What is Inconel®
There are many ‘super’ metals in use throughout the industry; Stainless Steel, Titanium, and Aluminum alloys. Many of these metals are formulated for some of the most rigorous applications; car racing, airplanes, spacecraft, etc.
Inconel® is the name for a group of nickel-based superalloys. The Inconel family of alloys was first developed before 30 December 1932, when its trademark was registered by the International Nickel Company of Delaware and New York. Significant early use was found in support of the development of the Whittle jet engine, during the 1940s by research teams at Henry Wiggin & Co of Hereford, England a subsidiary of the Mond Nickel Company, which merged with Inco in 1928. The Hereford Works and its properties including the Inconel trademark were acquired in 1998 by Special Metals Corporation.
Though Inconel Metal is not familiar to the general public the first Inconel alloys were originally formulated for demanding, high-temperature environments in the middle of the 20th century,
Inconel® Metal Composition
Inconel metal is actually a group of superalloys and not just one type of superalloy, its chemical composition varies across the different alloy types. However, all alloys in the Inconel® family are nickel-based. Additionally, chromium is typically the second most abundant element in the chemical makeup of Inconel®. Practically every form of Inconel® has some amount of iron in its composition, almost always greater than 1%. Other elements that are found in some Inconel® alloys include:
Properties of Inconel®
The metal possesses some extraordinary properties, some on par with other superalloys and some which set it apart.
One of these special properties is Inconel metal’s ability to withstand incredibly high-temperature environments, whilst it is true that some forms of steel also have high melting temperatures than nickel, their performance will deteriorate at higher temperatures; loss of strength and corrosion resistance in particular. Inconel® maintains its excellent strength at higher temperatures as well as forming a protective layer of oxide to resist corrosion.
The strength of a high-temperature alloy is developed through solid solution strengthening and precipitation strengthening, or age hardening. The heat treatment used depends on the specific superalloy.
These techniques combine nickel with varying amounts of niobium to form gamma double prime (γ″). This intermetallic compound creates tiny cubic crystals that prevent creep deformation and slip at high temperatures.
The formation of these crystals continues to increase with time, especially after 3-72 hours of exposing the alloy to temperatures of 1,560°F (850°C) and up. This thick, passivating oxide layer allows the metal to remain exceptionally strong over a wide range of temperatures.
Here’s an overview of the mechanical characteristics of a sheet of 718 after undergoing precipitation heat treatment.
- Yield strength in its annealed condition (Min with 0.2% offset): 80,000 psi (550 MPa)
- Ultimate tensile strength (Min): 140,000 psi (965 MPa)
- Elongation (Min): 30 (percentage in 2 in.)
- Coefficient of thermal expansion at 77°F to 1400°F: 8.9
In addition, Inconel® has excellent strength properties at room temperature. A high percentage of chromium gives it superb corrosion resistance at room temperatures as well. The other alloys mentioned above in the composition section provide additional properties such as toughness and hardness. Several Inconel alloys, such as Inconel 718®, are precipitation hardened to increase strength further still.
The main reason for the alloy’s very high-temperature resistance is due to the formation of an intermetallic compound Ni3Nb in the gamma double prime (ɣ’’) phase. This intermetallic phase acts as a ‘glue’ on the grain boundaries, preventing the grains from increasing in size at high temperatures.
Grades of Inconel®
Inconel® has a variety of grades varying in composition and properties developed for specific applications. Information supplied by Special Metals, full information on their range of Nickel Alloys is available here. The common Inconel® grades are as follows:
INCONEL ALLOY 600
A nickel-chromium alloy with good oxidation resistance at high temperatures and resistance to chloride-ion stress-corrosion cracking, corrosion by high-purity water and caustic corrosion. Used for furnace components, chemical and food processing, nuclear engineering and sparking electrodes.
INCONEL ALLOY 601
A nickel-chromium alloy with an addition of aluminum for outstanding resistance to oxidation and other forms of high-temperature corrosion. It also has high mechanical properties at elevated temperatures. Used for industrial furnaces; heat-treating equipment such as baskets, muffles, and retorts; petrochemical and other process equipment; and gas-turbine components.
INCONEL ALLOY 617
A nickel-chromium-cobalt-molybdenum alloy with an exceptional combination of metallurgical stability, strength, and oxidation resistance at high temperatures. Resistance to oxidation is enhanced by an aluminum addition. The alloy also resists a wide range of corrosive aqueous environments. Used in gas turbines for combustion cans, ducting and transition liners; petrochemical processing; heat-treating equipment; and nitric acid production.
INCONEL ALLOY 625
A nickel-chromium-molybdenum alloy with an addition of niobium that acts with the molybdenum to stiffen the alloy’s matrix and thereby provide high strength without strengthening heat treatment. The alloy resists a wide range of severely corrosive environments and is especially resistant to pitting and crevice corrosion. Used in chemical processing, aerospace and marine engineering, pollution control equipment, and nuclear reactors.
