Cobalt Alloys and Refractory Metals for Aerospace Hardware

Updated October 12, 2026

The alloys that show up on aerospace drawings

These materials are bought in small quantities, from few producers, with long mill lead times. Knowing exactly which grade, form and specification you need saves weeks.

Compositions above are from the producer pages and the ASTM scopes listed in the sources. The Haynes and Carpenter specification lists are what those producers publish for their own product; other mills may certify to a subset, so ask which specification a given heat is certified to.

L-605 (Haynes 25): hot sheet structure that work hardens fast

L-605 is a solid solution strengthened cobalt-chromium-tungsten-nickel alloy. The producer describes good oxidation resistance in long exposure up to 1800 F (982 C), with higher temperatures acceptable for short exposures, and notes that newer alloys such as Haynes 188 and 230 have largely replaced it in modern gas turbine engines. It still appears on legacy engine and hot structure drawings, and the producer also lists bearing balls and races among its uses.

Fabrication notes from the producer worth flowing down:

• Hot working requires the whole piece to reach about 2200 F (1204 C) and be held long enough to soak through.
• Cold forming is possible because ductility is good, but the alloy work hardens very rapidly, so complex forming needs intermediate anneals.
• Heat treatment. It is supplied solution treated, typically at 2150 to 2250 F (1177 to 1232 C) followed by rapid air or water cooling. Lower temperature anneals can precipitate carbides. Heat treat flow down for cobalt alloy parts is covered by AMS 2774, discussed on the heat treatment page.
• Welding by GTAW, GMAW, SMAW, electron beam and resistance methods with matching filler. No preheat, interpass at or below 200 F (93 C), and post weld heat treatment usually not needed. Submerged arc is not recommended because of high heat input and restraint.
• Thermal stability. Long exposure at intermediate temperatures can embrittle the alloy through Co2W Laves phase precipitation, which matters for hardware with long service at moderate heat.

Because it work hardens so quickly, plan machining with the same care as the nickel alloys on the nickel superalloys page, with sharp tools, positive feed and no dwelling.

MP35N: ultra high strength without hydrogen worries

MP35N is a nickel-cobalt-chromium-molybdenum alloy, nominally equal parts nickel and cobalt, that reaches tensile strength up to 300 ksi (2068 MPa) through work hardening, a phase transformation and aging, while keeping useful ductility and toughness. The producer lists it as nonmagnetic, with excellent corrosion resistance and exceptional resistance to sulfidation, high temperature oxidation and hydrogen embrittlement. In the fully work hardened condition, service up to 750 F (399 C) is suggested.

Carpenter lists aerospace fasteners among its markets, and the appeal is clear: a high strength steel fastener would need plating and an embrittlement relief bake and still carry hydrogen embrittlement risk. The cost is in the strength path: because strength comes largely from cold work, the bar size and cold reduction set the properties, and a later anneal would remove the cold work the strength depends on. Specify the AMS spec and the strength level, and require the cert to show cold work and aging condition. Carpenter lists AMS 5758, AMS 5844 and AMS 5845 for its product.

For fasteners made from MP35N, flow down the fastener standard and the material spec separately, and see fasteners for how we source traceable hardware.

Stellite and cobalt wear alloys

Stellite is a family of cobalt base alloys with significant chromium, up to 33 percent, and tungsten, up to 18 percent, formulated for wear resistance, corrosion resistance or high temperature service. They displaced earlier materials for poppet valves and valve seats, and they are widely applied as hardfacing overlays. Kennametal acquired the Stellite trademark in 2012.

Three buying points matter more than the grade number:

• Form. A cobalt wear alloy can be a solid part or a hardfaced overlay deposited on a steel or nickel substrate, and the two are inspected differently. Say which one the drawing assumes.
• Finishing. Many Stellite alloys are hard enough that they are mainly ground rather than cut, because cutting causes heavy tool wear even with carbide. Leave grinding stock on seats and sealing faces, and see EDM and precision grinding for burn control.
• Overlay qualification. For hardfaced parts, flow down the welding or overlay procedure, dilution limits, minimum finished overlay thickness and penetrant inspection, because a ground overlay that is too thin exposes the softer substrate.

Refractory metals: niobium C-103, tantalum, tungsten, molybdenum

The refractory metals are defined by very high melting points. Their shared weakness is oxidation at temperature and, for most of them, poor fabricability at low temperature.

Niobium C-103 (Nb-10Hf-1Ti) was developed in the early 1960s by Wah Chang and Boeing, was used for the Apollo Lunar Module descent engine nozzle, and is used on the SpaceX Merlin Vacuum nozzle. Hafnium provides most of its high temperature strength. Niobium oxidizes in air from about 200 C, and above roughly 400 C a protective coating is necessary or the alloy embrittles. Niobium also has a strong affinity for carbon, nitrogen and oxygen, so it is melted and processed in vacuum or inert atmosphere, and welds need inert gas shielding on both sides of the joint. Note that ASTM B392 and B393, the common niobium bar and sheet specifications, cover unalloyed niobium and Nb-1Zr grades, not C-103, so a C-103 drawing must name its own material specification.

Tantalum is one of the most corrosion resistant metals available. ASTM B708 covers unalloyed tantalum by electron beam or vacuum arc melting or by powder metallurgy, plus Ta-10W, Ta-2.5W and Ta-40Nb alloys in plate, sheet and strip.

Tungsten has the highest melting point of these metals and is used for rocket nozzles and counterweights. ASTM B760 covers unalloyed tungsten plate, sheet and foil, and ASTM B777 covers tungsten base high density metal.

