Facts at your Fingertips: Nickel-based Superalloys

Superalloys, also known as high-performance alloys, are considered as materials of construction for process equipment in applications involving high temperatures, highly corrosive environments and those requiring high strength properties. Many commonly used superalloys in the chemical process industries (CPI) use nickel as their primary metal. This one-page reference provides information on Ni and reviews common Ni superalloys, including combinations of Ni with chromium, copper, iron, molybdenum, cobalt, titanium and other elements.

Nickel

Ni appears as a silvery-white metal, and is magnetic at ambient temperature. Its abundance in the earth’s crust is 0.009%, a substantial amount of which arrived with meteorites. The world’s nickel resources are currently estimated at almost 300 million tons, according to the Nickel Institute (Toronto, Ont.; www.nickelinstitute.org). Australia, Indonesia, South Africa, Russia and Canada account for more than 50% of global nickel resources. The minerals from which most nickel is extracted are iron/nickel sulfides such as pentlandite. It is also found in other minerals, including garnierite.

Under the unified numbering system (UNS) for metals, nickel and Ni-based alloys are designated with the prefix “N.” Commercially pure (99.6%) wrought nickel (Nickel 200; UNS N02200) has good mechanical properties and corrosion resistance. Duranickel (UNS N03301) is the tradename for an alloy of nickel (94.0% Ni) with aluminum (4.5%) and titanium (0.5%) developed by Special Metals Corp. (New Hartford, N.Y.; www.specialmetals.com) that has greater strength and hardness than Ni 200.

Nickel retains an austenitic, face-centered-cubic crystal lattice structure up to its melting point (Table), so it avoids a ductile-to-brittle transition.

Ni-based alloys

Several properties of the element nickel have made it useful in the development of high-performance alloys. Chief among them is nickel’s ability to alloy with most other metals. For example, complete solid solubility between nickel and copper, and wide solubility ranges between nickel and elements such as iron, chromium and others, exist, making many alloy combinations possible. The Ni content in Ni-based superalloys generally ranges from 53 to 99%. Once made, Ni-based alloys offer outstanding ductility, malleability and formability, and are easily weldable.

Many nickel alloys perform well in temperatures up to 1,000°C, even for structural use. Ni-based superalloys exhibit excellent creep strength (resistance to deformation), oxidation resistance and fracture toughness. Commercial Ni-based alloys can be categorized generally into those made to withstand high temperatures and dry or gaseous corrosion, and a second group made for low-temperature (aqueous) applications.

Ni-Cu alloys

Nickel-copper alloys can have a range of mechanical properties, and are generally free from stress-corrosion cracking. They also exhibit good strength and toughness at sub-zero temperatures and are highly resistant to corrosion by saltwater.

Monel is a tradename for a series of Ni-Cu alloys made by Special Metals Corp. The most common of this series of alloys are Monel 400 (UNS N04400), Monel R-405 (a free-machining alloy), and Monel K-500 (improved strength and hardness compared to Monel 400).

Ni-Cr-Mo alloys

Nickel-chromium-molybdenum alloys provide high corrosion resistance, especially with reducing acids, such as hydrochloric acid (HCl) and sulfuric acid (H₂SO₄). Ni-Cr-Mo alloys are used for their ability to maintain high strength at high temperatures and corrosive environments.

Hastelloy is a tradename used by Haynes International (Kokomo, Ind.; www.haynesintl.com) for a series of Ni-Cr-Mo alloys. There are several grades of Hastelloy, and they are used in the construction of reactors, agitators, heat exchangers, reboilers, columns, trays, packing, pipework, couplings, valves, bolts, pumps, filters, dryers, and thermal oxidizers. These alloys provide improved performance over stainless steels at low and high temperatures.

An example composition of a common Ni-Cr-Mo alloy includes primarily Ni, with 15–17% Mo, 14.5–16.5% Cr, 4–7% Fe, 3–4.5% tungsten, and smaller concentrations of other elements, such as manganese.

Ni-Cr and Ni-Cr-Fe

Nickel-chromium alloys are noted for strength and corrosion resistance at high temperatures. Some of these alloys are derived from the Ni-Cr group by adding aluminum or titanium or both for precipitation hardening (a heat-treatment process that results in combinations of elements coming out of solution as small particles within the alloy’s microstructure). Controlled precipitation hardening of Ni-Cr and Ni-Cr-Fe alloys allows increased strength and hardness. Several alloy variations in this class have been developed by Special Metals Corp. and are sold under the tradenames Inconel (a series that generally contains Ni and Cr with smaller amounts of Fe and other elements) and Incoloy (a Ni-, Cr- and Mo-containing series with Fe as the primary element).

_{Department Editor: Scott Jenkins}