Inconel is a nickel-chromium-based superalloy often utilized in extreme environments where components are subjected to high temperature, pressure or mechanical loads. Inconel alloys are oxidation- and corrosion-resistant. 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 aluminium and steel would succumb to creep as a result of thermally-induced crystal vacancies. Inconel's high-temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy.[1][2]
Inconel alloys are typically used in high temperature applications. Common trade names for
The Inconel family of alloys was first developed before December 1932, when its trademark was registered by the US company International Nickel Company of Delaware and New York.[5][6] A significant early use was found in support of the development of the Whittle jet engine,[7] during the 1940s by research teams at Henry Wiggin & Co of Hereford, England a subsidiary of the Mond Nickel Company,[8] which merged with Inco in 1928. The Hereford Works and its properties including the Inconel trademark were acquired in 1998 by Special Metals Corporation.[9]
| Alloy | Solidus °C (°F) | Liquidus °C (°F) |
|---|---|---|
| Inconel 600[10] | 1354 (2,469) | 1413 (2,575) |
| Inconel 617[11][12] | 1332 (2,430) | 1377 (2,511) |
| Inconel 625[13] | 1290 (2,350) | 1350 (2,460) |
| Inconel 690[14] | 1343 (2,449) | 1377 (2,511) |
| Inconel 718[15] | 1260 (2,300) | 1336 (2,437) |
| Inconel X-750[16] | 1390 (2,530) | 1430 (2,610) |
Inconel alloys vary widely in their compositions, but all are predominantly nickel, with chromium as the second element.
| Inconel | Element, proportion by mass (%) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ni | Cr | Fe | Mo | Nb & Ta | Co | Mn | Cu | Al | Ti | Si | C | S | P | B | |
| 600[17] | ≥72.0[a] | 14.0–17.0 | 6.0–10.0 | — | ≤1.0 | ≤0.5 | ≤0.5 | ≤0.15 | ≤0.015 | ||||||
| 617[18] | 44.2–61.0 | 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 | 0.05–0.15 | ≤0.015 | ≤0.015 | ≤0.006 | |
| 625[19] | ≥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[20] | ≥58 | 27–31 | 7–11 | ≤0.50 | ≤0.50 | ≤0.50 | ≤0.05 | ≤0.015 | |||||||
| Nuclear grade 690[20] | ≥58 | 28–31 | 7–11 | ≤0.10 | ≤0.50 | ≤0.50 | ≤0.50 | ≤0.04 | ≤0.015 | ||||||
| 718[1] | 50.0–55.0 | 17.0–21.0 | Balance | 2.8–3.3 | 4.75–5.5 | ≤1.0 | ≤0.35 | ≤0.3 | 0.2–0.8 | 0.65–1.15 | ≤0.35 | ≤0.08 | ≤0.015 | ≤0.015 | ≤0.006 |
| X-750[21] | ≥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 | |||
When heated, Inconel forms a thick and stable passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high-temperature applications where aluminium 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 double prime (γ″). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures. The formation of gamma-prime crystals increases over time, especially after three hours of a heat exposure of 850 °C (1,560 °F), and continues to grow after 72 hours of exposure.[22]
The most prevalent hardening mechanisms for Inconel alloys are precipitate strengthening and solid solution strengthening. In Inconel alloys, one of the two often dominates. For alloys like Inconel 718, precipitate strengthening is the main strengthening mechanism. The majority of strengthening comes from the presence of gamma double prime (γ″) precipitates.[23][24][25][26] Inconel alloys have a γ matrix phase with an FCC structure.[25][27][28][29] γ″ precipitates are made of Ni and Nb, specifically with a Ni3Nb composition. These precipitates are fine, coherent, disk-shaped, intermetallic particles with a tetragonal structure.[24][25][26][27][30][31][32][33]
Secondary precipitate strengthening comes from gamma prime (γ') precipitates. The γ' phase can appear in multiple compositions such as Ni3(Al, Ti).[24][25][26] The precipitate phase is coherent and has an FCC structure, like the γ matrix;[33][27][30][31][32] The γ' phase is much less prevalent than γ″. The volume fraction of the γ″ and γ' phases are approximately 15% and 4% after precipitation, respectively.[24][25] Because of the coherency between the γ matrix and the γ' and γ″ precipitates, strain fields exist that obstruct the motion of dislocations. The prevalence of carbides with MX(Nb, Ti)(C, N) compositions also helps to strengthen the material.[25] For precipitate strengthening, elements like niobium, titanium, and tantalum play a crucial role.[34]
