Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Monel 400 vs Monel K-500 - Chemistry, Strength, Cost & Selection
Monel 400 (UNS N04400, ASTM B164) and Monel K-500 (UNS N05500, ASTM B865) are the two production grades of the nickel-copper alloy family at 63 to 70 percent Ni and 28 to 34 percent Cu. Monel 400 is the single-phase solid solution supplied annealed at 25 ksi (170 MPa) yield minimum. Monel K-500 is the precipitation-hardened sister grade, carrying 2.30 to 3.15 percent aluminium and 0.35 to 0.85 percent titanium for a Ni3(Al,Ti) gamma-prime response that lifts yield to 90 to 130 ksi (620 to 895 MPa) after solution treatment plus age. Corrosion behaviour is shared: both qualify in hydrofluoric acid, in flowing seawater, in brine, and in sour service to NACE MR0175 / ISO 15156-3 at the 35 HRC cap. The trade-off is mechanical, commercial and fabrication. K-500 buys roughly four times the yield and costs 20 to 30 percent more on bar, machines poorly, and cannot be field-welded in the aged condition. Monel® is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals. Numbers cited on this page are from ASTM B164, ASTM B564, ASTM B865, ASTM F468 and NACE MR0175 / ISO 15156-3.
Selection Summary: When to Specify Monel 400 vs K-500
| Service requirement | Specified grade | Reason |
|---|---|---|
| Flange bolting B16.5 / B16.47, ambient and process | Monel 400 (ASTM F468 Class 4 to 8) | Class 4 to 8 cold-worked stud bolting clears B16.5 tightening loads. |
| HF piping, plate, sheet, tubing | Monel 400 | Stress is low; weldability is the constraint, and N04400 welds without PWHT. |
| Brackish and seawater heat exchangers | Monel 400 (B163, B165) | Tube-wall stress is shell-side pressure only; annealed yield suffices. |
| Downhole pump shafts, valve stems, propeller shafts | Monel K-500 | Design yield exceeds 60 ksi; cold-worked N04400 cannot meet load at safe diameter. |
| Sour-well springs and high-strength fasteners | Monel K-500 (ASTM F468 N05500) | Spring rate and bolt preload need aged 90 to 130 ksi yield at the 35 HRC NACE cap. |
| Marine non-magnetic structural fittings | Monel 400 | Aged K-500 raises magnetic permeability slightly; N04400 is the cleaner non-magnetic choice. |
Source: ASTM B164 Table 1, ASTM B865 Table 1, NACE MR0175 / ISO 15156-3 Annex A. See Monel 400 bolts and Monel 400 stud bolts for fastener-specific selection.
Chemistry Comparison per ASTM B164 vs ASTM B865
Both grades share the nickel-copper backbone. K-500 splits from N04400 on three lines: aluminium added, titanium added, and tighter caps on iron, manganese, carbon and sulphur to keep the gamma-prime precipitation clean. The Al-and-Ti addition is the entire reason K-500 exists.
| Element, % | Monel 400 (N04400) | Monel K-500 (N05500) |
|---|---|---|
| Nickel + Cobalt, Ni | 63.0 to 70.0 | 63.0 to 70.0 |
| Copper, Cu | 28.0 to 34.0 | balance (typically 27 to 33) |
| Aluminium, Al | none | 2.30 to 3.15 |
| Titanium, Ti | none | 0.35 to 0.85 |
| Iron, Fe, max | 2.5 | 2.0 |
| Manganese, Mn, max | 2.0 | 1.5 |
| Carbon, C, max | 0.30 | 0.25 |
| Sulphur, S, max | 0.024 | 0.010 |
Source: ASTM B564 Table 1 Nickel-Copper Alloys (N04400 and N05500). Detail on the Monel 400 chemical composition page.
Mechanical Properties: Annealed vs Age-Hardened
Annealed Monel 400 sits at 25 ksi yield minimum on hot-worked rod per ASTM B164; cold work lifts it to 85 ksi at roughly 30 percent reduction. Aged Monel K-500 reaches 90 to 130 ksi yield depending on age time and section, with hardness controlled to the 35 HRC NACE cap.
| Property | Monel 400 annealed | Monel 400 cold-worked | Monel K-500 aged |
|---|---|---|---|
| Tensile min, ksi (MPa) | 70 (480) | 85 (585) | 130 to 160 (895 to 1100) |
| Yield 0.2% offset min, ksi (MPa) | 25 (170) | 55 to 85 (380 to 585) | 90 to 130 (620 to 895) |
| Elongation in 2 in. min, % | 35 | 15 to 25 | 15 to 20 |
| Hardness typical | 110 to 150 HBW | 175 to 230 HBW | 250 to 330 HBW (24 to 35 HRC) |
| Density, g/cm³ | 8.80 | 8.80 | 8.44 |
| Specification | ASTM B164 | ASTM B164 | ASTM B865 |
Source: ASTM B164 Table 2, ASTM B865. See Monel 400 mechanical properties.
