Nickel Superalloys and Nickel-Copper Alloys

Which nickel alloy fits which job

Nickel alloys split into precipitation-hardened grades that get their strength from an aging heat treatment, and solid-solution grades that rely on alloying and cold work. The split matters for welding, heat treatment flow-down and machining.

Every alloy in that table appears in MSFC-STD-3029 Table I, high resistance to stress corrosion cracking in salt environments, with 718 and K-500 rated in all conditions including weldments, 625 and C-276 rated in the annealed condition, and Hastelloy X in all conditions. Corrosion is rarely what pushes an engineer off a nickel alloy. Cost, lead time and machinability are.

Inconel 718: specification minimums

Alloy 718 is popular because it ages slowly. The niobium-bearing strengthening phase precipitates sluggishly, so the alloy can be welded and then age-hardened without the strain-age cracking that troubles aluminum-titanium hardened alloys. The producer bulletin describes its resistance to postweld cracking as outstanding.

The two anneal ranges are not interchangeable. The producer notes that the lower 1700 to 1850 F anneal gives the best rupture life, fatigue strength and highest tensile properties, while the higher 1900 to 1950 F anneal gives better transverse ductility in heavy sections and low-temperature notch toughness, at the cost of possible notch brittleness in stress rupture. A drawing that just says "718, aged" leaves that choice to whoever heat treats it.

Other 718 numbers from the same producer bulletin: density 0.296 lb/in3 annealed, melting range 2300 to 2437 F, and chemistry of 50 to 55 percent nickel plus cobalt, 17 to 21 percent chromium, 4.75 to 5.50 percent niobium plus tantalum and 2.80 to 3.30 percent molybdenum.

Inconel 625, Haynes 230 and Hastelloy

Alloy 625 gets its strength from molybdenum and niobium in solid solution, so it needs no aging and holds properties through welding. The producer gives nominal annealed rod, bar and plate properties of 120 to 150 ksi ultimate and 60 to 95 ksi yield, with the caution that these composite values are not for specification purposes. Its density is 0.305 lb/in3. Engineers choose it for formed and welded parts such as bellows, flex hoses and ducting, and as a filler or overlay.

Haynes 230 is a nickel-chromium-tungsten-molybdenum alloy for long exposure at high temperature. It is supplied solution treated under AMS 5878 for sheet, strip and plate and AMS 5891 for bar and forgings. The producer's indicative plate data shows about 55.5 ksi yield, 123.6 ksi ultimate and 46 percent elongation at room temperature. That low room temperature yield is a reminder that solid-solution alloys are chosen for strength retention and oxidation resistance at temperature, not for room temperature strength.

Hastelloy X fills a similar hot-sheet role. Hastelloy C-276 and C-22 are corrosion alloys, selected for aggressive chemical service rather than temperature. For both, stock is often limited to particular forms and sizes, so confirm availability before freezing the design.

Monel K-500 and oxygen compatibility

Monel K-500 is nickel-copper alloy 400 with aluminum and titanium added so it can be age hardened. Producer literature describes it as having roughly three times the yield strength and twice the tensile strength of alloy 400. Typical hot-finished and aged rod is listed at 140 to 190 ksi ultimate and 100 to 150 ksi yield, typical values rather than minimums. The producer also notes that aged K-500 has a greater tendency toward stress corrosion cracking than alloy 400 in some environments, though MSFC-STD-3029 rates it highly resistant in salt.

The reason nickel-copper alloys appear in oxygen hardware is flammability. Published promoted combustion test work in high-pressure gaseous oxygen gives a consistent picture:

Ranking is not the same as acceptability. Whether a given part is safe depends on pressure, temperature, thickness, flow velocity, particle impact and cleanliness. NASA-STD-6001B requires an oxygen compatibility assessment that considers those ignition mechanisms, and it points engineers to the MAPTIS database for test history. Thin sections, sharp edges and high-velocity flow points are where metal fires start, which is why valve seats, poppets and filter elements in oxygen service so often end up in Monel or nickel. Parts for oxygen service also need cleaning to a specified level; see precision and oxygen cleaning.

Machining nickel alloys

Nickel alloys keep their strength at the high temperatures generated at the cutting edge, and they work harden quickly. The producer of 718 notes that it machines readily if tool material, geometry, speeds and coolant are chosen for its high strength and work hardening, that annealed material gives easier machining and longer tool life, and that aged material gives a slightly better finish and better chip control.

• Rough annealed, finish aged is a common route for 718: rough machine in the solution-treated condition, age, then finish machine to remove distortion. This adds a setup but saves tools and holds tolerance.
• Keep the tool cutting. A light pass or dwell over a work-hardened surface glazes it, and the next tool sees harder material. Depth of cut should get under the previous pass's hardened layer.
• Expect low surface speeds and frequent tool changes. Coated carbide is the usual finishing tool; tool life per edge is short, so shops plan edge changes into the program.
• Plan for residual stress. Thin-wall 718 parts move after aging and after heavy material removal. Leave stock and fixture for the aged condition.

These factors make cycle time, not material cost, the biggest variable in a nickel quote. Quotes from different shops on the same part can differ widely depending on tooling strategy and whether they rough before or after aging.

Heat treatment, welding and what to call out

For precipitation-hardened alloys, specify the condition and the heat treat spec rather than a hardness alone. For 718 that means naming the AMS material spec and stating whether parts are delivered solution treated, aged, or solution treated and aged after machining or welding. If welded assemblies must be aged after welding, say so; 718's sluggish aging is the reason this is possible at all.

• Alloy, UNS number and procurement spec by product form.
• Heat treatment condition and the anneal range where it matters, with the heat treat spec and pyrometry requirements flowed to the processor.
• Weld filler and weld procedure spec, and post-weld heat treatment.
• Oxygen-service cleaning level if applicable.
• DFARS specialty metals requirement on defense work, since nickel alloys with more than 10 percent of other alloying metals fall under DFARS 252.225-7009.

Require mill certs showing heat number, chemistry and tensile results at the specified heat treatment, plus the heat treater's cert if aging is done after machining.

Get nickel alloy parts quoted

Nickel parts need shops with the right tooling and processors with the right heat treat approvals. Send your drawing through a CNC quote request and we will match it to suppliers who machine these alloys routinely, flow down the heat treat and cleaning requirements, and check every cert before shipment. See also the materials hub, stainless and specialty steels and titanium alloys.

Questions

Related

• Aerospace Materials: Selection, Specs and Paperwork
• Stainless and Specialty Steels for Aerospace Parts
• Titanium Alloys for Aerospace Hardware
• Engineering Plastics for Flight and Ground Hardware

Sources

• Special Metals, INCONEL alloy 718 technical bulletin
• Special Metals, INCONEL alloy 625 technical bulletin
• Haynes International, HAYNES 230 alloy at a glance
• HP Alloys, Monel K-500 specifications and typical properties
• Steinberg, Rucker, Beeson, Promoted Combustion of Nine Structural Metals in High-Pressure Gaseous Oxygen, ASTM STP (1989)
• Benz, Shaw, Homa, Burn Propagation Rates of Metals and Alloys in Gaseous Oxygen, ASTM STP (1986)
• NASA-STD-6001B, Flammability, Offgassing, and Compatibility Requirements
• NASA MSFC-STD-3029A, Table I-D nickel alloys