ADVANCED MATERIALS & PROCESSES | JULY 2026 26 as logging-while-drilling (LWD) and measurement-while-drilling (MWD). These tools that are placed behind the drill package are sophisticated electronic instruments contained in a capsule that must be both strong and non-magnetic. These applications typically involve relatively short-time exposure to corrosive fluids and are not well safety critical. During the 1980s, the oil and gas industry began to exploit fields with more challenging environments. Alloy 718 was considered for critical production components that would experience long exposure to concentrated brines and production fluids that were hot, acidic, and often contained H2S, and CO2. Applications included hangers, subsurface safety valves, packers, and side pocket mandrels. These were long cylindrical tools with intricate internal structures usually bored out of solid bar and weighing thousands of pounds. They control fluid flow and the injection of chemicals. Cold worked nickel-base corrosion resistant alloys (CRAs) were useful for small diameter well strings, but they could not be manufactured in the section size required for deeper high production wells. Only heat treatable alloy 718 could satisfy the requirement for the deepest wells[16]. One complicating factor for specification of materials for oil and gas production service is that the environment in each well is different and sometimes changes during the life of the well. Misapplication of CRAs has resulted in expensive and sometimes fatal well failures. Extensive testing of alloy 718 over an approximately 30-year period has resulted in American Petroleum Institute (API) specifications that have made oilfield grades of alloy 718 distinct from aerospace grades[17,18]. Returning to the notion of delta phase being detrimental to hydrogen embrittlement, stress corrosion resistance, and fracture toughness, the new specifications prohibit the presence of intergranular delta phase particles and minimize carbide coverage. A super delta solvus anneal and water quench followed by a single-step overaging treatment was developed to optimize properties. In addition, limits on carbon and phosphorus content make it difficult to use aero grade alloy 718 scrap when melting oilfield grades. From an alloy forger’s perspective, maintaining a delta-free microstructure became increasingly difficult as larger tools were needed for deeper wells. A recent trend has been to increase the minimum yield strength of oil- field alloy 718. Initially it was limited to 120 ksi (827 MPa), but through control of the aging temperature, yield strength up to 150 ksi is now acceptable for some applications. While alloy 718 is the highest volume heat treatable nickel-base alloy used in the oil and gas industry, it is inadequate for the most corrosive environments. Derivatives of alloys 625 and 718 have been developed for these severe applications[19]. FASTENERS An important but often overlooked use of alloy 718 is engineered fasteners for early fabricated engine structures. Its high yield and fatigue strength, notch ductility, resistance to stress corrosion and hydrogen embrittlement coupled with use temperature range from liquid helium to about 1200°F (649°C) made it ideal for a wide range of applications. Its commercial availability makes it a practical alternative when bespoke alloys are unavailable. Alloy 718 was applied for airframe and landing gear assembly to replace cadmium coated high-strength steel[20]. Simple threaded bolts and nuts have evolved into complex engineered systems and grown from a few ounces to thousand-pound tie bolts for land-based gas turbines. The first fasteners were simply machined from bar and given one of the two standard heat treatments. Later it was found that yield strength of 220 ksi (1517 MPa) could be achieved by first cold working annealed bar and then directly aging it[21]. Even higher strength is possible but at a severe loss in ductility so more expensive alloys such as MP35N are preferred. Initially cold working was performed by cold drawing, but later a variety of cold working techniques were applied for specialty structures. Additional deformation and compressive residual stress may be applied by thread rolling. This article considered only the most prominent non-aerospace applications for alloy 718. There are numerous other interesting uses including racing car engines and new ones made possible by additive manufacturing. It is reasonable to expect that alloy 718 A display of fasteners. Courtesy of Howmet Aerospace.
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