Monel 400 (UNS N04400, DIN 2.4360) is a single-phase nickel-copper solid-solution alloy. Roughly 63% nickel, 28–34% copper, with controlled amounts of iron, manganese, and silicon. Unlike duplex or precipitation-hardenable grades, there are no secondary phases to precipitate during welding - no sensitization, no sigma phase, no intermetallic formation at grain boundaries. That makes Monel 400 genuinely forgiving to weld compared to most nickel alloys.
Welded Monel 400 pipe is produced under ASTM B725 / ASME SB725. The manufacturing route: strip or plate is formed into a cylinder and joined with a longitudinal weld seam - typically autogenous GTAW (gas tungsten arc welding) for lighter walls, or GTAW with filler for heavier schedules. The weld seam must be annealed after welding to restore corrosion performance in the heat-affected zone (HAZ).
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Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
| Parameter | ASTM B165 (Seamless) | ASTM B725 (Welded) |
|---|---|---|
| Manufacturing | Extruded / cold-drawn from billet | Formed strip + longitudinal weld |
| ASME B31.3 joint efficiency | 1.0 (seamless) | 0.85 (welded, radiographed) or 0.80 (spot RT) |
| Pressure derating | Full schedule rating | Typically 85% of seamless equivalent |
| Standard NPS range | 1/8" – 10" common | 1/8" – 30"+ (economical above 4") |
| Cost vs seamless (same size) | Baseline | 15–35% lower above 4" NPS |
| Lead time (large diameters) | Longer | Shorter |
| Weld seam NDE | N/A | Eddy current or UT per ASTM E213/E309; RT when specified |
UNS N04400 Composition (per ASTM B725)
| Element | Content (wt%) | Role |
|---|---|---|
| Nickel (+ Co) | 63.0 min | Base - drives corrosion resistance |
| Copper | 28.0 – 34.0 | Solid-solution strengthener, seawater resistance |
| Iron | 2.50 max | Controlled - minor strengthening |
| Manganese | 2.00 max | Deoxidizer, hot workability |
| Silicon | 0.50 max | Weldability control |
| Carbon | 0.30 max | Kept low to avoid carbide precipitation |
| Sulfur | 0.024 max | Tight control for weld soundness |
Mechanical Properties - Annealed Condition
| Property | ASTM B725 Minimum | Typical Annealed Range |
|---|---|---|
| Tensile strength | 480 MPa (70 ksi) | 482 – 620 MPa |
| Yield strength (0.2% offset) | 195 MPa (28 ksi) | 195 – 310 MPa |
| Elongation in 50 mm | 35% | 35 – 50% |
| Hardness | - | 125 – 190 HB (65 – 80 HRB) |
| Density | - | 8.80 g/cm³ |
| Melting range | - | 1300 – 1350 °C |
| Modulus of elasticity | - | 179 GPa |
Welding Monel 400: Filler Metals, Procedures, and HAZ Behavior
This is where most supplier pages go thin. The welding of Monel 400 welded pipes deserves real attention because the weld seam is the defining feature of B725 product.
Filler Metal Selection
| Process | Filler | AWS Classification | Notes |
|---|---|---|---|
| GTAW / GMAW | ERNiCu-7 wire | AWS A5.14 | "Monel 60" - matching filler, most common |
| SMAW | ENiCu-7 covered electrode | AWS A5.11 | "Monel 190" - for field welds and repair |
ERNiCu-7 contains a small titanium addition (typically 2.5–3.5%) which acts as a deoxidizer and prevents weld metal porosity. It's the go-to filler for both shop fabrication of welded pipe and field girth welds in piping systems. If you're welding Monel 400 to a dissimilar metal (e.g., to carbon steel or stainless), ERNi-1 (AWS A5.14, pure nickel) is an alternative that reduces dilution cracking risk.
Key Welding Parameters and Considerations
Preheat: Not required. Monel 400's high thermal conductivity (21 W/m·K - about 4× that of austenitic stainless steel) means heat dissipates quickly, so you need adequate amperage to maintain a stable puddle, but no preheating is necessary.
Interpass temperature: Keep below 150 °C. Excessive interpass heat can widen the HAZ and soften the weld-adjacent zone.
Shielding gas: 100% argon for GTAW is standard. For GMAW, argon with small helium additions (e.g., 75Ar/25He) improves penetration and travel speed.
Post-weld heat treatment (PWHT): Monel 400 does not require PWHT after welding. No stress-relief anneal is mandated by ASME B31.3 or by ASTM B725. However, the longitudinal weld seam on B725 pipe must be annealed in-process by the manufacturer to restore full corrosion resistance in the HAZ - this is a production requirement, not a field one.
Pickling: After welding, light oxide tint in the HAZ should be removed by pickling (a nitric-hydrofluoric acid mixture is standard for nickel alloys) or by mechanical cleaning followed by passivation. Skipping this step leaves a surface oxide layer that can act as a preferential corrosion initiation site, particularly in seawater and chloride environments.
Frequently Asked Questions
What is the difference between ASTM B165 and ASTM B725 for Monel 400 pipe?
ASTM B165 covers seamless Monel 400 pipe (produced by extrusion or cold drawing from a solid billet, with no weld seam). ASTM B725 covers welded Monel 400 pipe (produced by forming strip or plate into a cylinder and joining with a longitudinal weld seam). B165 seamless has a joint efficiency factor of 1.0 in ASME B31.3; B725 welded has 0.85 (with full RT) or 0.80 (with spot RT). B725 is typically 15–35% cheaper than B165 in equivalent sizes above 4" NPS.
Can Monel 400 welded pipe be used in sour service (H₂S environments)?
Yes. Monel 400 is listed in NACE MR0175 / ISO 15156 for sour service applications. The alloy is resistant to sulfide stress cracking (SSC) in the annealed condition. For welded pipe in sour service, specify that the weld seam has been annealed and that the heat chemistry meets NACE requirements. Sour service documentation should accompany the MTC.
Does Monel 400 welded pipe require post-weld heat treatment?
No. Monel 400 is a solid-solution, single-phase alloy with no phase transformations. No stress-relief anneal or PWHT is required after welding - neither by ASTM B725 nor by ASME B31.3. The longitudinal weld seam on B725 pipe must be annealed during production by the manufacturer, but this is a manufacturing step, not a field requirement.
What is the maximum operating temperature for Monel 400 welded pipe?
The recommended upper service temperature is approximately 480 °C (900 °F) for continuous service. Short-term excursions above this are tolerable. At temperatures above 540 °C, Monel 400 can experience oxidation and should be replaced with a higher-nickel alloy (such as Inconel 600 or 625) for sustained high-temperature service.
How much does Monel 400 welded pipe cost in 2025?
Indicative 2025 pricing for ASTM B725 Monel 400 welded pipe ranges from approximately 30–45/kgex−works(China/Indiamill)to30–45/kgex−works(China/Indiamill)to38–55/kg from European distributors. Landed cost to Middle East destinations (including freight, duty, and third-party certification) typically runs $41–48/kg. Pricing tracks LME nickel with a 30–60 day lag - request a live quote for current market rates.
What welding filler should I use for Monel 400 pipe?
Use ERNiCu-7 (AWS A5.14, trade name "Monel 60") for GTAW/GMAW, or ENiCu-7 (AWS A5.11, trade name "Monel 190") for SMAW. ERNiCu-7 contains a titanium addition that prevents weld porosity. No preheat is required. Remove surface oxides from the HAZ by pickling or mechanical cleaning after welding to prevent preferential corrosion initiation.
