Monel 400 (UNS N04400), also designated Alloy 400 or W.Nr. 2.4360, is a nickel-copper solid-solution alloy containing 63–70% nickel and 28–34% copper, governed by ASTM B564 / ASME SB-564 for forgings. It is one of the most widely used corrosion-resistant engineering alloys in the world, prized for its exceptional resistance to seawater, hydrofluoric acid, and alkaline environments across temperatures from cryogenic service to 500°C.
Originally developed by the International Nickel Company (INCO) in the early 20th century and named after company president Ambrose Monell, the alloy has accumulated a field service record spanning over 100 years across marine, chemical, nuclear, and oil & gas industries. This guide covers everything an engineer or procurement professional needs to know: alloy history and designation, chemical composition, mechanical and physical properties, corrosion performance, heat treatment, machinability, welding, industry applications, and comparison with related alloys.
This article is published by Jiangsu Liangyi Co., Limited, an ISO 9001:2015 certified manufacturer of custom Monel 400 (UNS N04400) forged components — open-die forgings and seamless rolled rings per ASTM B564 — founded in 1997, Jiangyin, Jiangsu, China.
History and Alloy Designations
Monel 400 traces its origins to the early 1900s when metallurgists at the International Nickel Company (INCO) discovered that nickel ore smelted from the Sudbury Basin in Ontario, Canada produced a natural nickel-copper alloy with exceptional corrosion resistance. This alloy was commercialised around 1905 and named "Monel" in honour of company president Ambrose Monell.
The alloy was initially used for ship fittings and chemical plant components where seawater and acid resistance were critical. Over the following century, it became the alloy of choice for HF alkylation units, marine propeller shafts, nuclear plant components, and oil & gas wellhead equipment — accumulating a field service record unmatched by any competing nickel-copper alloy.
| Designation System | Designation | Description |
|---|---|---|
| UNS (USA) | N04400 | Unified Numbering System — primary designation |
| Trade Name | Monel 400 | Registered trademark of Special Metals Corporation |
| DIN / W.Nr. (Europe) | 2.4360 | German material number — equivalent alloy |
| EN (Europe) | NiCu30Fe | European standard nickel alloy designation |
| ASTM / ASME (Forgings) | B564 / SB-564 | Primary forging standard for UNS N04400 |
| ASTM / ASME (Bar & Rod) | B164 / SB-164 | Bar, rod and wire standard |
| AMS (Aerospace) | AMS 4675 | Aerospace Material Specification — forgings |
| Generic Name | Alloy 400 | Common generic reference used by non-INCO producers |
Chemical Composition of Monel 400 (UNS N04400)
The composition of Monel 400 is tightly controlled by ASTM B564 / ASME SB-564 (forgings) and ASTM B164 / ASME SB-164 (bar and rod). The alloy is a binary nickel-copper solid solution with nickel as the dominant element, giving the alloy its fundamental corrosion resistance and mechanical properties.
UNS N04400 Elemental Composition
ASTM B564 / ASME SB-564The high nickel content (≥63%) gives Monel 400 its thermodynamic nobility — nickel is inherently resistant to corrosion in reducing environments. The addition of 28–34% copper lowers the alloy's position in the galvanic series further, and the Ni-Cu solid solution provides immunity to chloride stress corrosion cracking (Cl-SCC) that makes austenitic stainless steels like 316L vulnerable in marine and sour gas environments.
Mechanical Properties of Monel 400
Monel 400 (UNS N04400) achieves a useful combination of strength and ductility in the annealed condition. The alloy is a single-phase solid solution that does not transform or precipitate secondary phases during heat treatment — unlike age-hardenable alloys. Its mechanical properties remain remarkably stable from cryogenic temperatures (−196°C) up to its continuous service limit of 500°C.
