Early applications tended towards those requiring higher temperature operation, such as aerospace, jet engines and gas turbines. Mechanical properties are retained over a wide range of temperatures; 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 too. Nickel-based alloys exhibit low expansion at these high temperatures, offer a degree of creep resistance under high-stress conditions, and are corrosion resistant. These days, many of these applications would calls for Alloy 625 to be supplied in accordance with the specification AMS 5666. AMS is the Aerospace Materials Specification, and 5666 covers the supply of bars, rings, forgings and some tube sizes. Whilst there is negligible difference in the composition or mechanical properties compared with other common specifications, there is a degree of different testing and certification.
Alloy 625 is now widely used in the oil and gas industry. It has excellent corrosion resistance, illustrated by its Pitting Resistance Equivalent Number (PREN) of greater than 45, which is significantly higher than Alloy 718, and super duplex stainless steels. It is acknowledged as being suitable for sour service (in H2S-containing environments) within NACE MR1075 / ISO 15156-3, and API 6A for “for wellhead and tree equipment”. 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. Risers, tube hangers and high-strength fasteners all make use of Alloy 625’s properties. Natural gas processing equipment such as fluid separation units will also use Alloy 625, on account of its oxidation resistance and good thermal fatigue strength. As it is effectively austenitic in terms of microstructure, fabrication and processing is reasonably straightforward with the appropriate tooling and set-ups.
Inconel 625 has been used in marine applications such as propellers, drive shafts and thrusters on account of its superior corrosion resistance in seawater. However, its relative cost means that it is more commonly substituted by super duplex or duplex stainless steels in these applications. These grades achieve more than acceptable levels of corrosion resistance and lifetime, and higher strengths.
The chemical processing industry provides a myriad of potential applications, due to good resistance to oxidising and reducing acids, excellent toughness and ease of working. Therefore, it is used for valve trim in the more demanding end uses found within this market segment. Sensors and thermowells utilise the high impact toughness and very wide operating temperature range of Incoloy 625, plus its ease of machining.
The construction of pollution control, gas scrubber and waste handling facilities will use Alloy 625 to withstand the corrosive and high temperature off gases, which may contain halides. High-performance sports cars will also utilise this alloy in their exhaust systems, but this is limited to Formula 1 and other racing classes on account of cost. More niche applications include nuclear energy components, such as control rods.
Perhaps the biggest challenge in the use of Alloy 625 is its relative cost compared with other grades. The superior corrosion resistance and ease of fabrication have made it the preferred candidate for the cladding of less corrosion-resistance grades. A thin layer of Alloy 625 is typically applied by weld deposition, and the layer machined to give a consistent finish. The automation of this process helps to keep it cost competitive.
Langley Alloys carries an extensive stock of Alloy 625, produced in accordance with API 6A and NACE MR1075 / ISO15156-3, in sizes from 5/8” (15.88mm) to 9” (228.6mm) diameter.
Inconel is a registered trademark of Special Metals Corporation.