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Monel K500 / (2.4375) / UNS N05500 / AMS 4676

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Monel K500 is the second most widely used Monel material after Monel 400. Designated 2.4375 in the DIN system and UNS N05500, Alloy K500 is, like the other Monel materials, fundamentally a nickel-copper alloy. Its corrosion resistance is at least as good as that of Monel 400. Beyond that, 2.4375 has a harder core and better mechanical properties than Monel 400. The aluminium and titanium it contains are what produce this harder, more durable structure. The material can be hardened by age (precipitation) hardening. Alloy K500 is also non-magnetic.

Monel Alloy K-500 is commonly used in special screws and nuts. It is also used in various special chains and springs, in yacht and marine equipment, pump components, valves, special shafts, various electronic components and sensors.​‌​​‌​

Machinability: Monel K-500 is a difficult alloy to machine because of its high hardness and strength. With the right machining techniques, however, these difficulties can be overcome.

Machining: Monel K-500 can be machined by conventional methods such as milling, turning and drilling. Because of the hardness of the alloy, hardened tooling and low cutting speeds are recommended. Cutting tools should generally be carbide or a hard alloy.​‌​​‌​

Welding: Monel K-500 can be welded by TIG and MIG methods, although heat treatment may be required during welding. An annealing (slow cooling) treatment after welding is generally recommended. Post-weld heat treatment allows the mechanical properties of the alloy to be preserved.

Cold and hot forming: Monel K-500 can be cold formed and hot formed, although these operations can be demanding because of its high hardness. Appropriate cooling should be used during forming.​‌​​‌​

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Standards by Product Form

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Monel K-500 (UNS N05500 / W.Nr. 2.4375) is the age-hardenable version of the basic Ni-Cu composition, made by adding aluminium and titanium; suitable heat treatment precipitates gamma-prime (γ′) Ni₃(Al,Ti) throughout the matrix and delivers two to three times the yield strength of alloy 400. This page has two centrepieces: the ageing cycle (including its controlled slow furnace cool) and hydrogen embrittlement in seawater — K-500’s real failure mode, and the thing buyers are least warned about.

NACE / ISO 15156 — the Major Trap

This is the history buyers get wrong. From 1975 to 1996 the “Nickel-Copper Alloys” clause of MR0175 listed N04400, N04405 and N05500, with N05500 acceptable at 35 HRC maximum in the hot-worked and aged, solution-annealed, or solution-annealed and aged condition. In the 2003 MR0175 / ISO 15156 harmonisation the nickel-copper alloys were removed from the general section: N04400 and cast Monel were removed entirely, and N05500 was retained only under restricted, enumerated equipment clauses. The stated driver is blunt: field failures in N05500 fasteners.​‌​​‌​

Current status (per secondary sources): 35 HRC maximum, age-hardened, for wellhead and Christmas-tree components. Sources diverge on the permitted applications — report, do not average: one gives wellhead and Christmas-tree components excluding bodies and bonnets, plus non-pressure-containing internal valve components, and adds the sting: “NACE defines valve shafts and stems as pressure-containing components, [so] this material could no longer be used for shafts and stems when compliance with MR0175 was required.” Another gives “internal, non-pressure-retaining components in valves, pressure regulators and level controllers”; a third says only “wellhead/Christmas tree components”. The full text of ISO 15156-3:2020 Annex A could not be verified — the table number, H₂S partial pressure, temperature, chloride and pH limits and any elemental-sulfur clause are not printed on this page. Confirm against the standard for your own equipment class before ordering.

And the most-copied error: the originator’s own 2004 bulletin still says “NACE listing: MR-01-75 for oil and gas service” with no restriction at all. That statement is more than twenty years stale and is the single most reproduced K-500 error on the web.​‌​​‌​

Composition: Ni (+Co) ≥63.0 % · Cu 27.0–33.0 % · Al 2.30–3.15 % · Ti 0.35–0.85 % · Fe ≤2.0 % · Mn ≤1.5 % · Si ≤0.50 % · S ≤0.010 %. There is a real split on carbon: the originator and three distributors publish C ≤0.25 % while ASTM B865, one European mill and one distributor publish ≤0.18 %. These are not the same acceptance limit: if your order says “ASTM B865”, the certificate must show ≤0.18 % C; a certificate at 0.22 % complies with the mill bulletin and is non-compliant with B865. Do not average, do not print one number.

