High performance nickel alloys
Inconel 71817th April 2020
Nickel-chromium alloys are probably the most widely used nickel-based alloys. They are characterised by their excellent corrosion resistance at both ambient and elevated temperatures – forming a protective film that prevents further oxidation and retention of their mechanical properties over a wide temperature range.
They are better known by the Special Metals Corporation tradename Inconel®, or the Haynes International Inc. trademark Hastelloy®. There are at least fifteen commercially-available nickel-chromium grades, with the potential for further development and refinement to support new applications.
Inconel 718 is perhaps the most widely known and specified grade within this family of alloys. It was introduced in the 1960’s and initially used in 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. Other nickel-based alloys such as Hastelloy C276 and Inconel 625 provided sufficient corrosion resistance, but insufficient strength without extensive cold working. Martensitic stainless steels were limited by their potential for hydrogen embrittlement and stress corrosion cracking. Therefore, Inconel 718 quickly became a default choice for many pressure-containing and load-bearing components in aggressive environments.
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. There are multiple terms for the precipitation treating of nickel alloys, including aged, age hardened or precipitation strengthened. The objective is to form very fine precipitates, widely dispersed throughout the bar. These act to ‘pin’ microstructural features (grain boundaries) and restrict them from moving when the metal is subjected to an external load. This process is achieved through a combination of chemical composition (selected additions to the alloy) with multi-stage heat treatments. It is available in a number of increasing strength levels depending upon the extent of heat treatment undertaken.
Nickel alloys such as Inconel 718 are well-suited to high temperature applications as most other metals are either brittle or oxidise when exposed to elevated temperatures. However, nickel-based alloys will quickly form a protective oxide layer when heated in an atmosphere containing oxygen, thereby preventing further oxidation. It also has a relatively high melting point. This means that only a handful of more exotic alloys are able to better the performance of nickel-based alloys at high temperatures. Inconel 718 retains its ambient temperature mechanical properties up to 650degC.
Inconel 718 offers very high levels of performance, both corrosion resistance and high strength. As such, applications will tend towards more aggressive environments as load-bearing components. The list of applications in oil and gas, plus chemical process industry, is extensive but includes wellhead and downhole components, sheathing, fasteners, gate valves, choke stems, packers, tubing hangers and other items for corrosive/sour service.
Langley Alloys stocks Alloy 718 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.
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Inconel is a trademark of the Special Metals Corporation group of companies.
Hastelloy is a trademark of Haynes International Inc.Incoloy 825Which specifications apply to Alloy 718?