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Cylinder of Inconel 718

Inconel is a family of austenitic nickel-chromium-based superalloys.[1] The name is a trademark of Special Metals Corporation.

Inconel alloys are typically used in high temperature applications. It is sometimes referred to in English as "Inco" (or occasionally "Iconel"). Common trade names for Inconel Alloy 625 include: Inconel 625, Chronin 625, Altemp 625, Haynes 625, Nickelvac 625 and Nicrofer 6020.[2]

The Inconel family of alloys was first developed in the 1940s by research teams at Wiggin Alloys (Hereford, England), which has since been acquired by SMC,[3] in support of the development of the Whittle jet engine.


Different Inconels have widely varying compositions, but all are predominantly nickel, with chromium as the second element.

Inconel Element (% by mass)
Ni Cr Fe Mo Nb Co Mn Cu Al Ti Si C S P B
600 [4] 72.0 14.0-17.0 6.0-10.0 1.0 0.5 0.5 0.15 0.015
617 [5] 44.2-56.0 20.0-24.0 3.0 8.0-10.0 10.0-15.0 0.5 0.5 0.8-1.5 0.6 0.5 0.15 0.015 0.015 0.006
625 [6] 58.0 20.0-23.0 5.0 8.0-10.0 3.15-4.15 1.0 0.5 0.4 0.4 0.5 0.1 0.015 0.015
690 [7] 59.5 30 9.2 0.35 0.01 0.02 0.35 0.019 0.003
718 [8] 50.0-55.0 17.0-21.0 balance 2.8-3.3 4.75-5.5 1.0 0.35 0.2-0.8 0.65-1.15 0.3 0.35 0.08 0.015 0.015 0.006
X-750 [9] 70.0 14.0-17.0 5.0-9.0 0.7-1.2 1.0 1.0 0.5 0.4-1.0 2.25-2.75 0.5 0.08 0.01


Inconel alloys are oxidation and corrosion-resistant materials well suited for service in extreme environments subjected to high pressure and kinetic energy. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high temperature applications where aluminum and steel would succumb to creep as a result of thermally-induced crystal vacancies (see Arrhenius equation). Inconel's high temperature strength is developed by solid solution strengthening or precipitation strengthening, depending on the alloy. In age hardening or precipitation strengthening varieties, small amounts of niobium combine with nickel to form the intermetallic compound Ni3Nb or gamma prime (γ'). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures.[10] The formation of gamma prime crystals increases over time, especially after three hours of a heat exposure of 850 C, and continues to grow after 72 hours of exposure.[11]


Inconel is a difficult metal to shape and machine using traditional techniques due to rapid work hardening. After the first machining pass, work hardening tends to plastically deform either the workpiece or the tool on subsequent passes. For this reason, age-hardened Inconels such as 718 are machined using an aggressive but slow cut with a hard tool, minimizing the number of passes required. Alternatively, the majority of the machining can be performed with the workpiece in a solutionized form, with only the final steps being performed after age-hardening.

External threads are machined using a lathe to "single point" the threads, or by rolling the threads using a screw machine. Holes with internal threads are made by welding or brazing threaded inserts made of stainless steel. Internal threads can also be formed using EDM (Electrical discharge machining).[citation needed]

Cutting of a plate is often done with a waterjet cutter. Internal threads can also be cut by the single point method on a lathe, or by threadmilling on a machining center. New whisker reinforced ceramic cutters are also used to machine nickel alloys. They remove material at a rate typically eight times faster than carbide cutters. 718 Inconel can also be roll threaded after full aging by using induction heat to 1300°F without increasing the grain size.[citation needed] Apart from these methods, Inconel parts can also be manufactured by selective laser melting.


Welding Inconel alloys is difficult due to cracking and microstructural segregation of alloying elements in the heat affected zone. However, several alloys have been designed to overcome these problems. The most common welding methods are gas tungsten arc welding and electron beam welding.[12]

New innovations in pulsed micro laser welding have also become more popular in recent years.