Two chemistry-linked process hazards. (1) Titanium carbide (TiC): process annealing for longer than about 1.5 hours in 760–871 °C “can result in the formation of titanium carbide. This compound is stable at the aging temperatures used to harden alloy K-500 such that the titanium cannot participate in the hardening reaction.” The remedy is a solution anneal at 1121 °C (2050 °F) for 30 minutes with a water quench — and one revision of QQ-N-286 exists specifically “for components requiring solution annealing at 2050 °F due to titanium carbide presence”. (2) A magnetic surface film: aluminium and copper oxidise selectively on heating, “leaving a magnetic nickel-rich film“, removed by pickling or bright dipping. That matters for instrument and naval acceptance.​‌​​‌​

The Age-Hardening System — the Centrepiece

The slow controlled furnace cool is not a formality; it is part of the precipitation. Every primary and standards source specifies a controlled furnace cool of 8–14 °C per hour from the soak temperature down to 482 °C (900 °F), after which uncontrolled cooling is permitted. Ageing continues during the descent — which is why the ramp rate is specified as tightly as the soak. Total cycle time (arithmetic from the published cycles): Procedure 1, 16 h soak plus about 8–13 h of descent ≈ 24–29 hours; Procedure 3 ≈ 10–17 hours; the European cycle ≈ 14–26 hours. A “2-hour age” on a mill certificate is not this alloy’s cycle. The originator publishes short-time 2/4/8-hour data but states explicitly that it is a “guide to short-time aging treatments and not suitable for specification purposes“.​‌​​‌​

Irreversible over-ageing — the hard limit. The originator, verbatim: “Material which has been heated for any appreciable length of time in the temperature range 1100° to 1400 °F (593–760 °C) will be overaged to an extent dependent on time and temperature of exposure. Overaged material will have lower mechanical properties than properly aged metal, and the properties cannot be raised by subsequent aging treatments.” A 2026 failure analysis confirms it in the field: failed K-500 screws measured 310–325 HV against a normal 340–360 HV.

Dimensional change on ageing: about 0.0002–0.00025 in/in contraction in annealed rod, with minimal warpage. Allow for it when setting the machining oversize.​‌​​‌​

Typical values (NOT minimums): hot-finished bar, aged 965–1310 MPa tensile / 690–1034 MPa yield / 30–20 % elongation / 27–38 HRC · cold-drawn, aged 931–1276 MPa / 655–1103 MPa / 30–15 % / 25–41 HRC · cold-rolled sheet, spring temper aged 1172–1517 MPa / 896–1345 MPa / 10–5 % / ≥34 HRC. Aged yield strength scatters by about ±30 % across publishers (690 / 676 / 790 / 786–904 MPa), and that is not sloppy reporting — it is the documented heat-to-heat variability that drives the hydrogen section below.​‌​​‌​

Impact and fatigue. Charpy V: 50 J at +21 °C, 46 J at −79 °C, 42 J at −196 °C; “no ductile-to-brittle transformation occurs even at temperatures as low as that of liquid hydrogen“. But ageing roughly halves Charpy keyhole energy (hot-finished longitudinal 100 J → 53 J aged). Fatigue (10⁸ cycles, room temperature): annealed hot-rolled 262 MPa · hot-rolled and aged 296 MPa · cold-drawn and aged 324 MPa. Surface finish decides it: aged and polished 345–393 MPa against aged with an oxidised surface only 272 MPa — “the oxide surface was produced by age hardening in air”. Practical consequence: age a cyclically loaded part in a protective atmosphere, or machine and polish after ageing; otherwise you take a ~20–30 % endurance-limit penalty.

Magnetic Behaviour — Different from Monel 400, and It Matters​‌​​‌​

K-500 is “virtually nonmagnetic, even at quite low temperatures“, with permeability ≈1.001–1.002 at 200 oersteds (one mill gives 1.0015 maximum). But the Curie point is condition-dependent: annealed and quenched ≈−134 °C (−210 °F); annealed and aged ≈−117 °C; cold-drawn and aged ≈−101 °C (−150 °F). So ageing raises the Curie temperature by about 35 °C, and the aged condition is the magnetically “worse” one. It is non-magnetic for all normal marine and downhole service, but a cryogenic or very-low-temperature instrument application must be assessed against the aged Curie point, not the annealed one. Separately, the nickel-rich surface film formed during heat treatment is magnetic and will fail a permeability check until pickled off. For contrast, Monel 400‘s Curie point sits around room temperature and it can be weakly ferromagnetic near ambient — that contrast is what makes K-500 the instrument, MWD and minesweeper alloy.