INCONEL ALLOY 625LCF
A nickel-chromium-molybdenum alloy that was developed as a fatigue-resistant bellows-quality version of INCONEL alloy 625. Alloying, melting, and processing of this alloy are specially designed and controlled to provide a sheet product with optimum resistance to low-cycle and thermal fatigue at temperatures up to 1200°F (650°C). Used in Aircraft exhaust and automotive flexible coupling bellows and expansion joints in various types of process or transport piping.
INCONEL ALLOY 686
An alloy designed for outstanding corrosion resistance in a wide range of severe environments. The alloy is used in the most severe environments encountered in chemical processing, pollution control, pulp and paper production, and treatment of industrial and municipal wastes. Chemical processing uses include heat exchangers, reaction vessels, evaporators, and transfer piping. Air pollution control applications are stack liners, ducts, dampers, scrubbers, stack-gas reheaters, fans, and housings.
INCONEL ALLOY 690
INCONEL alloy 690 is a high-chromium nickel alloy with excellent resistance to many corrosive aqueous media and high-temperature atmospheres. The alloy’s high chromium content gives it excellent resistance to aqueous corrosion by oxidizing acids (especially nitric acid) and salts, and to sulfidation at high temperatures. In addition to its corrosion resistance, alloy 690 has high strength, good metallurgical stability, and favorable fabrication characteristics.
INCONEL ALLOY 693
A nickel-chromium-aluminum alloy offers the best resistance to a metal dusting of any available conventional alloy. Alloy 693 also offers exceptional resistance to oxidation and carburization at temperatures up to 2100°F (1150°C) with excursions to even higher temperatures. The alloy’s ability to resist metal dusting is especially useful in systems used to reform hydrogen and generate synthesis gas for various industries, including gas and liquid fuel production. Alloy 693 is protected by U.S. Patent Number 4,882,125.
INCONEL ALLOY 706
INCONEL alloy 706 is a precipitation-hardenable nickel-iron-chromium alloy that provides high mechanical strength in combination with good fabricability. The properties of the alloy are similar to those of INCONEL alloy 718 (N07718) except that alloy 706 is more readily fabricated, particularly by machining. Primary uses of the alloy are aerospace and land-based gas turbine parts and components, requiring resistance to creep, and stress rupture up to 1300˚F (704˚C), oxidation resistance, and good fabricability.
INCONEL ALLOY 718 & 718SPF
Inconel alloy 718 is a precipitation-hardenable nickel-chromium alloy also containing significant amounts of iron, niobium, and molybdenum along with lesser amounts of aluminum and titanium. It combines corrosion resistance and high strength with outstanding weldability, including resistance to post-weld cracking. The alloy has excellent creep-rupture strength at temperatures to 1300°F (700°C). Used in gas turbines, rocket motors, spacecraft, nuclear reactors, pumps, and tooling. INCONEL alloy 718SPF is a special version designed for superplastic forming.
INCONEL ALLOY 725
A nickel-chromium-molybdenum-niobium alloy that is highly resistant to corrosion and is age hardenable for extremely high strength. The strength of this alloy is developed by heat treatment to achieve high ductility and toughness. The alloy is resistant to hydrogen embrittlement and stress-corrosion cracking. Used for hangers, landing nipples, side pocket mandrels, and polished bore receptacles in sour gas service. Also used for high-strength fasteners in marine applications.
INCONEL ALLOY 740H
A nickel-chromium-cobalt superalloy age hardened by the precipitation of a gamma prime second phase. Alloy 740H exhibits excellent high-temperature strength in age-hardened conditions up to 1500°F (815°C). With its high contents of chromium and cobalt, alloy 740H offers excellent resistance to oxidation, carburization, and sulfidation at elevated temperatures. Alloy 740H is targeted for use as advanced power production boiler tubes.
INCONEL ALLOY X-750
A nickel-chromium alloy similar to INCONEL alloy 600 but made precipitation hardenable by additions of aluminum and titanium. It has good resistance to corrosion and oxidation along with high tensile and creep-rupture properties at temperatures to 1300°F (700°C). Its excellent relaxation resistance is useful for high-temperature springs and bolts. Used in gas turbines, rocket engines, nuclear reactors, pressure vessels, tooling, and aircraft structures.
INCONEL ALLOY 75
A nickel-chromium alloy similar to INCONEL alloy X-750 but with increased aluminum content for greater precipitation hardening. This alloy was designed for use as exhaust valves in internal-combustion engines. In that application, the alloy offers high strength at operating temperatures, high hot hardness for wear resistance, and corrosion resistance in hot exhaust gases containing lead oxide, sulfur, bromine, and chlorine.
INCONEL ALLOY 783
An oxidation-resistant low coefficient of thermal expansion (low CTE) superalloy developed for gas turbine applications. The alloy is strengthened by a precipitation-hardening heat treatment made possible by additions of niobium and aluminum. In addition, the aluminum content provides excellent resistance to oxidation at high temperatures. The alloy’s density is 5% less than those of superalloys such as INCONEL alloy 718. The combination of low expansion, high strength, and excellent resistance to oxidation makes the alloy especially useful for gas turbine and steam turbine components. The low expansion enables closer control of clearances and tolerances for greater power output and fuel efficiency.
