ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2026 21 perties and processing characteristics of alloy 718. The strengthening precipitate is a stable form of gamma prime that is also less liable to transform to delta at 1300°F[15]. Although 718Plus has successfully filled a design gap, initially for static engine parts, its higher intrinsic metal cost will limit its use to areas of need. The most recent development has been VDM alloy 780. The iron in this derivative has been totally replaced by cobalt and aluminum increased to 3%[16]. This alloy has initially been promoted in powder form and is still in qualification for engine applications. SPECIAL PURPOSE ALLOYS The evolution of alloy 718 composition, heat treatment, and microstructure for use in oil and gas drilling and production was described in part two of this series. Alloy 718 has moderate resistance to pitting corrosion in hot sour-halide environments, but for the most severe conditions more corrosion resistant alloys are required. In the late 1980s, the oil and gas equipment manufacturers expressed the need for an alloy with “625 corrosion resistance and 718 strength.” Within a few years three alloys were approved for use in NACE MR-01-75: Carpenter alloy 625 PLUS, Inconel alloy 725, and Crucible Compaction Metals alloy 625M[17-19]. The latter, a high niobium content gamma prime strengthened modification of alloy 625, met the corrosion requirements but was segregation prone as a cast/wrought alloy. It was successfully used as a powder alloy. The other two alloys were designed to form gamma double prime with a higher iron and low Al/ Ti ratio along with 8% molybdenum to combat crevice corrosion. These alloys have been considered the “gold standard” for use in the most severe sour well environments in components such as hangers, packers, and sub- surface safety valves. However, in addition to being relatively expensive compared to alloy 718, uncertainty about their resistance to hydrogen embrittlement has limited their use. Recently, lower nickel alloys with superior strength and corrosion resistance than the oilfield grade of alloy 718 have been introduced and qualified for sour service. These include INCOLOY alloys 945, 945X, and Feroni alloy AD955[20,21]. These alloys have lower niobium contents and are super delta solvus heat treated and aged so that they are free from intergranular delta phase that is detrimental to stress corrosion resistance. These alloys fill niche applications in the oil patch, but alloy 718 remains the preferred nickel- base alloy due to its availability. The most unique family of alloy 718 derivatives are the low coefficient of thermal expansion alloys. These alloys make use of the “INVAR effect” to provide low thermal expansion over various temperature ranges. While the classic 36%Ni-64%Fe alloy is not age hardenable, Eiselstein and Bell created a high strength alloy with 3% niobium and smaller additions of aluminum and titanium[22]. This alloy known as INCOLOY 903 was the first of a series of developments for aircraft engine rings and seals to improve fuel efficiency. Carpenter Technology Corp. independently developed CTX-1, a very similar alloy. Alloy 903 is unique in that it requires warm work for its properties and hence is inherently anisotropic. The alloy contains no chromium, an element that raises the expansion coefficient, and thus has little oxidation resistance and is very susceptible to stress assisted grain boundary oxidation (SAGBO) cracking. To improve SAGBO resistance, Inco developed alloy 907 and shortly thereafter alloy 909[23,24]. These alloys have higher niobium, restricted aluminum, and in the case of alloy 909 a small but critical addition of silicon. The metallurgy of alloy 909 is more conventional and it was preferred in the aircraft industry although 903 persists in older engines. The primary product forms are hot rolled seamless or flash butt welded rings. Alloys 903 and 909 have also been used in rocket engines where their low burn characteristic complements their low expansion and high strength. Other bespoke age hardened low expansion alloys have been developed for specific cryogenic applications such as alloy 908 developed in a collaboration by Inco and the Massachusetts Institute of Technology for sheathing for superconducting magnet coils intended Example of sheathing from ITER’s magnet systems. Courtesy of ITER. Typical tensile properties of Allvac 718Plus. Courtesy of Alloy Digest, Allvac 718Plus, Fig. 3, ASM International, March 2007.
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