Molybdenum and TZM. TZM adds about 0.5 percent titanium and 0.1 percent zirconium to molybdenum for higher creep resistance and high temperature strength, with service temperatures above 1060 C possible. ASTM B387 lists TZM as vacuum arc cast Alloy 363 and powder metallurgy Alloy 364 for bar, rod and wire. TZM is also the classic die material for isothermal forging of nickel superalloys, which must be done in vacuum or inert gas because hot TZM oxidizes; see forging.

Oxidation protection and coatings

Any refractory metal part that sees air or an oxidizing exhaust at high temperature needs a protective coating or a protective atmosphere. For a C-103 thruster nozzle that means a coating system qualified by the program, and the coating is as much a part of the design as the alloy.

What to settle on the drawing and PO for a coated refractory part:

• The coating specification or qualified process by name and revision, applied by an approved source
• Surface preparation and edge radii, because sharp edges are where thin coatings fail first
• Coating thickness range and how it is measured, and any coupon processed with the part
• Post coating inspection and acceptance, including visual and any oxidation test on coupons
• Handling after coating, since coated refractory parts are easily damaged, and packaging to protect coated surfaces
• Rework rules: whether a damaged coating may be stripped and recoated, and how many times

For hardware that also needs cleaning to an oxygen or propellant service level, sequence the cleaning with the coating; see precision and oxygen cleaning.

Machining and joining notes

• Cobalt alloys such as L-605 work harden very rapidly, per the producer. Ask the machine shop how it avoids rubbing passes that harden the surface ahead of the next cut, the same discipline used on nickel alloys.
• Stellite type wear alloys are often finished by grinding; plan grind stock and burn inspection.
• Refractory metals are described in published references as having poor fabricability at low temperature. Before releasing formed or drilled tungsten, molybdenum or TZM parts, ask the supplier what forming temperature, edge preparation and inspection for cracking they use.
• Niobium picks up carbon, nitrogen and oxygen when hot, so heat treatment is done in vacuum or inert gas and welds need inert gas shielding on both sides of the joint. Flow those shielding requirements down to the welder.
• Joining dissimilar metals. Where a refractory part is brazed or welded to steel, nickel or titanium structure, treat the joint as a qualified special process with its own procedure, and ask engineering whether a mechanical joint would avoid the qualification.

DFARS specialty metals and export notes

Under DFARS 252.225-7009, specialty metals include steels above stated alloy limits, nickel or iron-nickel alloys with more than 10 percent of other alloying metals, cobalt alloys with more than 10 percent of alloying metals other than cobalt and iron, titanium and titanium alloys, and zirconium and zirconium alloys. L-605, MP35N and Stellite alloys therefore fall inside the definition for covered defense contracts. Niobium, tantalum, tungsten and molybdenum alloys are not listed as their own categories in that definition, although niobium, tungsten and molybdenum appear in it as alloying elements that make a steel a specialty metal. Confirm applicability with the contracting officer for your contract; the DFARS specialty metals page explains the clause.

Export classification of refractory alloy products and of the hardware made from them depends on the item, form and end use, and the drawings for propulsion hardware are often controlled. We do not make classification determinations. Controlled technical data never goes through the website and moves only by a secure method to authorized U.S. persons and authorized suppliers; read controlled programs before sending files.

What to call out on the drawing or PO

• Alloy name and UNS number, product form and material specification with revision
• Melt practice where it matters (vacuum arc, electron beam, powder metallurgy), since ASTM refractory specs separate grades this way
• Condition: solution treated, cold worked and aged level for MP35N, or recrystallized versus stress relieved for refractory sheet
• Coating specification, thickness and acceptance for oxidation protected parts
• Weld or braze procedure and filler specification, with shielding requirements
• Mill certification with heat number, chemistry including interstitials, and mechanical results
• DFARS specialty metals compliance where the contract invokes it
• Packaging that protects coated surfaces and keeps heat identification with each piece

For cert content and traceability, see material certs and traceability. For related families, see controlled expansion alloys and the materials hub.

Find certified cobalt and refractory material

Short lengths of L-605 sheet, MP35N bar, C-103 or TZM are often what holds up a prototype or a repair. Send the alloy, form, size and specification and we look for certified material with traceable mill certs through qualified sources, including certified surplus, and can combine the material with machining or fabrication on one PO. A person replies within one business day, and we flow your specification and cert requirements to the supplier. See also certified aerospace material and obsolete aerospace parts.

Questions

Related

• Nickel Superalloys and Nickel-Copper Alloys
• Controlled Expansion Alloys and Copper Alloys for RF
• Material Certs, Specialty Metals and Lot Traceability
• Aerospace Forgings: Processes, Grain Flow, Specs and Callouts

Sources

• Haynes International: HAYNES 25 (L-605) alloy
• Carpenter Technology: MP35N alloy
• Stellite alloys: composition, uses, machining
• Niobium: C-103 history, oxidation, processing
• Niobium alloys: C-103 strength, welding shielding
• Refractory metals: properties, TZM, tantalum, tungsten
• ASTM B387 molybdenum and molybdenum alloy bar, rod and wire
• ASTM B386 molybdenum and molybdenum alloy plate, sheet, strip and foil
• ASTM B708 tantalum and tantalum alloy plate, sheet and strip
• ASTM B365 tantalum and tantalum alloy rod and wire
• ASTM B393 niobium and niobium alloy strip, sheet and plate
• ASTM B760 tungsten plate, sheet and foil
• DFARS 252.225-7009 specialty metals definition