INCONEL ALLOY 22
By virtue of its contents of chromium, molybdenum, tungsten, and controlled iron, this alloy exhibits excellent resistance to both oxidizing and reducing acid environments, as well as those containing mixed acids. It is particularly useful for resistance to pitting and crevice corrosion in acid-halide environments. Applications include chemical processing, pollution control, flue gas desulfurization, waste incineration, and pulp and paper processing industries.
INCONEL ALLOY C-276
A nickel-molybdenum-chromium alloy with an addition of tungsten has excellent corrosion resistance in a wide range of severe environments. The high molybdenum content makes the alloy especially resistant to pitting and crevice corrosion. The low carbon content minimizes carbide precipitation during welding to maintain corrosion resistance in as-welded structures. Used in pollution control, chemical processing, pulp and paper production, and waste treatment.
INCONEL ALLOY G-3
A nickel-chromium-iron alloy with additions of molybdenum and copper. It has good weldability and resistance to intergranular corrosion in the welded condition. The low carbon content helps prevent sensitization and consequent intergranular corrosion of weld heat-affected zones. Used for flue-gas scrubbers and for handling phosphoric and sulfuric acids.
INCONEL ALLOY HX
A nickel-chromium-iron-molybdenum alloy with outstanding strength and oxidation resistance at temperatures to 2200°F (1200°C). Matrix stiffening provided by the molybdenum content results in high strength in a solid-solution alloy having good fabrication characteristics. Used in gas turbines, industrial furnaces, heat-treating equipment, and nuclear engineering.
INCONEL ALLOY N06230
A carbide strengthened nickel-chromium tungsten alloy with an exceptional combination of strength, stability, and resistance to corrosion at very high temperatures. Alloy N06230 offers particularly good resistance to oxidation at temperatures greater than 1800°F (980°C). It also offers good resistance to carburization and nitridation. Potential applications for this alloy include equipment and components for land-based gas turbines, thermal and petrochemical processing, heat treating, and ore refining.
Inconel® Alloys Applications
Oil & Gas Extraction
It is used widely in oil and gas extraction, mainly due to the high-temperature resistance that it offers and its ability to resist oxidation. Corrosive gases present during the extraction process can damage even the most resistant of metals, Inconel 625® is especially suited for the processing systems needed for natural gas production, being used for the separation of extracted fluids.
Due to its higher costs in comparison to steel and aluminum the automotive industry has been slow to take up the use of the superalloy, instead, they are machining smaller, important parts in the metal.
Tesla is now claiming to use Inconel® in place of steel in the main battery pack contactor of its Model S so that it remains springy under the heat of heavy current. Tesla claims that this allows these upgraded vehicles to safely increase the maximum pack output from 1300 to 1500 amperes, allowing for an increase in power output (acceleration) that Tesla refers to as “Ludicrous Mode”. Other manufacturers using Inconel are Ford, Jaguar, and BMW.
It is also used for after-market products, especially for high-temperature exhaust work. It copes with heat far better than stainless steel but can be used to create components even lighter than Titanium.
Heat Treat Applications
All forms of the alloy are resistant to extreme temperatures, and retain enough tensile strength at high temperatures to continue holding moderate loads (Inconel 625® retains 13.3 KSI tensile strength at 2,000°F). This makes Inconel® the ideal basket material for heat treat applications—comparing favorably to stainless steel alloys such as Grade 304, 316, and 330 SS.
Rapid Temperature Fluctuation Environments
A large temperature differential can be seen in some manufacturing processes, and this can happen in rapid succession. A number of Inconel alloys are able to retain their excellent oxidation resistance during the process even during fluctuations from near-cryogenic lows to high-temperature highs.
Some grades of stainless steel exhibit excellent resistance to salt water, and they are certainly more cost-effective. However, Inconel®, in addition to high corrosion resistance is able to handle the environment if the temperature exceeds 1,000°F, retaining far better corrosion resistance at higher temperatures than 316 stainless steel.
Jet engines operate under the most extreme of temperatures, both high and low during their day-to-day use. At 36,000 ft the air temperature falls as low as -69.3°F yet the combustion process can exceed 2120°F.
Various cooling technologies are used to rapidly reduce the temperature of the engine’s components but it is common for the temperature to exceed the tolerances of many metals. Inconel 600® has the ability to retain its high resistance to oxidation and maintains good tensile strength despite rapid fluctuations in temperature.
Inconel® is an ideal material whenever extreme temperature and chemical resistance are a must, and for any process where temperature highs would normally degrade the oxidation resistance of other metals. This has led to its use in space vehicles from SpaceX, NASA, and Rocketdyne.
Inconel is just one superalloy, or super material, read about the others HERE.