfield applications employing conventional sub-solvus annealed material included fasteners, valve stems, shafts, and drill tools, sometimes using material downgraded from aerospace applications. Later applications included sensitive resource exploration tooling such being developed may in time comprise a significant new market for the alloy. Eiselstein’s patent specified only a “super delta phase solvus” annealing treatment that generates a relatively coarse grain size. In the precommercial development period an alternative annealing treatment was developed that optimized fatigue strength and notch ductility desired for aircraft engine structures. This was achieved through the controlled precipitation of delta phase that restricted grain growth during the annealing treatment. By controlling the final forging below the delta solvus and specifying a relatively high and narrow niobium content, the unique grain size of ASTM 8-12 ideal for turbine rotors could be achieved. However, notch toughness, notch tensile strength, and transverse ductility Cutaway drawing of a nuclear fuel rod assembly. Courtesy of Nuclear Regulatory Commission. Schematic drawing of a sub-surface safety valve (not drawn to scale). Courtesy of Drilling Manual.
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