Inconel, a name synonymous with high-performance superalloys, has carved out a niche in some of the most demanding industrial applications. This article delves into Inconel’s chemical composition, its grades, and the unique features that make it an indispensable material in aerospace, nuclear, automotive, and other industries.
Introduction to Inconel
Inconel refers to a family of nickel-chromium-based superalloys, developed and trademarked by Special Metals Corporation. The inception of Inconel alloys traces back to the 20th century, with the primary intent to withstand extreme temperatures and corrosive environments. Over time, these high-performance metals have found applications in diverse fields, including aerospace, nuclear reactors, and chemical processing plants.
The Chemical Composition of Inconel
Inconel alloys are essentially nickel-based, with chromium appearing as the second most abundant element. While the chemical composition varies across different Inconel types, all members of the family share a common trait – a nickel-chromium matrix. Other elements such as iron, cobalt, molybdenum, niobium, titanium, and small quantities of manganese, silicon, aluminum, and titanium also make their way into Inconel’s composition.
General Chemical Composition
Here is a general composition of Inconel:
- Nickel: 58% minimum
- Chromium: 20-23%
- Iron: 5% maximum
- Molybdenum: 8-10%
- Niobium (plus Tantalum): 3.15-4.15%
- Cobalt: 1% maximum
- Manganese: 0.5% maximum
- Silicon: 0.5% maximum
- Aluminum: 0.4% maximum
- Titanium: 0.4% maximum
This composition, however, may vary depending on the specific grade of Inconel.
The Grades of Inconel
Inconel’s family includes various grades, each designed to serve specific applications. These grades differ in their chemical composition, offering unique properties. The following sections delve into the most common Inconel grades – Inconel 600, Inconel 718, Inconel 825, Inconel 800, and Inconel 625.
Inconel 600
Inconel 600 is a non-magnetic, nickel-based high-temperature alloy possessing an excellent combination of high strength, hot and cold workability, and resistance to ordinary form of corrosion. This alloy also displays good heat resistance and freedom from aging or stress corrosion throughout the annealed to heavily cold worked condition range.
- Nickel: 72% minimum
- Chromium: 14-17%
- Iron: 6-10%
Inconel 718
Inconel 718 is a high-strength, corrosion-resistant nickel chromium material used at -423° to 1300°F. The age-hardenable alloy can be readily fabricated, even into complex parts. Its welding characteristics, especially its resistance to postweld cracking, are outstanding.
- Nickel: 50-55%
- Chromium: 17-21%
- Iron: Balance
- Molybdenum: 2.8-3.3%
- Niobium (plus Tantalum): 4.75-5.5%
Inconel 825
Inconel 825 or Alloy 825 is a nickel-iron-chromium alloy with additions of molybdenum, copper and titanium. This nickel steel alloy’s chemical composition is designed to provide exceptional resistance to many corrosive environments.
- Nickel: 38-46%
- Chromium: 19.5-23.5%
- Iron: 22% minimum
- Molybdenum: 2.5-3.5%
- Copper: 1.5-3%
Inconel 800
Inconel 800 is a combination of nickel and chromium alloy that has excellent tensile strength and resistance to oxidation as well as carburzation properties. This alloy is extremely functional as it maintains a stable structure, even when exposed to high temperature.
- Nickel: 30-35%
- Chromium: 19-23%
- Iron: 39.5% minimum
Inconel 625
Inconel 625 is a corrosion and oxidation resistant nickel alloy that is used both for its high strength and outstanding aqueous corrosion resistance. Its outstanding strength and toughness is due to the addition of niobium which acts with the molybdenum to stiffen the alloy’s matrix.
- Nickel: 58% minimum
- Chromium: 20-23%
- Iron: 5% maximum
- Molybdenum: 8-10%
- Niobium (plus Tantalum): 3.15-4.15%
| Grade | Nickel % | Chromium % | Iron % |
| Inconel 600 | 72% minimum | 14-17% | 6-10% |
| Inconel 718 | 50-55% | 17-21% | Balance |
| Inconel 825 | 38-46% | 19.5-23.5% | 22% minimum |
| Inconel 800 | 30-35% | 19-23% | 39.5% minimum |
| Inconel 625 | 58% minimum | 20-23% | 5% maximum |
Inconel’s Unique Properties
Inconel’s distinctive properties come from its nickel-chromium base. The niobium and molybdenum in the alloy offer high strength without the need for precipitation hardening treatment. This alloy matrix imparts excellent resistance to a variety of severe corrosive environments. Additionally, Inconel retains its structural integrity in high-temperature atmospheres, resisting oxidation and scaling.
High-Temperature Strength
Inconel’s high-temperature strength is derived from its solid solution strengthening or precipitation hardening, depending on the alloy. The stiffening effect of molybdenum and niobium on its nickel-chromium matrix gives Inconel 625 its high strength.
Corrosion Resistance
A key advantage of Inconel is its outstanding resistance to corrosive substances. This makes it an ideal choice for components exposed to high temperatures, such as gas turbine blades and nuclear reactor components.
Fabricability
Another significant benefit of Inconel is its high level of fabricability. This makes it an excellent material for various components in plant equipment, such as heat exchangers, transfer piping, and valves.
Applications of Inconel
Inconel’s unique features make it an indispensable material in a variety of industries. Its high strength and excellent fabricability make it a popular choice for parts that require resistance to temperature and stress, such as aircraft ducting systems, engine exhaust systems, and heat-exchanger tubing.
Inconel also plays a significant role in the nuclear industry due to its high strength, excellent uniform corrosion resistance, and resistance to stress cracking. In the chemical processing field, Inconel is widely accepted due to its remarkable resistance to corrosive environments.
Conclusion
Inconel’s unique combination of high-temperature strength, corrosion resistance, and ease of fabrication makes it an essential material in some of the most demanding applications. Its various grades, each with its unique chemical composition, ensure that there is an Inconel alloy for every need.
