What is the difference between Monel 400 and Monel K-500?

Monel 400 (UNS N04400) is a single-phase nickel-copper solid solution with 63 to 70 percent Ni and 28 to 34 percent Cu, supplied annealed at 25 ksi (170 MPa) yield minimum per ASTM B164. Monel K-500 (UNS N05500) keeps the Ni-Cu backbone but adds 2.30 to 3.15 percent aluminium and 0.35 to 0.85 percent titanium for Ni3(Al,Ti) gamma-prime precipitation hardening, supplied solution-treated and aged at 90 to 130 ksi (620 to 895 MPa) yield per ASTM B865. Corrosion resistance is similar; K-500 trades cost, machinability and weldability for roughly 4 times the yield.

Why is Monel K-500 stronger than Monel 400?

K-500 owes its strength to aluminium and titanium dissolved in the Ni-Cu matrix during a 980 degrees Celsius solution treatment, then precipitated as coherent Ni3(Al,Ti) gamma-prime particles during an 8 to 16 hour age at 593 degrees Celsius, furnace-cooled to 482 degrees Celsius. The precipitates pin dislocations and raise yield from the annealed 40 to 50 ksi range to 90 to 130 ksi. Monel 400 carries no aluminium and no titanium per ASTM B164 Table 1, so the precipitation route is closed; cold work is the only strengthening path and tops out near 85 ksi yield.

What is the price difference between Monel 400 and Monel K-500?

Monel K-500 bar typically sits 20 to 30 percent above Monel 400 bar by weight in the Indian market, driven by the aluminium and titanium additions, the longer melt route, and the mandatory solution-anneal plus age cycle. Premium widens on machined parts because K-500 hardens further during machining and demands lower feeds, slower speeds and more frequent tool changes than Monel 400. For flange bolting, HF piping and most wetted service where annealed 25 ksi yield is adequate, Monel 400 is the cheaper correct answer; K-500 is justified only where high yield is genuinely required.

When is Monel K-500 mandatory over Monel 400?

K-500 is mandatory when the design yield exceeds roughly 60 ksi (415 MPa) in a service that also needs the nickel-copper corrosion envelope. Typical landings: downhole tools and pump shafts in sour wells, propeller and impeller shafts in seawater, valve stems carrying high tightening loads, springs in marine and chemical service, and high-strength fasteners (ASTM F468 N05500) where Monel 400 Class 4 to 8 cold-worked grades cannot meet the load. For flange bolting at standard B16.5 tension, HF piping internals, brine handling and most plate-sheet-tube service, Monel 400 is the specified alloy.

Can Monel K-500 be welded like Monel 400?

K-500 welds, but the heat-affected zone overages and loses the gamma-prime hardening. Monel 400 welds in any thickness without post-weld heat treatment using ENiCu-7 / ERNiCu-7 fillers per AWS A5.11 and A5.14. K-500 needs the joint to be solution-treated at 980 degrees Celsius and re-aged at 593 degrees Celsius after welding to restore the aged yield, 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.

Do both Monel 400 and K-500 qualify for NACE MR0175 sour service?

Yes, both nickel-copper grades qualify under NACE MR0175 / ISO 15156-3 for sour service at the 35 HRC hardness ceiling. Monel 400 (N04400) clears the cap easily in the annealed condition; K-500 (N05500) is age-hardened to roughly 27 to 35 HRC and must be controlled to stay at or below the 35 HRC limit. Both are listed in Annex A as acceptable CRA grades without prequalification testing for hydrogen sulphide partial pressure, chloride and pH within the standard envelope.

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Monel 400 vs Inconel 625 — Chemistry, Strength & Corrosion Selection

Monel 400 (UNS N04400) and Inconel 625 (UNS N06625) are both nickel-base alloys widely specified for severe-service fasteners, but they solve different corrosion problems and carry very different price points. Monel 400 is a Ni-Cu binary (~67 Ni, 32 Cu) selected for hydrofluoric acid, seawater, and reducing chloride service. Inconel 625 is a Ni-Cr-Mo-Nb superalloy chosen where chloride pitting, sour service, or temperatures above 540 °C drive selection. Choosing wrong over-spends 3-4x without performance gain or under-specs and fails in service.

Chemistry — UNS N04400 vs UNS N06625

Element	Monel 400 (N04400)	Inconel 625 (N06625)
Ni	63 min	58 min
Cu	28-34	0.50 max
Cr	—	20-23
Mo	—	8-10
Nb (+ Ta)	—	3.15-4.15
Fe	2.5 max	5.0 max
C	0.30 max	0.10 max

Monel 400 has zero chromium and zero molybdenum — that is precisely why it resists hydrofluoric acid (HF cracks chromium-rich grain boundaries). Inconel 625's chromium-molybdenum-niobium combination places it among the most corrosion-resistant nickel alloys available, especially against chloride-induced pitting and crevice attack. See full Monel 400 element ranges.