Heat-Treatment Route: Anneal Only vs Solution + Age
Monel 400 has one thermal cycle: soft-anneal at 871 to 982 degrees Celsius followed by water quench (sheet, soft bar) or air cool (thick section). No quench-hardening, no temper, no age. Strength is set in the mill by hot-finish plus controlled cold work; ASTM F468 Class 4 to Class 8 fastener grades use this cold-work response.
Monel K-500 needs a two-step cycle. Solution treatment at roughly 980 degrees Celsius dissolves Al and Ti into the matrix and quenches to a super-saturated single phase. Age-hardening follows at 593 degrees Celsius for 8 to 16 hours, then furnace-cool to 482 degrees Celsius and air cool. Coherent Ni3(Al,Ti) gamma-prime particles precipitate during the age, pin dislocations, and raise yield from the solution-treated 40 to 50 ksi to the aged 90 to 130 ksi.
| Step | Monel 400 | Monel K-500 |
|---|---|---|
| Solution treatment | 871 to 982 deg C, anneal only | ~980 deg C, water quench |
| Age treatment | None (no Al or Ti) | 593 deg C, 8 to 16 h, FC to 482 deg C |
| Strength route | Cold work (mill) for Class 4 to 8 | Gamma-prime precipitation |
| Post-weld response | No PWHT required | Joint over-ages; re-solution + age needed |
| Specification | ASTM B164 | ASTM B865 |
See Monel 400 heat treatment for the full envelope on N04400.
Cost Ratio: K-500 Sits 20 to 30 Percent Above Monel 400
On bar stock by weight, Monel K-500 typically sits 20 to 30 percent above Monel 400 in the Indian market and similar bands abroad. Three drivers stack: the Al and Ti additions enter the melt at premium element prices, the melt route is tighter with sulphur held at 0.010 max against 0.024 max for N04400, and the supplied condition is solution-treated plus aged rather than annealed. On finished machined parts the premium widens to 30 to 50 percent or more because K-500 work-hardens during machining and consumes more tooling. On flange bolting where Monel 400 Class 4 to 8 clears the design load, the K-500 premium is paid for yield that the joint does not use. See Monel 400 pricing for the LME-to-quote bridge on N04400.
Machinability: K-500 Hardens Further During Cutting
Monel 400 is already tough to machine: a single-phase Ni-Cu solid solution at roughly 8 to 14 percent the machinability index of free-cutting brass (B1112 = 100). Standard CNC turning runs at 30 to 60 m/min with carbide, rigid setup, and sharp tooling. The free-machining variant Monel R-405 (UNS N04405) at 0.025 to 0.060 percent sulphur is specified for high-volume threading.
Monel K-500 is worse by roughly a factor of two. Aged gamma-prime particles raise base hardness to 24 to 35 HRC, and the matrix work-hardens further at the chip interface during cutting. Recommended turning is 15 to 30 m/min with carbide, heavy positive rake, and continuous coolant flood; threading is single-point only, with the work re-solution-treated and aged after rough machining where geometry allows. Tool life on K-500 is typically half that of N04400 at the same parameter set. Detail on the Monel 400 machinability page.
Weldability: Monel 400 Welds Freely, K-500 Over-Ages
Monel 400 welds without PWHT up to 38 mm thickness using ENiCu-7 (SMAW) and ERNiCu-7 (GTAW, GMAW) per AWS A5.11 and A5.14. The single-phase Ni-Cu matrix does not form a hardened HAZ; there is no susceptibility to delayed hydrogen cracking and no precipitation-related fissuring.
Monel K-500 can be welded with the same ERNiCu-7 filler, but the heat-affected zone over-ages: gamma-prime particles coarsen, dissolve back into solution, or grow past the coherent size and lose their pinning action. The result is a soft band in the HAZ at roughly 50 to 70 percent of the aged base-metal yield. To restore aged properties the joint must be re-solution-treated at 980 degrees Celsius and re-aged at 593 degrees Celsius, which is impractical for field welds. For welded fabrications, specify Monel 400. K-500 is for machined, threaded or hot-formed parts that ship in the aged condition.
Corrosion: Both Hold in HF, Seawater and Brine
The corrosion envelope is essentially identical. Both grades carry the same nickel-copper film and behave the same way in the services that drive Monel selection. Hydrofluoric acid at any concentration: both hold; both are specified in HF alkylation internals per API RP 751. Flowing seawater above 30 degrees Celsius: both hold; K-500 is specified for shafts because of yield, not because of corrosion. Brackish heat-exchanger tubing, sour service to NACE MR0175, NaOH caustic and anhydrous ammonia: same behaviour, same suitability. Detail on Monel 400 corrosion resistance, HF acid resistance and seawater corrosion.