Cold-Working and Strengthening
Monel 400 cannot be hardened by heat treatment. However, it responds well to cold working — cold-drawn bars and rings can achieve tensile strengths of 690–860 MPa with proportionally higher yield strengths, at the cost of reduced ductility. For applications requiring both high strength and NACE MR0175 compliance (maximum 35 HRC), the annealed condition is mandatory. For high-strength applications without hardness restrictions, consider UNS N05500 (Monel K-500) — the age-hardenable variant reaching 900–1100 MPa tensile.
| Temperature | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| Room Temp. (21°C) | 517–620 | 172–345 | ≥35 |
| 200°C (392°F) | ~480 | ~165 | ~38 |
| 300°C (572°F) | ~460 | ~150 | ~40 |
| 400°C (752°F) | ~430 | ~138 | ~42 |
| 500°C (932°F) | ~390 | ~125 | ~44 |
Physical and Thermal Properties
The physical properties of Monel 400 make it suitable for demanding thermal and electrical environments. Its relatively low thermal expansion compared to austenitic stainless steels reduces thermal fatigue risk in cyclic service, while its moderate thermal conductivity suits heat exchanger and reactor applications.
| Property | Metric Value | Imperial Value |
|---|---|---|
| Density | 8.80 g/cm³ | 0.318 lb/in³ |
| Melting Range | 1300–1350°C | 2372–2462°F |
| Thermal Expansion (20–100°C) | 13.9 µm/m·°C | 7.7 µin/in·°F |
| Thermal Conductivity (20°C) | 21.8 W/m·K | 151 BTU·in/hr·ft²·°F |
| Specific Heat (20°C) | 427 J/kg·K | 0.102 BTU/lb·°F |
| Electrical Resistivity (20°C) | 0.511 µΩ·m | 20.1 µΩ·in |
| Magnetic Permeability | ~1.001 (non-magnetic) | ~1.001 |
| Poisson's Ratio | 0.32 | 0.32 |
Monel 400 is essentially non-magnetic (magnetic permeability ~1.001) in the annealed condition. However, heavy cold work can induce a small degree of magnetism. For applications requiring strict non-magnetic specifications — such as compass housings or MRI-adjacent equipment — use Monel 404 (UNS N04404), a low-permeability variant produced under controlled composition limits.
Corrosion Resistance of Monel 400
The corrosion resistance of Monel 400 (UNS N04400) is the central reason for its widespread adoption across demanding industries. Unlike stainless steels that rely on a passive oxide film that can be destroyed by chlorides or H₂S, Monel 400 derives its corrosion resistance from the thermodynamic nobility of the nickel-copper matrix itself — making its protection mechanism far more robust and reliable across a wider range of environments.
Seawater and Chloride Environments
Monel 400 is one of the most seawater-resistant engineering alloys available. In flowing seawater, corrosion rates are consistently below 0.025 mm/year — negligible over a 20–30 year design life. It is completely immune to chloride stress corrosion cracking (Cl-SCC), the failure mode that disqualifies austenitic stainless steels from high-chloride, high-temperature marine applications. Pitting susceptibility is minimal in properly designed components that avoid stagnant zones.
Hydrofluoric Acid (HF) — Preferred Alloy
Monel 400 is the industry standard alloy for hydrofluoric acid service below 80°C — including HF alkylation units, acid storage vessels, piping, and pump internals. Corrosion rates are below 0.05 mm/year across concentrations from 10% to anhydrous HF at temperatures up to 80°C. Above 80°C, corrosion rates increase significantly and engineering assessment is required.
Alkaline and Caustic Environments
Monel 400 shows outstanding resistance to sodium hydroxide (NaOH) and other caustic solutions across all concentrations, including hot concentrated caustic soda at temperatures approaching boiling point — conditions that attack most stainless steels through caustic SCC. This makes it the preferred alloy for caustic storage tanks, evaporators, and processing equipment.
