Inconel 625 (Alloy 625, UNS N06625, 2.4856) is a nickel-based alloy that provides good levels of strength and excellent corrosion resistance at elevated temperatures. Its composition is predominantly nickel (58% min.), with additions of chromium, then lesser amounts of molybdenum, niobium, iron, tantalum, cobalt, and trace amounts of manganese, silicon, aluminium, and titanium.
Inconel 625 was developed in the 1960s by Inco (now part of Special Metals Corporation), hence the associated Inconel trademark. The Inconel trademark has become ubiquitous for a family of nickel-chromium alloys, even though versions are now produced by other manufacturers. Its original development was driven by the need for alloys to work at ever-increasing temperatures in power generation (supercritical steam) – as this increases the thermodynamic efficiency of the units. Initial applications would have included steam piping, but these have broadened significantly over time to include aerospace, motorsport exhausts, industrial gas off-takes, heat exchangers, fuel lines and even rocket components. As metallurgical understanding and steelmaking practices have improved, the alloy’s composition has been slightly modified with time. The current specification is more creep-resistant and weldable than the very first iteration, encouraging its use in pump and valve components for power stations at high temperatures and pressures.
Nickel alloys are well-suited to high-temperature applications due to the formation of a stable passive oxide layer that prevents further oxidation of the metal occurring. Adding chromium improves resistance to corrosion, but does little to enhance the mechanical properties. In the case of Inconel 625, additions of niobium and manganese are responsible for the uplift in yield strength. It achieves a minimum yield strength of 50ksi (345N/mm2) in diameters greater than 4” (100mm) and 60ksi (415N/mm2) for smaller diameters. The minimum tensile strength is 110ksi (760N/mm2) and 120ksi (830N/mm2) respectively.
Small quantities of manganese, typically < 0.5% maximum, provide a degree of solid solution strengthening. In metallurgical terms, this means that the manganese goes into solution with the nickel and chromium, but stresses the crystallographic structure slightly. This is due to a mismatch in the relative size of the atoms that make up the structure. This disturbance hinders the movement of dislocations and other imperfections in the crystal structure, thereby increasing its strength under load.
Niobium is similarly soluble in nickel-chromium alloys, but is most commonly used in other alloys such as Inconel 718 to provide precipitation strengthening. It will combine with carbon to form small particles of niobium carbide after an ageing heat treatment that greatly increases the yield strength. However, in the case of Inconel 625, it is used in higher quantities, typically 3-4%. Here it adds to the overall solution strengthening, and would improve creep performance at higher temperatures and longer operating times, but is not fully exploited through ageing. There are some customer-specific requirements for aged Alloy 625, taking the solution annealed product and subjecting it to additional ageing and testing, but this is a relatively infrequent request.
Although Inconel 625 does not provide the very high strength levels of Inconel 718 and other precipitation strengthened alloys, the yield strength is comparable with duplex stainless steels, and just below that of super duplex stainless steels. However, properties are more consistent over a wider temperature range. Impact strength is retained down to cryogenic temperatures (-196degC), whilst there is a transition to brittle fracture for ferritic and austenitic-ferritic duplex stainless steels somewhere between -70 to -50degC. Similarly, their mechanical properties are reasonably consistent up to 650degC, above which temperature there is a significant drop in yield and tensile strength. The nickel content ensures continued resistance to oxidation up to nearly 1000degC, so it can be used in non-structural or load-bearing applications.
Resistance to creep and fatigue is good up to 650degC, as there is a very slow precipitation process that takes place at or above this temperature that reduces mechanical properties slightly. Cold working can be used to moderately increase strength levels. Fabrication and welding is also straightforward with the correct equipment, settings and weld qualifications.
Although primarily used for applications that exploit its mechanical properties over the wide temperature range, this is ably supported by excellent corrosion resistance in an array of media. With more than 20% chromium and up to 10% molybdenum, Inconel 625 has a pitting resistance equivalent number (PREN) of 45 or more. This ensures high pitting and crevice corrosion resistance. Therefore, it is ideal for seawater, brackish or polluted water exposure, and used in the construction of marine components and oceanographic equipment.
The chemical processing industry is a common application. Besides its corrosion resistance and tolerance of elevated temperatures, the ease of welding and fabrication make it sought after for the production of manufacturing equipment such as heat exchangers, valves and connectors.
Alloy 625 is included within NACE MR-1075 (ISO 15156-3) as suitable for use in environments contains hydrogen sulphide i.e. H2S. Therefore, it is commonly specified in oil and gas applications where the well is deemed ‘sour’, and sulphide stress cracking is a risk. The microstructure and inherent toughness of this grade is well-suited to tooling, downhole tubing and hangers.
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