Mechanical Properties (Annealed)

Property	Monel 400	Inconel 625
Tensile strength (min)	70 ksi / 485 MPa	120 ksi / 827 MPa
0.2% yield (min)	28 ksi / 195 MPa	60 ksi / 414 MPa
Elongation (min)	35%	30%
Hardness (HRB)	60-80	92-100
Max service temp (continuous)	480 °C / 900 °F	980 °C / 1800 °F

Inconel 625 is roughly 70% stronger and rated for twice the service temperature. If the application is hot oil-and-gas downhole, gas-turbine combustor, or any sour-service flange ≥ 540 °C, Monel 400 cannot meet API/NACE qualification.

Corrosion Performance — When Each Wins

Monel 400 wins on: hydrofluoric acid (any concentration up to boiling), hydrofluosilicic acid, sulfuric acid < 80%, sea water (especially de-aerated), brine, ammonia and amine service, and reducing acids generally. Inconel 625 corrodes in HF.

Inconel 625 wins on: oxidizing chloride environments, sour gas (H₂S + CO₂) per NACE MR0175, high-temperature flue gas, marine atmosphere with hot chlorine, sulfuric acid > 80%, and pitting/crevice resistance in aerated seawater. Monel 400 pits in hot oxidizing chlorides.

Cost & Lead Time

Monel 400 raw stock typically prices at 1.0× baseline (call it 100%). Inconel 625 prices around 3.0-4.0× the same forms. Lead times track availability: Monel 400 round bar / plate are stocked widely across India, US, and EU; Inconel 625 above 6″ section often requires 8-12 weeks mill order. Request a quote for indicative pricing on either grade.

Applicable Standards

Monel 400: ASTM B127 (sheet/plate), ASTM B164 (rod/bar), ASTM B165 (pipe), ASTM F468 (bolts), ASTM B564 (forgings).
Inconel 625: ASTM B443 (sheet/plate), ASTM B446 (rod/bar), ASTM B704 (welded tube), API 6A (well-head), NACE MR0175 / ISO 15156 (sour service).

Selection Guide

Pick Monel 400 if your service is HF, hot seawater, reducing acids, or any non-chromium-suitable chemistry, and your temperature stays below 480 °C. Pick Inconel 625 if your service is sour gas, oxidizing chlorides, hot flue gas, or temperatures above 540 °C. For ambient chloride seawater fasteners both will work — Monel 400 is the lower-cost choice.

FAQs

Can Monel 400 substitute for Inconel 625 in sour-gas service?

No. NACE MR0175 / ISO 15156 qualifies Inconel 625 for sour service; Monel 400 is not qualified above NACE's threshold partial pressures. Substitution risks SSC and SCC failure.

Why does Monel 400 cost less than Inconel 625?

Copper (Monel's ~32% alloying element) is significantly cheaper than chromium + molybdenum + niobium (Inconel 625's alloying load). Different metallurgy, different price point.

Is one easier to weld than the other?

Both weld readily, but Monel 400 uses ERNiCu-7 filler (Monel 60) while Inconel 625 uses ERNiCrMo-3 (matching filler INCONEL Filler 625). Heat input and interpass control matter for both. Monel 400 is more forgiving of weld restraint.

Where can Monel 400 fasteners be sourced?

TorqBolt manufactures Monel 400 anchor bolts, stud bolts, hex bolts, hex nuts, and washers per ASTM F468 + B164 from our Mumbai + Rajkot facilities. Lead time 4-6 weeks for standard sizes. Request quote.

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Monel 400 vs 316/316L Stainless — baseline marine comparison
UNS N04400 specification — complete Monel 400 data sheet
Monel 400 anchor bolts — product page

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Monel 400 vs Inconel 625 — Chemistry, Strength & Corrosion Selection

Chemistry — UNS N04400 vs UNS N06625

Mechanical Properties (Annealed)

Corrosion Performance — When Each Wins

Cost & Lead Time

Applicable Standards

Selection Guide

FAQs

Specifications

Surface Treatments

Certifications

About Us

Satisfaction Survey

Monel 400 vs Inconel 625 — Chemistry, Strength & Corrosion Selection

Chemistry — UNS N04400 vs UNS N06625

Mechanical Properties (Annealed)

Corrosion Performance — When Each Wins

Cost & Lead Time

Applicable Standards

Selection Guide

FAQs

Related Comparisons

Specifications

Surface Treatments

Certifications

About Us

Satisfaction Survey