| Corrosive Medium / Environment | Performance | Typical Corrosion Rate | Notes |
|---|---|---|---|
| Seawater (flowing) | ✔ Excellent | <0.025 mm/yr | Preferred over 316L; immune to Cl-SCC |
| Seawater (stagnant / crevice) | △ Good | 0.03–0.10 mm/yr | Design to eliminate stagnant zones |
| HF Acid — all conc., <80°C | ✔ Excellent | <0.05 mm/yr | Industry standard alloy for HF service |
| HF Acid — >80°C | △ Moderate | 0.10–0.60 mm/yr | Engineering assessment required |
| HCl — dilute, de-aerated, RT | ✔ Good | <0.10 mm/yr | Suitable for dilute service |
| HCl — concentrated or aerated | ✘ Not recommended | Rapid attack | — |
| H₂SO₄ — dilute, de-aerated, RT | ✔ Good | <0.10 mm/yr | Non-oxidising conditions only |
| Nitric Acid (HNO₃) | ✘ Not recommended | Rapid attack | Use Inconel 625 instead |
| Caustic Soda (NaOH) | ✔ Excellent | <0.03 mm/yr | All concentrations; resistant to caustic SCC |
| Ammonia / Amines (anhydrous) | ✔ Excellent | <0.025 mm/yr | Refrigeration and ammonia handling |
| Phosphoric Acid (dilute) | ✔ Good | <0.10 mm/yr | Non-oxidising conditions |
| High-Temperature Steam (<500°C) | ✔ Excellent | Negligible | Nuclear and power generation service |
| Moist Chlorine Gas (Cl₂) | ✘ Risk of SCC | Variable | Use Hastelloy C-276 instead |
| H₂S — sour gas service | ✔ Excellent | <0.02 mm/yr | NACE MR0175 listed; immune to SSC |
Heat Treatment of Monel 400 Forgings
Monel 400 is a single-phase alloy — it cannot be hardened by precipitation or transformation heat treatment. The only applicable heat treatments are full annealing (to restore properties after forging or cold work) and stress relieving (to reduce residual stresses without recrystallisation).
Monel 400 is susceptible to sulfur embrittlement at forging and annealing temperatures. Never heat Monel 400 in a sulfur-bearing atmosphere (e.g. from contaminated furnace fuel or sulfur-bearing lubricants). Sulfur deposits on the surface at high temperature cause intergranular attack — permanently damaging mechanical properties and corrosion resistance at the surface layer. Always use a clean, sulfur-free furnace atmosphere.
Machinability and Welding of Monel 400
Machinability
Monel 400 has a machinability rating of approximately 20–30% of free-machining brass (AISI B1112). Its primary challenge is a strong tendency to work-harden during cutting — the material becomes significantly harder at the cut surface, making subsequent passes more difficult. This characteristic requires sharp tooling, consistent chip contact, and adequate coolant flood.
- Tooling: Carbide inserts with positive rake angles (8–15°) are preferred. High-speed steel is suitable for low-speed operations and drilling.
- Turning speeds: 30–75 m/min (carbide); feed 0.15–0.40 mm/rev. Never allow the tool to dwell — it work-hardens the surface instantly.
- Drilling: 8–15 m/min; feed 0.08–0.18 mm/rev. Use a chip-breaking geometry to prevent chips wrapping the drill.
- Coolant: Flood coolant is mandatory — sulphurised cutting oil for turning and drilling; water-soluble emulsion (10–15%) for milling. Never dry-machine Monel 400.
- Workpiece condition: Always machine in the fully annealed state. If work-hardening has occurred, re-anneal at 870–982°C before continuing.
Welding
Monel 400 is readily welded by GTAW (TIG), GMAW (MIG), and SMAW (stick) processes using matching nickel-copper filler metal. It does not sensitise during welding — the single-phase FCC structure has no carbon-chromium sensitisation mechanism, making it simpler to weld than austenitic stainless steels in that respect.
- Filler metal: ERNiCu-7 (AWS A5.14) for GTAW/GMAW; ENiCu-7 for SMAW. Do not use stainless steel fillers — galvanic incompatibility in corrosive service.
- Preheat: Not required for sections below 25 mm. For heavy sections (>25 mm): warm to 100–150°C to reduce thermal gradient.
- Interpass temperature: Maximum 150°C. Minimise heat input — use stringer beads.
