High performance nickel alloys
Inconel 62527th April 2020
Alloy 625 is part of the family of nickel alloys based upon a mostly nickel-chromium composition and is widely in demanding applications such as oil and gas. It provides 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. It is better known by the Special Metals Corporation tradename Inconel®.
Inconel 625 was developed in the 1960s and achieves reasonably high strength through the addition of molybdenum and niobium to the nickel-chromium base. This imparts solution-strengthening, meaning that no additional heat treatment stage is required. It can comfortably be used up to 650degC with little loss of mechanical properties, and actually retains much of its strength up to 800degC and down to cryogenic temperatures.
Corrosion resistance is excellent in the most aggressive of environments, as it is particularly resistant to pitting and crevice corrosion with a pitting resistance equivalent number (PREN) of over 45. This is attributed to the high level of chromium and molybdenum content.
Inconel 625 is 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.
Due to its excellent corrosion resistance, Inconel 625 is widely used in wellhead components, downhole equipment for corrosive/sour service and chemical process equipment. More niche applications include nuclear energy production for reactor core and control rod components, plus occasionally marine shafts and fasteners. As well as excellent corrosion resistance, Inconel 625 is non-magnetic and spark resistant. This set of physical properties is useful in applications such as downhole equipment and tooling for oil and gas exploration. As it is effectively austenitic in terms of microstructure, fabrication and processing is reasonably straightforward with the appropriate tooling and set-ups.
For applications where Inconel 625 properties are ideal, but the level of mechanical properties insufficient then Inconel 725 (Alloy 725, UNS N07725) can be specified. Based upon Inconel 625, controlled additions of titanium combined with further heat treatment can increase the strength 2-3 times. This precipitation treated condition is also known as precipitation strengthened, age-hardened or aged. 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.
Langley Alloys stocks Alloy 625 as solid bars between 1/2” (12.7mm) and 10” (254mm) diameter. Our stock is certified to API 6A CRA and NACE MR0175 / ISO 15156-3.
Alloy 625 is also described or covered by the following terms; NiCr22Mo9Nb, BS 3076 NA21, Werkstoff 2.4856, UNS N06625 and ASTM B564/B466.
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