Monel Alloys
Monel is the nickel-copper family — about 63–67 % nickel with 28–34 % copper — that stays a single solid-solution phase from cryogenic temperatures to about 480 °C. Monel 400 yields about 240 MPa annealed with 40 % elongation; it is practically immune to chloride stress-corrosion cracking, resists high-velocity seawater, hydrofluoric acid at all concentrations and deaerated sulfuric and hydrochloric acid, and is the most machinable and formable of the nickel alloys. Monel K500 adds aluminum and titanium and age-hardens to about 700 MPa yield.
Popular Monel grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Monel 400 | 240 | 550 | 8.8 | Poor | Very good | The default; seawater, HF acid, valve trim and tube |
| Monel K500 | 310 | 690 | 8.44 | Poor | Good | Age-hardened to 700 MPa; shafts and oil tools |
| Monel R-405 | 450 | 620 | 8.83 | Fair | Poor | Free-machining 400 for screw-machine fittings |
| Monel 401 | 170 | 470 | 8.86 | Poor | Good | Copper-rich electrical grade for resistors |
| M-35-1 | 205 | 485 | 8.84 | Poor | Very good | Cast 400 for valve bodies and pump casings |
| ERNiCu-7 | 315 | 510 | 8.8 | Poor | Excellent | Matching TIG and MIG filler for Monel 400 |
| M25S | 550 | 760 | 8.36 | Not recommended | Not recommended | Hardest cast Ni-Cu grade |
Choose Monel for
Seawater pump shafts, impellers, valve trim, propeller shafts, marine fasteners and fittings that must last decades immersed; hydrofluoric-acid, fluorine and alkylation plant; heat-exchanger tubes in brackish and salt water; caustic and deaerated acid service; and non-magnetic, high-strength oil-tool components (Monel K500).
Look elsewhere when
The medium is oxidizing — nitric acid, ferric or cupric salts, aerated acids, wet chlorine — where Inconel 625 or Hastelloy C276 belong; the temperature exceeds about 480 °C (Inconel 600); the seawater is stagnant and fouled for long periods, where crevice attack starts and 2507 or 254 SMO resist it better; mercury or moist hydrofluoric-acid vapour with air is present (both crack Monel under stress); or 316L or 2205 already survives — nickel-copper is several times their cost.
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Monel by application
- Seawater shafts and valve trim
- Monel 400 for stems, seats, sleeves and fasteners in flowing seawater; Monel K500 when the shaft carries torque or the seat must resist galling and wire-drawing. Cast bodies in M-35-1.
- Hydrofluoric acid and fluorides
- Monel 400 is the reference alloy for HF at every concentration to boiling when the acid is free of air, for HF alkylation units and fluorine handling; the vapour space has to stay free of oxygen to avoid stress cracking.
- Non-magnetic downhole tools
- Monel K500 for MWD collars, housings and instrument sleeves — non-magnetic to −130 °C with about 700 MPa yield — within the 35 HRC ceiling of NACE MR0175.
- Turned fittings in volume
- Monel R-405 for adapters, nuts, plugs and screw-machine parts that need the corrosion resistance of Monel 400 without its machining time.
Working with Monel
- Machining
- Monel 400 cuts like a gummy, work-hardening austenitic stainless at about 60 % of its speed — sharp positive carbide, heavy steady feed, rigid setup and sulfurized oil; cold-drawn bar machines cleaner than annealed. Monel K500 is best machined in the aged condition for a good finish, or roughed annealed, aged, then finished, since aging moves dimensions slightly. Monel R-405 machines at roughly twice the speed of 400.
- Welding
- Monel 400 welds readily by TIG, MIG and SMAW with ERNiCu-7 or ENiCu-7, no preheat and no post-weld treatment; joints to steel take ERNiCu-7 or ERNi-1. Monel K500 is welded in the annealed condition and aged afterwards, and the weld itself stays at Monel 400 strength. Monel R-405 is not welded. Degrease and grind to bright metal — sulfur, lead and zinc embrittle nickel welds.
- Heat treatment
- Monel 400 is annealed at 870–980 °C and hardens only by cold work (to about 700 MPa tensile). Monel K500 is solution-treated at about 980 °C, water-quenched, then aged around 595 °C and cooled slowly in steps to 480 °C over roughly a day, reaching 27–35 HRC; a softer age keeps oil-tool parts at or below the 35 HRC sour-service limit.
- Forming
- Monel 400 forms, spins and deep-draws like a stiff copper alloy, with high springback and strong work hardening — anneal between severe operations. Monel K500 is formed annealed and then aged; sheet and tube of both bend cold to normal stainless radii.
Other nickel alloy series
Monel Alloys — frequently asked
What is Monel?
Monel is the nickel-copper family — about 63–67 % nickel with 28–34 % copper — that stays a single solid-solution phase from cryogenic temperatures to about 480 °C. Monel 400 yields about 240 MPa annealed with 40 % elongation; it is practically immune to chloride stress-corrosion cracking, resists high-velocity seawater, hydrofluoric acid at all concentrations and deaerated sulfuric and hydrochloric acid, and is the most machinable and formable of the nickel alloys. Monel K500 adds aluminum and titanium and age-hardens to about 700 MPa yield.
Which grade of Monel should I use?
Monel 400 for valve trim, pump internals, seawater piping, heat-exchanger tubes, fasteners, hydrofluoric-acid and alkylation equipment — the general-purpose grade in every mill form. Monel K500 when the same environment needs strength or hardness: pump and propeller shafts, doctor blades, oil-tool and MWD components, springs and non-magnetic instrument housings, at about 700 MPa yield and 27–35 HRC after aging. Monel R-405 is 400 with sulfur added for screw-machine work — the same corrosion resistance, roughly twice the cutting speed, not for welding. Monel 401 is the copper-rich electrical grade for resistors and electronic leads. Castings are M-35-1 and M35-2 (the 400 equivalents for valve bodies and pump casings) and M25S, the silicon-bearing hardenable grade for galling-resistant wear parts. ERNiCu-7 and ENiCu-7 are the matching filler wire and electrode.
Which grade of Monel is the strongest?
M25S has the highest yield strength in the series at 550 MPa; M30H follows at 520 MPa. Values are for each grade's default condition.
Which grade of Monel is easiest to machine?
Monel R-405 (machinability fair, 50/100), then M30C (35/100).
Monel 400 or Monel K500?
Monel 400 for everything that does not need strength: it is cheaper, welds and forms easily, and its corrosion resistance is the reference for the family. Monel K500 is the same chemistry plus 2.7 % aluminum and 0.6 % titanium, aged to roughly three times the yield strength (about 700 MPa against 240) and 27–35 HRC, at the cost of a heat-treatment cycle, harder machining and a weld zone that reverts to 400 strength. K500 for shafts, springs, blades, fasteners and downhole tools; 400 for piping, trim and vessels.
Is Monel magnetic?
Monel 400 sits right at the edge: its Curie temperature is around room temperature (roughly 20–50 °C depending on exact composition and cold work), so a cold-drawn 400 bar is weakly magnetic on a cold day and non-magnetic when warm. Monel K500 has a Curie point near −130 °C and is reliably non-magnetic in service, which — together with its strength and seawater resistance — is why it is the standard alloy for non-magnetic drill collars and MWD housings.