What is Inconel?

1st June 2020

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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, associated with the original development of 18 different alloys.

The one thing that each Inconel alloy has in common is that chromium is the main alloying element after nickel, although a far wider range of additional elements are used to provide varying levels of properties and performance.

The very first Inconel alloy brought to market was Inconel 600. It now finds use in nuclear reactors, but the original target application was far humbler: milk cans. Based upon a chemical composition of >72% nickel, with 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 latest alloy to carry the Special Metals Corporation trademark is Inconel 693. This alloy was launched in 2006 and was developed specifically to resist metal dusting – a phenomenon that can occur in gas synthesis reactors. It can resist oxidation and carburisation up to as high as 1150degC, partly due to its very high chromium content of up to 31%.

Nickel-based alloys offer good performance at higher temperatures. The melting temperature of nickel is 1453degC, which is far higher than metals such as copper (1084degC) and aluminium (660degC). The ability to operate at elevated temperatures though is not only linked to metals melting temperature, otherwise iron (1150degC) or steel (1400degC) would be more widely used. The additional property of nickel and nickel alloys which allows them to operate at elevated temperatures is their ability to form a thick, stable, passivating oxide layer when heated which protects them from further attack. This oxide layer can be several microns in thickness, dependent upon the temperature and environment to which the metal has been exposed.

Perhaps the most widely specified alloys, particularly in Oil and Gas related applications, are Inconel 718 and Inconel 625.

Inconel 718 was launched in 1962, initially for the aerospace industry, but due to its ready availability in the market it was taken-up by the oil and gas industry. This was associated with wells moving offshore, drilling deeper with higher temperatures, pressures and corrosive contaminants. Inconel 718 utilises precipitation treatment to achieve significantly increased strengths, whilst still retaining much of the toughness (impact strength) and formability (elongation) of the base alloy.

Inconel 625 was launched just after Inconel 718 in 1964. Inconel 625 achieves reasonably high strength through the addition of molybdenum and niobium to the nickel-chromium base, but nowhere as high as Inconel 718. However, due to the high level of chromium and molybdenum it is corrosion resistant in the most aggressive of environments. It is particularly resistant to pitting and crevice corrosion with a pitting resistance equivalent number (PREN) of over 45.

Langley Alloys stocks 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. Our stock is certified to API 6A CRA and NACE MR0175 / ISO 15156-3.

Inconel 725 was developed in 1989 and seeks to address the obvious limitation of Inconel 625 by increasing its strength. Retaining the same basic composition as Inconel 625, and therefore the same excellent corrosion resistance, additions of titanium are used in combination with further heat treatment to achieve age-hardening.

Langley Alloys stocks Alloy 725 (UNS N07725) as solid bars between 1” (25.4mm) and 4” (101.6mm) diameter, suitable for fastener manufacturer. Our stock is certified to API 6A CRA and NACE MR0175 / ISO 15156-3.