Introduction to Inconel 718
Inconel 718, designated UNS N07718, is the most widely known and specified grade in the nickel-chromium alloy family. Nickel-chromium alloys are the most extensively used nickel-based alloys in manufacturing because they combine excellent corrosion resistance at ambient and elevated temperatures with retention of mechanical properties over a wide temperature range. The corrosion resistance comes from a protective surface film that prevents further oxidation, while the alloy base retains strength and stability during prolonged service.
History and Market Adoption
Inconel 718 was introduced in the 1960s and was initially used in the aerospace industry. As the alloy became readily available in the market, the oil and gas industry also adopted it, which allowed wells to move offshore and drill deeper while dealing with increasingly higher temperatures, pressures, and corrosive contaminants. Other nickel-based alloys such as Hastelloy C276 and Inconel 625 provided sufficient corrosion resistance, but they lacked the required strength without extensive cold working, while martensitic stainless steels were limited by the risk of hydrogen embrittlement and stress corrosion cracking. Inconel 718 therefore became a default choice for many pressure-containing and load-bearing components in aggressive environments.
Precipitation Hardening: The Source of High Strength
Inconel 718 achieves significantly increased strength through precipitation treatment, also described as aging, age hardening, or precipitation strengthening, while retaining much of the toughness and formability of the base alloy. The objective is to form very fine precipitates dispersed throughout the microstructure. These precipitates pin the grain boundaries and restrict their movement when the metal is subjected to an external load. The process is achieved through a combination of controlled chemical composition and multi-stage heat treatments, and the alloy is available in a number of increasing strength levels depending on the extent of heat treatment.
High-Temperature Performance
Nickel alloys such as Inconel 718 are well suited to high-temperature applications because most other metals become brittle or oxidize rapidly at elevated temperatures. When heated in an oxygen-containing atmosphere, nickel-based alloys form a protective oxide layer that prevents further oxidation, and Inconel 718 retains its ambient-temperature mechanical properties up to approximately 650 °C. Its relatively high melting point means that only a small group of more exotic alloys can outperform it at high temperatures.
Typical Applications in Oil and Gas and Chemical Processing
Because Inconel 718 offers both corrosion resistance and very high strength, it is used in load-bearing components in aggressive environments. In oil and gas and the chemical process industry, typical applications include wellhead and downhole components, sheathing, fasteners, gate valves, choke stems, packers, tubing hangers, and other items for corrosive and sour service. The same properties support aerospace gas-turbine disks, shafts, and fasteners, as well as energy-sector equipment that must survive high pressure, high temperature, and corrosive media.
FAQ
Q: What is Inconel 718 made of?
A: Inconel 718 is a nickel-chromium alloy containing iron, niobium, molybdenum, titanium, and aluminum. The niobium, titanium, and aluminum additions enable precipitation hardening, which produces its characteristic high strength.
Q: What temperature can Inconel 718 withstand?
A: Inconel 718 retains its room-temperature mechanical properties up to approximately 650 °C, which is why it is specified for gas turbines, fasteners, and pressure components operating at elevated temperatures.
Q: Why is Inconel 718 used in oil and gas?
A: It combines high strength, toughness, and resistance to corrosive and sour service, making it suitable for wellhead components, downhole tools, valves, and fasteners in high-pressure, high-temperature wells.
Q: How is Inconel 718 strengthened?
A: Through precipitation hardening: fine precipitates formed during multi-stage heat treatment pin grain boundaries and restrict movement under load, raising yield and tensile strength while preserving toughness.
Q: Is Inconel 718 the same as Inconel 625?
A: No. Both are nickel-chromium alloys, but 625 is strengthened by solid-solution hardening and offers superior corrosion resistance, while 718 is precipitation-hardened and offers higher strength at elevated temperatures.
Q: Can Inconel 718 be welded?
A: Yes, with appropriate filler metal and qualified procedures. Post-weld heat treatment is often required to restore strength and control the properties of the heat-affected zone in critical applications.
