Monel 400
Also: Alloy 400, Monel 400, UNS N04400, Nickel-copper alloy 400, W.Nr. 2.4360, NiCu30Fe, Monel, Nicorros, QQ-N-281
Monel 400 is the nickel-copper alloy for seawater and hydrofluoric acid: 550 MPa (80 ksi) tensile, ductile, easy to weld and form. Values annealed.
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At a glance
| Density | 8.8 g/cm³ (0.318 lb/in³) |
|---|---|
| Tensile strength | 483 MPa (70 ksi) min; typically 550 MPa (80 ksi) |
| Yield strength | 195 MPa (28 ksi) min; typically 240-275 MPa (35-40 ksi) |
| Hardness | 60-75 HRB (110-150 HB) |
| Elongation | 35% min; typically 40-45% |
| Modulus of elasticity | 179 GPa (26 Msi) |
| Thermal | Melts at 1300-1350 °C (2370-2460 °F); thermal conductivity ~22 W/m·K; Curie temperature 20-50 °C (70-120 °F), so it is weakly magnetic at room temperature; service to about 480 °C (900 °F) |
| Composition | Ni 63% min, Cu 28-34%, Fe up to 2.5%, Mn up to 2.0%, Si up to 0.5%, C up to 0.3% |
Typical values, Annealed (hot rolled or cold drawn and annealed per ASTM B127 / B164). Cold drawn bar: 585-690 MPa (85-100 ksi) tensile, 380-585 MPa (55-85 ksi) yield. Confirm against the mill certificate and the purchase spec.
- MachinabilityFair
- Similar to 304 stainless: gummy and work-hardening, so use sharp tools, positive rake, steady feed and sulfur-based cutting oil. About 25-30% of the free-machining steel benchmark. Specify Monel R-405 (with sulfur) for screw machine parts.
- WeldabilityExcellent
- TIG and MIG with ERNiCu-7 (Monel 60) filler, no preheat and no post-weld heat treatment. Clean off all sulfur, lead and paint before heating; nickel alloys embrittle from sulfur pickup.
- FormabilityExcellent
- Very ductile annealed: bends at about 1x thickness, deep draws, spins and cold heads. High work-hardening means intermediate anneals on severe draws and heavy springback on bends.
- Corrosion resistanceExcellent
- Outstanding in flowing seawater, brines, hydrofluoric acid, de-aerated sulfuric and hydrochloric acid, and caustic. Immune to chloride stress-corrosion cracking. Not for oxidizing acids (nitric), ammonia and ammonium salts, sulfides or moist chlorine.
Monel 400 is a solid-solution alloy of about two-thirds nickel and one-third copper, the oldest of the commercial nickel alloys and still the standard answer to a few specific problems: fast-flowing seawater, hydrofluoric acid, brines, and de-aerated acids. It resists erosion-corrosion and cavitation in seawater far better than copper alloys, is immune to chloride stress-corrosion cracking, and handles hydrofluoric acid in all concentrations up to the boiling point, which no stainless steel can do.
Its strength is moderate, about 550 MPa (80 ksi) tensile annealed, with very high ductility, and it cannot be hardened by heat treatment; cold drawn bar and strip reach 585-690 MPa (85-100 ksi). When higher strength is needed with the same corrosion resistance, its age-hardenable cousin Monel K-500 (UNS N05500) roughly doubles the yield strength for pump shafts, valve stems and fasteners.
It fabricates easily for a nickel alloy: welds with ERNiCu-7 filler and no post-weld heat treatment, forms and deep draws in the annealed condition, and machines about as well as an austenitic stainless, with a free-machining variant (R-405) for screw machines. It is weakly magnetic at room temperature, which surprises people, and it should not be used in oxidizing acids, ammonia solutions or sulfide-bearing environments.
Heat treatment
Not hardenable by heat treatment; strength is raised only by cold work. Anneal at about 870-980 °C (1600-1800 °F) and cool rapidly. Stress relieve cold-worked parts at about 540-650 °C (1000-1200 °F) before machining or service in stress-corrosion environments (mercury and hydrofluoric acid vapour can crack cold-worked Monel). For an age-hardened version, specify Monel K-500, which ages at about 595 °C (1100 °F).
Tempers and conditions
- Annealed
- Hot finished
- Cold drawn (bar, higher strength)
- Stress relieved
- Monel R-405 (free-machining, sulfur-bearing variant)
Stock forms
- Round bar and hex bar
- Plate
- Sheet and strip
- Seamless and welded pipe and tube
- Forgings
- Wire and fastener stock
- Fittings and flanges
Relative cost
Tier 4 of 5. Roughly 5-8x the price of 304 stainless per pound, driven by the nickel and copper content; about a third to a half of Inconel 625. Machining and forming costs are close to stainless, so fabricated Monel parts are cheaper than their material price suggests.