- Back purge: Argon back purge recommended for GTAW root passes on tube/pipe.
- Post-weld: Full anneal at 870–982°C for pressure-retaining welds; stress relieve at 538–566°C if full anneal is impractical.
- Key risk — hot cracking: Caused by sulfur contamination in parent metal or filler. Ensure all surfaces are clean, sulfur-free, and degreased before welding.
Monel 400 Industry Applications — Six Key Sectors
Monel 400 (UNS N04400) serves demanding applications across six major industries, each exploiting a different combination of its corrosion resistance, mechanical properties, and temperature capability.
Jiangsu Liangyi Co., Limited holds ISO 9001:2015 quality management certification. References to API 6A, NACE MR0175, ABS, DNV-GL, ASME, and EN standards in this section describe product standards that our forgings are manufactured to meet, based on customer specifications — not additional factory-level certificates held by Jiangsu Liangyi. Third-party inspection (TUV, Bureau Veritas, DNV GL, ABS, Lloyd's Register) is available via customer-nominated inspectors on individual orders.
Monel 400 is the alloy of choice for seawater-immersed and splash-zone marine components. Its corrosion rate below 0.025 mm/year in flowing seawater, combined with immunity to Cl-SCC and biofouling resistance, makes it superior to any grade of stainless steel for direct marine service. Our forgings are supplied with EN 10204 Type 3.1 certificates; Type 3.2 via customer-nominated third-party inspector (including ABS / DNV-GL surveyors).
Key components: Marine propeller shafts, seawater pump shafts and impellers, heat exchanger tubes and tube sheets, valve bodies for seawater systems, rudder stocks, keel bolts, and hydraulic cylinder rods on offshore vessels.
In sour gas service where H₂S is present above NACE MR0175 threshold, Monel 400 is the technically superior choice over 316L stainless steel. Its immunity to sulfide stress cracking (SSC), Cl-SCC, and pitting in chloride-rich formation water provides service lives of 20–30 years versus 12–24 months for 316L in comparable environments.
Key components: Wellhead valve bodies, Christmas tree components, choke bodies, mud motor housings, submersible pump shafts, and downhole instrument housings.
Monel 400 is indispensable in HF alkylation units — the only commercial structural alloy with adequate resistance to hydrofluoric acid below 80°C. It is also widely used in caustic soda plants, pickling equipment, and phosphoric acid processing, where its resistance to reducing acids and alkaline environments far exceeds that of stainless steels.
Key components: HF alkylation reactor flanges and valve bodies, caustic evaporator shells, pickling tank liners, phosphoric acid pump casings, and hydrochloric acid handling equipment.
Monel 400 has been used in nuclear power plants since the 1950s, primarily in reactor coolant systems where resistance to high-temperature demineralised water, boric acid, and pressurised steam is essential. Its non-magnetic character is additionally advantageous in environments sensitive to magnetic interference.
Key components: Reactor coolant pump shafts and impellers, pressuriser nozzles, steam generator tube sheets, control rod drive mechanism components, and primary circuit valve internals.
Power Generation: Heat exchanger tube sheets, turbine components, boiler feedwater systems. | General Engineering: Valve balls and seats, mechanical seal faces, cryogenic fluid handling (−196°C LNG service), instrument bodies, and pressure vessel nozzles. | Desalination: Multi-stage flash evaporator components and brine heater tubes.
Monel 400 vs Monel K-500 — When to Upgrade
Monel K-500 (UNS N05500) is an age-hardenable variant of the same Ni-Cu base alloy, with additions of aluminium (2.3–3.15%) and titanium (0.35–0.85%) that allow precipitation hardening. The two alloys share virtually identical corrosion resistance — the choice between them is purely a strength decision.
