Inconel 625

What is Inconel 625?

Inconel 625 is a nickel-chromium-molybdenum-niobium solid solution alloy. Its designations include UNS N06625 and material number 2.4856. It is used from cryogenic temperatures to 982 degrees Celsius where a component requires high strength and resistance to severe corrosion. Molybdenum and niobium strengthen the nickel-chromium matrix. The alloy therefore does not require precipitation hardening and has high strength in the annealed condition.

Inconel 625 is widely used for seawater and subsea hardware. The alloy originator identifies four relevant properties: resistance to local attack by pitting and crevice corrosion, high corrosion-fatigue strength, high tensile strength and resistance to chloride ion stress corrosion cracking. Applications include mooring cables, propeller blades, submarine fittings, undersea communication cable sheathing and steam line bellows.

Inconel 625 has a density of 8.44 grams per cubic centimetre and a melting range of 1290 to 1350 degrees Celsius. NACE MR0175 lists the alloy for sour service.

Laxcon Steels lists Inconel 625 in its grades reference as a nickel alloy grade. The same steel is written Alloy 625, N06625 and 2.4856.

What is the chemical composition of Inconel 625?

Composition limits in weight percent are nickel 61.0 typical, chromium 20.5 typical, molybdenum 9.0 typical, carbon 0.05 typical, iron 2.5 typical and niobium plus tantalum 3.15 to 4.15.

What is Inconel 625 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

Corrosion and temperature behaviour

Chromium, molybdenum and nickel provide different forms of corrosion resistance. Chromium at around 20 percent forms and maintains the passive oxide film. Molybdenum at around 9 percent resists local breakdown of this film. This property limits pitting and crevice attack in stagnant chlorides, including locations beneath gaskets and within lap joints. The high nickel content provides resistance to chloride ion stress corrosion cracking, which can damage austenitic stainless steel in hot chlorides.

The alloy has excellent pitting resistance in water at 260 to 316 degrees Celsius. It is therefore considered for reactor core and control rod components. Its high allowable design strength at elevated temperature also permits consideration in the 649 to 760 degrees Celsius range. The alloy originator reports resistance to high-temperature oxidation and carburisation.

Strength affects material selection in chemical plant. The combination of strength and corrosion resistance allows thinner vessel walls or tubing than a weaker material would require for the same duty. Reduced wall thickness improves heat transfer and reduces weight.

Mechanical properties

ASTM B446 covers rod and bar in two conditions: Grade 1 annealed and Grade 2 solution treated. These conditions have different mechanical properties and must be distinguished when ordering. The specification minimums for annealed bar are 120 ksi tensile strength, equivalent to 827 N/mm squared, and 60 ksi yield strength, equivalent to 414 N/mm squared. The alloy originator also provides nominal room-temperature ranges.

Nominal room-temperature mechanical properties for rod, bar and plate up to 4 inches, from the alloy originator's technical bulletin. Composite values, not for specification use.

Solution treatment reduces yield strength by roughly one third compared with the annealed condition. It improves creep and rupture performance during prolonged high-temperature service. For service above about 650 degrees Celsius, the alloy originator permits selection of either condition according to the required short-term strength or long-term rupture life.

Intermediate-temperature exposure

Intermediate-temperature exposure changes the mechanical properties of Inconel 625. Annealed rod samples held at 650, 760 and 870 degrees Celsius for 2000 hours showed increased room-temperature strength and hardness and reduced ductility. Before exposure, the material had a tensile strength of 965 N/mm squared, a yield strength of 479 N/mm squared and an elongation of 54 percent. The specimens also contracted by about 0.048 percent.

These changes result from the precipitation of carbides and intermetallic phases from the supersaturated matrix. They may be acceptable for components that operate continuously at elevated temperature without an impact requirement. They must be included in the design of components that must retain toughness after prolonged service in this temperature range.

Standards and product forms

ASTM B446 and ASME SB446 cover rod, bar, wire and forging stock. ASTM B564 covers forgings. AMS 5666 covers bar, forgings and rings, while AMS 5837 covers wire. Other applicable standards include ISO 9723, BS 3076NA21 and DIN 17752. ASTM B443 and AMS 5599 cover plate, sheet and strip. ASTM B444 covers seamless pipe and tube.

Forming, machining and welding

Hot-formed or cold-formed components are normally annealed after forming at approximately 925 to 1040 degrees Celsius. Treatment time depends on the section. The alloy can be welded without post-weld heat treatment. Welded material retains strength and toughness from cryogenic temperatures to 982 degrees Celsius.

Inconel 625 work hardens rapidly, conducts heat poorly and remains strong at cutting-edge temperatures. Machining therefore uses rigid setups, sharp positive-rake tooling, low surface speeds, flood coolant and feeds that keep the tool below the work-hardened layer. Dwelling allows the surface to harden while the tool rubs against it, which damages cutting tools.

Applications

Seawater and subsea applications include mooring and wire rope, propeller blades, submarine fittings, transducer controls and cable sheathing. Chemical process applications include reaction vessels, distillation columns, heat exchangers, transfer piping, bubble caps and valves. Other applications include aerospace ducting, bellows and exhaust systems, flare stacks, offshore risers, nuclear reactor core and control rod components, springs, seals and flexure devices exposed to corrosion fatigue.

Where Inconel 625 is not the right choice

Inconel 625 is unsuitable where a less expensive alloy can meet the same service requirements. Duplex or super duplex stainless steel and six percent molybdenum austenitic grades cover much of the chloride service range at a fraction of the cost and have far better machinability. Inconel 625 becomes applicable where temperature, sour service or severe chloride exposure excludes the stainless steel families.

Inconel 625 is unsuitable for prolonged service in the intermediate-temperature range when ductility must be retained. Exposure for 2000 hours between roughly 650 and 870 degrees Celsius measurably increases hardness, reduces elongation and causes slight contraction. Components that must retain toughness during this exposure require these changes to be included in the design.

Inconel 625 is also unsuitable for strongly reducing acids at high concentration and temperature. A nickel-molybdenum alloy of the C family is appropriate for those conditions. The alloy is expensive to machine. Components requiring heavy metal removal, deep holes or fine threads can incur more machining cost than the bar cost by weight. Near-net forms reduce this requirement where the geometry permits.

Other nickel alloy grades