Hydrogen Embrittlement and Cracking in Seawater — the Real Failure Mode​‌​​‌​

What the mills say (and understate): the originator writes that corrosion resistance is “substantially equivalent to that of alloy 400 except that, when in the age-hardened condition, alloy K-500 has a greater tendency toward stress-corrosion cracking in some environments“. One European mill calls it “virtually immune” to chloride-induced SCC but notes that age-hardened material is sensitive to SCC in hot hydrofluoric acid vapour under high tensile stress. No mill datasheet reviewed warns about hydrogen embrittlement under cathodic protection. That silence is the gap this page fills.

The mechanism. K-500’s marine failure mode is not classic chloride SCC; it is hydrogen environment-assisted cracking (HEAC): cathodic protection generates atomic hydrogen at the surface, the hydrogen is absorbed into the γ′-strengthened matrix and concentrates at grain boundaries ahead of the crack tip. The fracture mode under cathodic polarisation is intergranular, against mixed in air. γ′ Ni₃(Al,Ti) supplies both the strength and the planar slip that localises strain; one study found the controlling variable was slip character, not grain-boundary precipitation. Hydrogen traps are TiC and M₂₃C₆ carbides; sulphur segregating to grain boundaries acts synergistically with hydrogen to promote embrittlement, while boron and zirconium are beneficial.​‌​​‌​

On the hardness threshold, honestly. Verified: 35 HRC maximum, age-hardened, is the NACE cap for N05500’s restricted listing; ASTM F468 caps K-500 fasteners at 37 HRC. What is not verified is the critical part: no published, standards-backed yield-strength or hardness threshold exists below which K-500 is immune to HEAC under cathodic protection. The research shows that yield strength, hydrogen diffusivity, grain-boundary character and grain size “do not independently control the observed variations” — susceptibility is set by the combination of strength, hydrogen uptake and grain-boundary impurity chemistry. One study reports that a ±15 % variation in yield strength (786–904 MPa) can change K_TH by as much as 65 MPa√m at mild potentials, and concludes that “strength limitations may be necessary”. Anyone who quotes you a single “safe hardness” for K-500 in cathodically protected seawater is going beyond the literature.​‌​​‌​

The procurement lever that does exist: the QQ-N-286 revision letter. QQ-N-286 Rev G requires slow strain rate tensile (SSRT) testing; material made to Rev E is not subject to it. SSRT is the standard screen for hydrogen-embrittlement susceptibility, and Rev G exists because of those failures. But Rev G is not a guarantee: one study tested four nominally peak-aged heats that all met QQ-N-286G and found hydrogen-embrittlement susceptibility “varied extensively“, because the specification “allows age hardening to occur by any process proved adequate to meet the mechanical property requirements“. And the commercial reality: distributors sell both Rev E and Rev G K-500 bar — so a buyer who writes “QQ-N-286” with no revision letter can legitimately be shipped unscreened Rev E material.

And be careful with a datum that looks good. The originator’s own test: “After 6 days of continuous immersion in saturated (3500 ppm) hydrogen sulfide solutions at acidic and basic pHs (ranging from 1.0 to 11.0), U-bend specimens of age-hardened sheet showed no cracking. Hardness of the specimens ranged from 28 to 40 HRC.” Do not let this sell the alloy: it is a six-day U-bend immersion, at hardnesses above the NACE cap, and it says nothing about cathodically generated hydrogen — which is the actual marine failure mode and the reason NACE restricted the alloy in 2003. Good sulphide-stress-cracking behaviour and poor HEAC behaviour coexist in this alloy.