Inconel is often encountered in extreme environments. It is common in gas turbine blades, seals, and combustors, as well as turbocharger rotors and seals, electric submersible well pump motor shafts, high temperature fasteners, chemical processing and pressure vessels, heat exchanger tubing, steam generators in nuclear pressurized water reactors, natural gas processing with contaminants such as H2S and CO2, firearm sound suppressor blast baffles, and Formula One, NASCAR and APR, LLC exhaust systems.[13][14] It is also used in the turbo system of the 3rd generation Mazda RX7, and the exhaust systems of high powered rotary engined Norton motorcycles where exhaust temperatures reach more than 1,000 degrees C.[15] Inconel is increasingly used in the boilers of waste incinerators.[16] The Joint European Torus and DIII-D tokamaks vacuum vessels are made in Inconel.[17] Inconel 718 is commonly used for cryogenic storage tanks, downhole shafts and wellhead parts.[18]

Several applications of inconel in aerospace include:

Rolled Inconel was frequently used as the recording medium by engraving in black box recorders on aircraft.[22]

Alternatives to the use of Inconel in chemical applications such as scrubbers, columns, reactors, and pipes are Hastelloy, perfluoroalkoxy (PFA) lined carbon steel or fiber reinforced plastic.

Inconel alloys[edit]

  • Inconel 600: Solid solution strengthened
  • Inconel 625: Acid resistant, good weldability. The LCF version is typically used in bellows.
  • Inconel 690: Low cobalt content for nuclear applications, and low resistivity[23]
  • Inconel 713: Newly developed alloy
  • Inconel 718: Gamma double prime strengthened with good weldability
  • Inconel 751: Increased aluminum content for improved rupture strength in the 1600 °F range[24]
  • Inconel 792: Increased aluminum content for improved high temperature corrosion properties, used especially in gas turbines
  • Inconel 939: Gamma prime strengthened with good weldability

See also[edit]


  1. ^ "High-Performance Alloys". Special Metals Corporation. Retrieved 2010-04-26. 
  2. ^ "Special Alloys: Inconel 625". Retrieved 2010-04-26. 
  3. ^ "Special Metals Corporation: History". Retrieved 2012-05-18. 
  4. ^ INCONEL alloy 600, Special Metals Corporation
  5. ^, High Temp Metals
  6. ^ INCONEL alloy 625, Special Metals Corporation
  7. ^ Stress Corrosion Cracking of Alloy 600 and Alloy 690 in Hydrogen/Steam at 380C. G. Sui, J. Titchmarsh, G. Heys, J. Congleton. Corrosion Science, Vol 39, No. 3, pp 565-587, 1997
  8. ^ INCONEL alloy 718, Special Metals Corporation
  9. ^ INCONEL alloy X-750, Special Metals Corporation
  10. ^ [1],
  11. ^ [2], University of Cambridge: Dissemination of IT for the Promotion of Materials Science (DoITPoMS).
  12. ^ Joining, retrieved 2009-10-09 .
  13. ^ Power Generation, Special Metals Corporation.
  14. ^ Chemical Processing, Special Metals Corporation.
  15. ^ Motorcycle Trader.Norton Rotary Revival.Cathcart.Dec 2007.
  16. ^ Inconell - state-of-the-art corrosion protection by Babcock & Wilcox Vølund, 2003
  17. ^ The Inconel JET vessel in use since 1983. A simple, sturdy structure.
  18. ^ Inconel Alloy, Inconel 718.
  19. ^ Robert S. Houston, Richard P. Hallion, and Ronald G. Boston, EDITOR'S INTRODUCTION, "Transiting from Air to Space: The North American X-15", The Hypersonic Revolution: Case Studies in the History of Hypersonic Technology, Air Force History and Museums Program, 1998.
  20. ^ Anthony Young, "The Saturn V Booster: Powering Apollo into History", Springer-Verlag, 2009.
  21. ^ "SpaceX Falcon 9". Space Launch Report. Retrieved 2013-08-13. 
  22. ^
  23. ^ INCONEL alloy 690, NDT Resource Center
  24. ^ INCONEL alloy 751, Special Metals Corporation