- Tensile strength of 517–620 MPa is sufficient for the design
- Good weldability is required (pressure vessels, fabrications)
- NACE MR0175 sour service compliance is needed (anneal to <35 HRC)
- Cost is a consideration — Monel 400 is 20–30% cheaper per kg than K-500
- Application: valve bodies, pump casings, flanges, heat exchanger components
- Cryogenic service — both alloys perform well, Monel 400 is standard choice
- Higher tensile strength required: 900–1100 MPa (age-hardened)
- High yield strength needed: 690–760 MPa minimum
- Application: pump shafts, propeller shafts requiring high torque capacity
- Springs, fasteners, and bolting in corrosive environments
- High-speed rotating components where fatigue strength is critical
- Cavitation-resistant impellers and wear-resistant pump internals
| Property | Monel 400 / UNS N04400 | Monel K-500 / UNS N05500 |
|---|---|---|
| Ni Content | 63–70% | 63–70% |
| Cu Content | 28–34% | 27–33% |
| Age-Hardening Elements | None | Al 2.3–3.15%; Ti 0.35–0.85% |
| Tensile Strength (typical) | 517–620 MPa | 900–1100 MPa (age-hardened) |
| Yield Strength (0.2%) | 172–345 MPa | 620–760 MPa (age-hardened) |
| Hardness | 65–85 HRB | 25–35 HRC (age-hardened) |
| Weldability | Excellent | Good (post-weld age required) |
| Corrosion Resistance | Excellent | Excellent (virtually identical) |
| Forging Standard | ASTM B564 / AMS 4675 | ASTM B564 / AMS 4676 |
| Relative Cost | Lower baseline | ~20–30% premium |
Monel 400 Forging Process and Procurement Guidance
Open-Die Forging of UNS N04400
Open-die forging is the preferred production method for Monel 400 valve bodies, flanges, rings, shafts, and custom components. The multi-directional deformation refines the grain structure throughout the full cross-section, closes residual porosity from the cast billet, and develops grain-flow patterns aligned with component geometry — critical for fatigue resistance and SCC immunity at notches and bore corners.
Monel 400 is forged in the temperature range 950–1180°C. The upper limit prevents excessive grain growth; the lower limit prevents cold-working effects that would elevate hardness above NACE limits. Post-forge full annealing at 870–982°C is mandatory for sour service compliance.
Key Procurement Specifications
- Material standard: ASTM B564 / ASME SB-564 (forgings); ASTM B164 / ASME SB-164 (bar and billet)
- Heat treatment: Full annealing at 870–982°C — documented furnace records required
- Hardness: Maximum 35 HRC per NACE MR0175 — physical test per piece, minimum 4 locations
- NDT: 100% volumetric UT per ASTM A388; surface PT per ASTM E165
- Certification: EN 10204 Type 3.1 standard; Type 3.2 via customer-nominated third-party inspector
- Lead time: 45 days standard from Jiangsu Liangyi; 30–35 days from stock billet
Jiangsu Liangyi Co., Limited manufactures custom Monel 400 (UNS N04400) open-die forgings and seamless rolled rings per ASTM B564, with ISO 9001:2015 quality management. We supply forged bars (OD 25–600 mm), seamless rings (OD up to 3,000 mm), shafts (up to 6,000 mm length), and custom shapes to customer drawings. EN 10204 Type 3.1 standard; Type 3.2 via customer-nominated inspector. 45-day delivery to USA, EU, UK, Australia, and Middle East. custom UNS N04400 open-die forgings and seamless rolled rings
Frequently Asked Questions
Common engineering and procurement questions about Monel 400 (UNS N04400).
Monel 400 (UNS N04400, also called Alloy 400 or W.Nr. 2.4360) is a nickel-copper solid-solution alloy containing 63–70% nickel and 28–34% copper, governed by ASTM B564/ASME SB-564 for forgings and ASTM B164/ASME SB-164 for bar and rod. It offers exceptional resistance to seawater (corrosion rate <0.025 mm/year), hydrofluoric acid, H₂S environments, and alkaline media, with service temperatures from cryogenic to 500°C. Originally developed by the International Nickel Company and named after company president Ambrose Monell. Note: Monel® is a registered trademark of Special Metals Corporation.