Because the γ″ phase is metastable, over-aging can result in the transformation of γ″ phase precipitates to delta (δ) phase precipitates, their stable counterparts.[25][27] The δ phase has an orthorhombic structure, a Ni3(Nb, Mo, Ti) composition, and is incoherent.[35][29] As a result, the transformation of γ″ to δ in Inconel alloys leads to the loss of coherency strengthening, making for a weaker material. That being said, in appropriate quantities, the δ phase is responsible for grain boundary pinning and strengthening.[33][32][29]
Another common phase in Inconel alloys is the Laves intermetallic phase. Its compositions are (Ni, Cr, Fe)x(Nb, Mo, Ti)y and NiyNb, it is brittle, and its presence can be detrimental to the mechanical behavior of Inconel alloys.[27][33][36] Sites with large amounts of Laves phase are prone to crack propagation because of their higher potential for stress concentration.[31] Additionally, due to its high Nb, Mo, and Ti content, the Laves phase can exhaust the matrix of these elements, ultimately making precipitate and solid-solution strengthening more difficult.[32][36][28]
For alloys like Inconel 625, solid-solution hardening is the main strengthening mechanism. Elements like Mo are important in this process. Nb and Ta can also contribute to solid solution strengthening to a lesser extent.[34] In solid solution strengthening, Mo atoms are substituted into the γ matrix of Inconel alloys. Because Mo atoms have a significantly larger radius than those of Ni (209 pm and 163 pm, respectively), the substitution creates strain fields in the crystal lattice, which hinder the motion of dislocations, ultimately strengthening the material.
The combination of elemental composition and strengthening mechanisms is why Inconel alloys can maintain their favorable mechanical and physical properties, such as high strength and fatigue resistance, at elevated temperatures, specifically those up to 650°C.[23]
Inconel is a difficult metal to shape and to machine using traditional cold forming 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 typically 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,[clarification needed] with only the final steps being performed after age hardening. However some claim[by whom?] that Inconel can be machined extremely quickly with very fast spindle speeds using a multifluted ceramic tool with small width of cut at high feed rates as this causes localised heating and softening in front of the flute.
External threads are machined using a lathe to "single-point" the threads or by rolling the threads in the solution treated condition (for hardenable alloys) using a screw machine. Inconel 718 can also be roll-threaded after full aging by using induction heat to 700 °C (1,290 °F) without increasing the grain size.[citation needed] Holes with internal threads are made by threadmilling. Internal threads can also be formed using a sinker electrical discharge machining (EDM).[citation needed]
Welding of some Inconel alloys (especially the gamma prime precipitation hardened family; e.g., Waspaloy and X-750) can be difficult due to cracking and microstructural segregation of alloying elements in the heat-affected zone. However, several alloys such as 625 and 718 have been designed to overcome these problems. The most common welding methods are gas tungsten arc welding and electron-beam welding.[37]
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 and core components in nuclear pressurized water reactors,[38] natural gas processing with contaminants such as H2S and CO2, firearm sound suppressor blast baffles, and Formula One, NASCAR, NHRA, and APR, LLC exhaust systems.[39][40] It is also used in the turbo system of the 3rd generation Mazda RX7, and the exhaust systems of high powered Wankel engined Norton motorcycles where exhaust temperatures reach more than 1,000 °C (1,830 °F).[41] Inconel is increasingly used in the boilers of waste incinerators.[42] The Joint European Torus and DIII-D tokamaks' vacuum vessels are made of Inconel.[43] Inconel 718 is commonly used for cryogenic storage tanks, downhole shafts, wellhead parts,[44] and in the aerospace industry -- where it has become a prime candidate material for constructing heat resistant turbines.[45]
Rolled Inconel was frequently used as the recording medium by engraving in black box recorders on aircraft.[64]
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.
Alloys of inconel include:
In age hardening or precipitation strengthening varieties, alloying additions of aluminum and titanium combine with nickel to form the intermetallic compound Ni3(Ti,Al) or gamma prime (γ′). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures.