Welding, Machining, Forming and Service Limits​‌​​‌​

Welding — the filler quietly decides the joint’s ceiling

GTAW is the primary process. The material must be solution-annealed before welding: the originator says “avoid welding age-hardened material (greatly reduced ductility)“, and a European mill states “material must be solution-annealed before welding”. Interpass temperature max 120 °C. No numeric preheat or heat-input limit could be verified in any source — the 120 °C interpass cap implies low heat input. After welding: anneal at 850–900 °C, then age.​‌​​‌​

The critical point is the filler. ERNiCu-7 (FM 60), which everyone quotes as the default, is not age-hardenable: the originator states flatly that “weldments are not age hardenable and, thus, do not have strength matching that of the hardened base metal”. ERNiCu-8 (FM 64) carries Al 2.0–4.0 % and Ti 0.25–1.0 % and “the filler metal will age harden on heat treatment“. So the filler choice permanently decides whether the joint can ever match the base metal. With the right filler and post-weld heat treatment the achievable joint efficiency is high: GTAW on 0.5 mm sheet, heat-treated after welding, gave 93 % at +26 °C, 93 % at −73 °C, 93 % at −196 °C and 95 % at −253 °C. Pitfalls: (1) welding aged material destroys ductility and the HAZ over-ages irreversibly in the 593–760 °C band; (2) post-weld ageing re-exposes the whole assembly to the ageing cycle — ageing in air oxidises the surface (~20–30 % endurance-limit penalty) and leaves a magnetic nickel-rich film; (3) the general nickel-alloy rule applies — sulphur, lead and zinc contamination cause hot cracking, so clean and degrease before welding.

Machining and forming​‌​​‌​

When to machine — the settled answer: “Heavy machining is best accomplished when the material is in the annealed condition or hot-worked and quenched. Age-hardened material can be finish-machined to close tolerances and fine finishes.” The recommended practice is to machine slightly oversize, age-harden, then finish to size — allowing for the 0.0002–0.00025 in/in ageing contraction. Cutting speeds diverge substantially across publishers: one (annealed, 88 HRB, 35 % machinability) gives turning 120–160 m/min, milling 90–120 m/min and drilling 90–120 m/min, and notes that CBN allows 2–4× the carbide speed; another (about 25 % machinability) gives annealed turning at 30–55 m/min (100–180 sfm), describes “the same gummy-chip fight as alloy 400, plus higher forces”, and warns the material is “springy” — watch deflection on slender parts. Do not average them; shop practice for aged K-500 runs at the low end. No published speeds for the aged condition were found. The common principles: rigid setup, sharp carbide, heavy positive feeds, soluble-oil coolant — and never dwell, never rub, because the alloy work-hardens rapidly.

Seawater, crevice corrosion and galling. In flowing, high-velocity seawater corrosion rates are “very low”, and combined with the high strength that makes K-500 suitable for pump shafts. Stagnant or low-velocity seawater is the weakness: “fouling may occur followed by pitting”; “pitting may occur in stagnant or low velocity seawaters”. K-500 inherits alloy 400’s stagnant-seawater pitting and crevice weakness; ageing buys strength, not crevice resistance. Quantified crevice-corrosion resistance (critical crevice temperature, PREN equivalent) could not be verified. And on galling: no mill or institute source reviewed makes any galling claim for K-500. What is published is an application list (valve stems, pump shafts, sleeves, wear rings), not a tribological claim. Distributor pages asserting “excellent galling resistance” are unsupported — do not print a galling claim.​‌​​‌​

Physical properties: density 8.44 g/cm³ · melting range 1315–1350 °C · modulus of elasticity 179–180 GPa · shear modulus 66 GPa · Poisson’s ratio 0.32 · thermal conductivity at 21 °C 17.2–17.5 W/m·K · specific heat about 419 J/kg·K · electrical resistivity 0.615 µΩ·m · mean linear expansion (20–100 °C) 13.7 µm/m·°C.

Frequently Asked Questions​‌​​‌​

K-500, Monel 400, Inconel 718 or 17-4 PH for high-strength seawater and subsea service