When Monel K-500 is Required
Specify Monel K-500 when the design yield exceeds roughly 60 ksi (415 MPa) in a service that also needs the nickel-copper corrosion envelope. Typical landings: downhole pump shafts and valve stems in sour wells, propeller and impeller shafts in seawater, marine and chemical springs where annealed Monel 400 cannot meet spring rate at safe wire diameter, high-strength fasteners to ASTM F468 N05500 where Class 4 to 8 cold-worked N04400 cannot meet bolt preload, and oilfield drill collars and logging-tool housings where strength and non-magnetic behaviour both matter.
When Monel 400 is the Specified Grade for Most Service
Specify Monel 400 for the bulk of nickel-copper service. Flange bolting to B16.5 and B16.47: Monel 400 bolts and Monel 400 stud bolts in ASTM F468 Class 4 to 8 clear the design tightening load without the K-500 premium. HF piping internals, HF alkylation reactors and HF storage: stress is low, weldability is high, and K-500 cannot be field-welded; N04400 is the only practical choice. Plate and sheet to ASTM B127, tubing to ASTM B163 and ASTM B165, forgings and flanges to ASTM B564: all annealed N04400. Brackish and seawater heat exchangers: shell-side pressure does not justify aged K-500 tubing.
Sour Service: Both Qualify per NACE MR0175 / ISO 15156-3
Both N04400 and N05500 are listed in NACE MR0175 / ISO 15156-3 Annex A as acceptable CRA for hydrogen sulphide environments at the 35 HRC hardness ceiling, without prequalification testing under the standard envelope of partial pressure, chloride and pH. Monel 400 in the annealed or hot-finished condition is comfortably below 35 HRC; ASTM F468 Class 4 to 8 stud-bolt grades clear the cap by margin. Monel K-500 in the aged condition runs at 24 to 35 HRC and must be hardness-tested per heat to confirm it stays at or below the cap. For ambient-temperature sour service flange bolting, N04400 is simpler. For sour-well downhole tools where strength is the constraint, N05500 at controlled hardness is the answer. See NACE MR0175 sour service.
Frequently Asked Questions
What is the difference between Monel 400 and Monel K-500? Monel 400 is a single-phase Ni-Cu solid solution supplied annealed at 25 ksi yield per ASTM B164. Monel K-500 keeps the Ni-Cu backbone but adds 2.30 to 3.15 percent aluminium and 0.35 to 0.85 percent titanium for gamma-prime precipitation hardening, supplied solution-treated and aged at 90 to 130 ksi yield per ASTM B865.
Why is K-500 stronger? The Al and Ti dissolve in the Ni-Cu matrix during solution treatment at 980 degrees Celsius and precipitate as coherent Ni3(Al,Ti) particles during an 8 to 16 hour age at 593 degrees Celsius. The particles pin dislocations and raise yield by roughly four times over annealed N04400. Monel 400 has no Al and no Ti, so the route is closed.
What is the price difference? Monel K-500 bar typically sits 20 to 30 percent above Monel 400 by weight, driven by the Al and Ti additions, the tighter melt, and the solution-anneal plus age cycle. The premium widens to 30 to 50 percent or more on machined parts.
Can both be welded? Monel 400 welds freely with ENiCu-7 and ERNiCu-7 fillers and needs no PWHT. K-500 welds but the HAZ over-ages and loses gamma-prime hardening; to restore aged yield the joint needs re-solution treatment plus re-age, impractical in the field.
Do both qualify for sour service? Yes. Both are listed in NACE MR0175 / ISO 15156-3 Annex A at the 35 HRC ceiling. N04400 clears the cap easily; N05500 must be hardness-tested per heat to confirm the aged condition stays at or below 35 HRC.
Related Pages
Continue the Monel 400 reference: chemical composition, mechanical properties, heat treatment, machinability, corrosion resistance, NACE MR0175, UNS N04400, DIN 17743, nickel-copper alloy family, vs Stainless Steel, vs Hastelloy C-276. Standards: ASTM B164, ASTM B127, ASTM B165, ASTM B564, ASTM F468, ASTM F467. Product forms: bolts, stud bolts, round bar, plate, forgings.
Request a Quote
For a Monel 400 versus Monel K-500 selection review on a specific service, send the design yield requirement, operating temperature, fluid composition (chloride ppm, H2S partial pressure, HF content), product form, quantity and certification class to info@torqbolt.com or WhatsApp +91-22-66157017. TorqBolt QA-QC returns the alloy-selection memo, EN 10204 type 3.1 or 3.2 MTC class, mill source and Ex-Mumbai price within one working day. Monel 400 ships from Mumbai stock against ASTM B127, B164, B165, B564, F467 and F468; Monel K-500 ships against ASTM B865 and F468 N05500 with solution-treatment plus age certified by heat number. References: ASTM B164, B127, B564, B865, F468; NACE MR0175 / ISO 15156-3 Annex A; AWS A5.11 and A5.14; Nickel Institute publications on Monel 400 and Monel K-500 ageing response and seawater service.
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