Inconel® is a family of high-performance nickel-based alloys developed by Special Metals Corporation. It is one of the most widely recognised trademarks in the industry, covering the original development of 18 different alloys.
What every Inconel alloy has in common is that chromium is the main alloying element after nickel. Beyond that, a wide range of further elements is used to tune properties and performance.
From milk cans to metal dusting
The first Inconel alloy was Inconel 600. It now finds use in nuclear reactors, but its original target application was far humbler: milk cans. Based on more than 72% nickel, up to 17% chromium and less than 10% iron, it was one of the first alloys to perform well at elevated temperatures of up to 600degC.
The most recent alloy to carry the Special Metals Corporation trademark is Inconel 693. Launched in 2006, it was developed specifically to resist metal dusting, a phenomenon that can occur in gas synthesis reactors. It resists oxidation and carburisation up to 1150degC, partly due to its very high chromium content of up to 31%.
Why nickel performs at high temperatures
Nickel-based alloys perform well at higher temperatures. The melting temperature of nickel is 1453degC, far higher than copper (1084degC) or aluminium (660degC). But melting point is not the whole story - otherwise iron (1150degC) or steel (1400degC) would be more widely used. The key is that nickel and its alloys form a thick, stable, passivating oxide layer when heated, which protects them from further attack. This layer can be several microns thick, depending on the temperature and environment.
The most widely specified grades
The most widely specified Inconel alloys, particularly in oil and gas applications, are Inconel 718 and Inconel 625.
Inconel 718 was launched in 1962, initially for aerospace, but its ready availability saw it taken up by the oil and gas industry as wells moved offshore and drilled deeper, with higher temperatures, pressures and corrosive contaminants. Inconel 718 uses precipitation treatment to achieve significantly increased strength while retaining much of the toughness (impact strength) and formability (elongation) of the base alloy.
Inconel 625 followed in 1964. It reaches reasonably high strength through additions of molybdenum and niobium to the nickel-chromium base - not as high as Inconel 718, but its high chromium and molybdenum content make it corrosion resistant in the most aggressive environments. It is particularly resistant to pitting and crevice corrosion, with a pitting resistance equivalent number (PREN) of over 45.
We stock Alloy 718 (2.4668, UNS N07718) and Alloy 625 (2.4856, UNS N06625) as solid bars between 5/8" (15.875mm) and 10" (254mm) diameter. This stock is certified to API 6A CRA and NACE MR0175 / ISO 15156-3.
Inconel 725 was developed in 1989 to address the obvious limitation of Inconel 625 by increasing its strength. It keeps the same basic composition, and so the same excellent corrosion resistance, using additions of titanium with further heat treatment to achieve age-hardening. We stock Alloy 725 (UNS N07725) as solid bars between 1" (25.4mm) and 4" (101.6mm) diameter, suitable for fastener manufacture. This stock is certified to API 6A CRA and NACE MR0175 / ISO 15156-3.