Monel 400 is the default when you need seawater corrosion resistance and nothing more: roughly a third of K-500’s yield strength, but better in one respect that matters — it is not precipitation hardened, so it is far less exposed to hydrogen embrittlement. Specify 400 for piping, valve bodies, sheathing and any part where strength is not the constraint. Note that NACE MR0175/ISO 15156 removed N04400 entirely in 2003 — it is not permitted for any sour-service application. K-500 earns its place only where you need alloy 400’s corrosion behaviour plus two to three times its yield strength, in a non-magnetic alloy: pump and propeller shafts, valve stems, drill collars, downhole and MWD instrument housings, springs, marine fasteners. Its low permeability (≤~1.002, Curie point around −101 °C aged) is the thing no competitor offers. Inconel 718 is γ″ Ni₃Nb-hardened, reaches higher strength and is codified by API 6A718 — but it is not exempt from hydrogen embrittlement under cathodic protection either: in subsea trees where an insulation blanket failed, flooding coupled 718 bolts to the CP system, and the documented root cause was furnace cooling instead of quenching after solution anneal, which raised grain-boundary delta phase and trapped hydrogen. 17-4 PH is a martensitic PH stainless: cheap and strong, but ferromagnetic, without the nickel margin of K-500 or 400 in seawater, and classed as stress-corrosion susceptible. The decision: corrosion only → 400; corrosion plus strength plus non-magnetic → K-500; higher strength with API codification → 718 (with the same hydrogen question); cheap and moderate → 17-4 PH, but not in seawater.​‌​​‌​

Hydrogen embrittlement: what hardness should I specify, and what does cathodic protection do?

First: there is no single “safe hardness”, and anyone who gives you one is going beyond the literature. The verified caps are 35 HRC maximum, age-hardened for the NACE restricted listing and 37 HRC for ASTM F468 fasteners. But the research is explicit that yield strength, hydrogen diffusivity, grain-boundary character and grain size “do not independently control the observed variations” — susceptibility is set by their combination; indeed a ±15 % variation in yield strength can shift the threshold stress intensity by as much as 65 MPa√m. Second, the variable that actually governs is the cathodic potential: “intergranular cracking is eliminated by reduced cathodic polarisation more positive than −750 mV(SCE); susceptibility occurs when cathodically polarised at −800 mV(SCE) and lower.” Conventional sacrificial-anode protection for subsea steel drives structures to −0.9 to −1.05 V(SCE) — squarely inside the cracking regime. And the K-500 part need not be the thing being protected: a K-500 bolt in a cathodically protected steel flange is electrically coupled and polarised. What to put on the order: (1) the QQ-N-286 REVISION LETTER — Rev G requires slow strain rate tensile testing, Rev E does not, and distributors sell both, so an order saying just “QQ-N-286” can legitimately be filled with unscreened material; (2) the ageing cycle (soak temperature, time, the 8–14 °C/hour controlled furnace cool down to 482 °C) — “aged” alone is not enough, and a 2-hour cycle is not this alloy’s cycle; (3) the measured hardness and yield strength, not the cap; (4) on the design side, electrically isolate K-500 from cathodically protected steel or control the potential, and avoid over-protection on impressed-current systems; (5) reduce stress levels — the threshold is only 5–14 % of the fracture toughness.​‌​​‌​

How much does a certificate that says “aged” actually tell you?

Not enough — and on this alloy the difference is measurable. First, ageing is a cycle, not a soak. The originator, ASTM B865 and a European mill all specify a controlled furnace cool of 8–14 °C per hour from the soak temperature down to 482 °C (900 °F); precipitation continues during the descent, which is why the ramp rate is defined as tightly as the soak. The total cycle, per the published procedures, runs roughly 10 to 29 hours. The originator itself publishes short-time 2/4/8-hour data while stating it is “not suitable for specification purposes“. So if the certificate says “aged 2 hours”, that is not this alloy’s cycle. Second, over-ageing is irreversible: material held any appreciable time in 593–760 °C is permanently weakened and “the properties cannot be raised by subsequent aging treatments“. A 2026 field failure shows exactly this: the failed screws measured 310–325 HV against a normal 340–360 HV — and a simple hardness check would have caught it. Third, the unaged hardness rows are caps, not floors: ASTM B865 sets ≤245 HB on unaged hot-worked bar and ≤185 HB on annealed product; reading these as strength minimums is a common error. Fourth, ageing in air leaves two marks: a ~20–30 % endurance-limit penalty (272 MPa with an oxidised surface against 345–393 MPa polished) and a magnetic nickel-rich surface film that will fail a permeability check until it is pickled off. Put on the order: the full ageing cycle, the cooling rate, the atmosphere or oxidation control, the measured hardness — and, on critical parts, the QQ-N-286 revision letter.​‌​​‌​

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