Per ASTM B564 (forgings), Monel 400 (UNS N04400) contains: Nickel (Ni) 63.0–70.0% minimum (the balance, comprising the largest single element); Copper (Cu) 28.0–34.0%; Iron (Fe) 2.50% max; Manganese (Mn) 2.00% max; Carbon (C) 0.30% max; Silicon (Si) 0.50% max; Sulfur (S) 0.024% max.
Monel 400 (UNS N04400) in the annealed condition per ASTM B564 has: Tensile Strength 517–620 MPa (75–90 ksi); Yield Strength (0.2% offset) 172–345 MPa (25–50 ksi); Elongation minimum 35%; Hardness 65–85 HRB (approximately 130–180 HBW). Modulus of Elasticity is 179 GPa (26,000 ksi). These properties are maintained from cryogenic temperatures (−196°C) to 500°C service temperature.
Monel 400 (UNS N04400) is used across six major industries: Marine & Shipbuilding (propeller shafts, seawater pumps — components manufactured to ABS/DNV class standards); Oil & Gas (wellhead valve bodies, Christmas trees, NACE MR0175 compliant); Chemical Processing (HF alkylation units, caustic plants, reactors); Nuclear Power (reactor coolant pump components, ASME Section III); Power Generation (heat exchangers, turbines); and General Engineering (valves, cryogenic equipment, pressure vessels).
Monel 400 (UNS N04400) has exceptional seawater corrosion resistance with a corrosion rate below 0.025 mm/year in flowing seawater — one of the lowest of any structural engineering alloy. It is completely immune to chloride stress corrosion cracking (Cl-SCC) and shows minimal pitting susceptibility, making it the preferred alloy for marine propeller shafts, seawater pumps, heat exchanger tube sheets, and offshore valve components. Stagnant seawater zones should be avoided in design to prevent localised crevice corrosion.
Monel 400 (UNS N04400) forgings are full annealed at 870–982°C (1600–1800°F) in a sulfur-free atmosphere, held for minimum 15 minutes per 25 mm of section thickness, then water quenched or rapidly air cooled. This produces complete recrystallisation, optimal corrosion resistance, and hardness below 35 HRC in compliance with NACE MR0175. Stress relieving at 538–566°C for 1–2 hours may be applied as an alternative when full anneal is impractical. Monel 400 cannot be hardened by heat treatment — it is a solid-solution alloy.
Both alloys share the same nickel-copper base and virtually identical corrosion resistance. Monel 400 (UNS N04400) is a solid-solution alloy with tensile strength 517–620 MPa in the annealed condition, excellent weldability, and is the standard choice for valve bodies, pump casings, heat exchangers, and flanges. Monel K-500 (UNS N05500) adds aluminium (2.3–3.15%) and titanium (0.35–0.85%), enabling age-hardening to achieve tensile strength 900–1100 MPa and yield strength 690–760 MPa — making it the preferred choice for high-load propeller shafts, springs, fasteners, and rotating components where strength is the priority. Monel K-500 costs approximately 20–30% more per kg. Note: Monel® is a registered trademark of Special Metals Corporation.
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Explore Materials →Trademark Notice: Monel® is a registered trademark of Special Metals Corporation, a PCC company. UNS N04400 and UNS N05500 are the standard alloy designations used by manufacturers worldwide. Jiangsu Liangyi Co., Limited has no affiliation with Special Metals Corporation.
Certifications: Jiangsu Liangyi Co., Limited holds ISO 9001:2015 quality management certification. ASTM, ASME, API, NACE, AMS, and EN references describe product manufacturing standards and testing requirements, not additional factory-level certificates held by Jiangsu Liangyi. Third-party inspection available via customer-nominated inspection bodies.
Standards referenced: ASTM B564, ASME SB-564, ASTM B164, ASME SB-164, AMS 4675, NACE MR0175/ISO 15156 Parts 1–3, ASTM A388, ASTM E165, EN 10204:2004, AWS A5.14.
All corrosion performance data are representative values based on published technical literature. Specific service environments should be evaluated by qualified corrosion engineers. Contact our engineering team for project-specific assessment.