Nickel is the fifth most common element on earth, but commercial use was limited until the last century by the difficulty of extracting and refining it. The development of the jet engine then became a major catalyst for nickel-based alloys, which combine high strength with performance at high temperatures.
Why nickel performs at high temperatures
The melting temperature of nickel is 1453degC - far higher than copper (1084degC) or aluminium (660degC), though far lower than tungsten (3400degC). Melting point alone is not the reason, or iron (1150degC) and steel (1400degC) would be more widely used. The key property is that nickel and its alloys form a thick, stable, passivating oxide layer when heated, protecting them from further attack. This layer can be several microns thick, depending on temperature and environment. Nickel alloys also resist carburisation, where carbon species are present at high temperature - for example during the cracking of gas in chemical processing or refinery operations.
Strengthening mechanisms
High-temperature strength is retained by solid-solution strengthening or precipitation strengthening, depending on the alloy.
Solid-solution strengthening adds atoms of the alloying element to the crystal lattice of nickel. This disturbance makes deformation more difficult by slowing or blocking the movement of dislocations. Elements such as molybdenum are added to Inconel 625 (Alloy 625, UNS N06625, 2.4856) to raise strength.
In precipitation strengthening, small amounts of niobium, titanium and aluminium combine with nickel to form intermetallic precipitates during a final heat treatment known as ageing. These precipitates also slow the movement of dislocations, increasing strength and toughness, and at higher temperatures reduce the likelihood of creep - which is exploited in aerospace. Nickel alloys that use precipitation strengthening include Inconel 718 (Alloy 718, UNS N07718, 2.4668), Inconel 725 (Alloy 725, UNS N07725), Incoloy 925 (Alloy 925, UNS N09925) and Monel K-500 (Alloy K-500, UNS N05500, 2.4375).
Alloys such as Inconel 718 and Inconel 625 retain much of their mechanical properties to 650degC (1200degF) and can be used selectively at temperatures approaching 1000degC (1800degF).
Corrosion resistance
Because nickel alloys readily with many other metals, corrosion resistance and other properties can be improved alongside strength. Chromium and molybdenum are commonly added to enhance resistance to corrosion and oxidation, with molybdenum well understood to improve resistance to pitting in particular. Copper is used in Incoloy 825 to enhance resistance to reducing acids such as sulphuric, phosphoric and hydrochloric acid.
We supply a range of nickel alloys, widely used in the oil, gas and chemical processing industries.
| Material | Melting point |
|---|---|
| Tungsten | 3400°C |
| Nickel | 1453°C |
| Steel | 1400°C |
| Iron | 1150°C |
| Copper | 1084°C |
| Aluminium | 660°C |