Typical applications
- Marine pump shafts, impellers, valve trim and propeller shafts
- Seawater piping, heat exchanger tubing and condensers
- Hydrofluoric acid alkylation and fluorine handling equipment
- Salt and brine production equipment
- Marine fasteners, fittings and wire rope
- Chemical process vessels and pickling equipment
- Fuel and water tanks on ships and aircraft
- Springs and musical instrument components (with K-500 and cold-drawn stock)
Process compatibility
How Monel 400 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| CNC Machining | FairFair | Machines like 304 stainless: gummy, work-hardening, needs sharp tools and steady feed. Expect 3-4x the cycle time of 6061. |
| Screw Machining | FairFair | Standard Monel 400 is stringy on screw machines; specify R-405, the sulfur-bearing free-machining version, for bar-fed parts. |
| Welding | ExcellentExcellent | ERNiCu-7 filler by TIG or MIG, as welded, no post-weld heat treatment; the weld resists seawater as well as the parent metal. |
| TIG Welding | ExcellentExcellent | Argon shielding, low heat input, and scrupulously clean joints; sulfur from oil, marking paint or grease embrittles nickel alloys. |
| Sheet Metal Fabrication | ExcellentExcellent | Sheet cuts, bends, draws and welds into tanks, hoods and liners; the most formable of the common nickel alloys. |
| Deep Drawing | ExcellentExcellent | Deep draws and spins in the annealed condition with lubricated tooling; anneal between severe draws. |
| Press Brake Forming | ExcellentExcellent | Bends at about 1x thickness inside radius with moderate overbend; forces about 1.5x those for 304 stainless. |
| Laser Cutting | GoodGood | Cuts sheet and plate cleanly with nitrogen assist; the copper content reflects a little but fiber lasers handle it. |
| Waterjet Cutting | ExcellentExcellent | Any thickness with no heat-affected zone; common for valve and pump plate parts. |
| Forging | GoodGood | Forged for flanges, valve bodies and pump parts at 925-1175 °C (1700-2150 °F); keep sulfur out of the furnace atmosphere. |
| Investment Casting | GoodGood | Cast as M-35-1 (ASTM A494) for valve bodies, pump casings and impellers in seawater service. |
| Hard Chrome Plating | FairFair | Used on Monel pump shafts and valve stems for wear resistance; needs a nickel strike for adhesion, and the plating does not add corrosion resistance where it cracks. |
| Heat Treating | Not recommendedNot recommended | Cannot be hardened by heat treatment; only anneal and stress relief exist. For a heat-treatable nickel-copper alloy, specify Monel K-500. |
Design and sourcing tips
- Specify "Monel 400 per ASTM B164, annealed" for bar, "ASTM B127" for plate and sheet; add AMS 4544 or 4675 for aerospace, and QQ-N-281 only if the customer still cites it.
- Use it for what it is best at: flowing seawater, hydrofluoric acid, brines and caustic. For oxidizing acids, ammonia or sulfides, choose a different alloy.
- Call out Monel R-405 for screw machine parts and Monel K-500 for shafts, stems and fasteners that need double the yield strength.
- Design welds as-welded with ERNiCu-7 filler; no post-weld heat treatment, but specify clean joints free of sulfur, lead and marking compounds.
- Stress relieve cold-worked parts that will see hydrofluoric acid vapour or mercury; cold-worked Monel can stress-corrosion crack in those.
- Do not assume it is non-magnetic: its Curie point is near room temperature, so it may pull a magnet weakly. Specify a different alloy if non-magnetic behaviour matters.
Frequently asked questions
- Is Monel 400 magnetic?
- Weakly, and it depends on the lot and the temperature. Its Curie point is about 20-50 °C (70-120 °F), so at room temperature it is slightly ferromagnetic and may attract a magnet, especially cold-worked stock; warm it and the attraction disappears. If the part must be non-magnetic, specify Inconel 625, 316 stainless or titanium instead.
- What is the difference between Monel 400 and Monel K-500?
- K-500 adds about 3% aluminum and 0.5% titanium to the 400 chemistry, which makes it age-hardenable: about 965 MPa (140 ksi) tensile and 690 MPa (100 ksi) yield after aging, roughly double the yield of annealed 400, with nearly the same corrosion resistance. Use 400 for fabricated and welded parts, K-500 for shafts, valve stems, springs and fasteners that need strength.
- Can Monel 400 be welded?
- Yes, easily. TIG and MIG with ERNiCu-7 filler (Monel 60) give sound welds with no preheat or post-weld heat treatment, and the weld metal resists seawater and hydrofluoric acid like the base metal. The one rule is cleanliness: any sulfur, lead or paint left on the joint embrittles the weld.
- Is Monel better than stainless steel for salt water?
- For flowing seawater, yes: Monel 400 resists erosion-corrosion, cavitation and chloride stress-corrosion cracking far better than 304 or 316, which is why it is used for pump shafts and impellers. In stagnant seawater it can pit under fouling, where a super-duplex or Inconel 625 does better. It costs several times as much as 316.
- What is Monel 400 used for?
- Marine pump and valve parts, propeller shafts, seawater piping and heat exchangers; hydrofluoric acid and fluorine equipment; salt and brine production; marine fasteners and wire; and chemical process vessels handling de-aerated acids and caustic. It is the alloy for hydrofluoric acid service.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM B127 and ASTM B164 (property minimums by form and condition)
- AMS 4544 and AMS 4675
- Special Metals Monel alloy 400 data sheet
- ASM Handbook, Volume 2 (Properties and Selection: Nonferrous Alloys)
