Nickel-chromium-molybdenum-tungsten alloy · Open-die forgings

NiCr21Mo16W Forgings (UNS N06686, W.Nr. 2.4606, Alloy 686)

Published 20 March 2018 · Last updated 23 August 2026 · Reviewed by the Jiangyin Jiangnan Metal metallurgical engineering team

Short answer

UNS

N06686

Werkstoff

2.4606

Tensile min

690MPa · 100 ksi

Yield min

310MPa · 45 ksi

Elongation min

45% · ASTM B564

Density

8.73g/cm³ · 0.315 lb/in³

PREN

≈80Cr+3.3(Mo+0.5W)

Solution anneal

1177°C min, water quench

Max ring OD

2,500mm

Max piece

8,000kg

What NiCr21Mo16W forged products can you buy?

Jiangyin Jiangnan Metal produces UNS N06686 by three routes, chosen by geometry and order size. Open-die forging covers shafts, blocks, hollow bars and heavy discs. Seamless ring rolling produces rings from 200 mm to 2,500 mm outside diameter, the usual route for valve bodies, pump casings and pressure-vessel flanges. Upset forging handles short, large-section hubs, tube sheets and flange blanks. The raw material costs roughly twenty times more per kilogram than carbon steel, so near-net-shape dies repay their cost faster in this grade than in any other we run. Removing 30 to 50% of the rough machining stock is normally the largest single influence on the finished price.

Every piece leaves solution annealed and water quenched, identified with heat number and specification, and accompanied by an EN 10204 3.1 certificate showing chemistry, mechanical results, heat-treatment chart records and any corrosion or NDE testing the order calls for.

What is NiCr21Mo16W / UNS N06686?

NiCr21Mo16W is a third-generation nickel-chromium-molybdenum alloy, introduced in 1992 to close a gap the earlier grades left open. Alloy 625 has the chromium (20–23%) to survive oxidising acids but only 8–10% molybdenum, so reducing acids attack it. C-276 has the molybdenum (15–17%) to survive reducing acids but only 14.5–16.5% chromium, so oxidising conditions attack it. NiCr21Mo16W takes the chromium level of 625 and the molybdenum level of C-276 at the same time, adds 3.0–4.4% tungsten, and holds iron and carbon down to keep the structure clean.

Four properties follow from that chemistry.

• It resists both directions of attack. High nickel and molybdenum carry reducing conditions such as hydrochloric and dilute sulphuric acid; 19–23% chromium carries oxidising conditions such as nitric acid and ferric or cupric chloride contamination. Mixed-acid service, where a plant swings between the two, is exactly what it was designed for.
• Localised corrosion resistance is exceptional. Molybdenum and tungsten together push the pitting resistance equivalent number to roughly 80, above C-276 (≈74), Alloy 59 (≈75), Alloy 22 (≈71) and far above Alloy 625 (≈51). Published data show useful service in media at pH 1 or below with more than 100,000 ppm chloride.
• The weld heat-affected zone stays corrosion resistant. Carbon is capped at 0.010% and iron at 5.0% (2.0% in European practice), which suppresses grain-boundary carbide and topologically close-packed phase precipitation during welding. On a welded vessel this is what determines whether the seams survive, and it is why the matched filler metals (AWS A5.14 ERNiCrMo-14, AWS A5.11 ENiCrMo-14) are also used as overmatched consumables on super-austenitic and super-duplex stainless steels.
• There is no ageing reaction. The alloy is single-phase austenitic and strengthened only by elements in solid solution. Strength comes from the annealed structure and, on forgings, from grain size. Extra strength cannot be heat-treated into a NiCr21Mo16W part, and any treatment other than a full solution anneal plus rapid quench lowers its corrosion resistance.

It is non-magnetic (relative permeability about 1.0001) and remains austenitic to cryogenic temperatures. It is a corrosion alloy rather than a high-temperature alloy. Its upper limit is set by creep and by TCP-phase stability rather than by oxidation, and ASME Section VIII Division 1 allowable stresses are published only up to 427 °C (800 °F).

What are the equivalent designations of NiCr21Mo16W?

Buyers meet this alloy under at least ten names. All of the designations below describe the same Ni-21Cr-16Mo-4W chemistry, and we accept purchase orders under any of them.

Standards are cited by number. Always work to the revision in force at your contract date.

What is the chemical composition of NiCr21Mo16W?

The composition below is the ASTM requirement common to B564, B462, B574 and B575, and it is what we buy raw material against unless a drawing calls for a tighter band. Chromium carries oxidising resistance, molybdenum carries reducing resistance, tungsten reinforces molybdenum against localised attack, and the carbon and iron ceilings exist to keep the grain boundaries clean after welding.

Nickel is reported as the remainder. Calculated by difference the band is roughly 49.5–62.9%, with production heats normally landing near 57%.

ASTM band against European pressure-equipment practice

These are not the same limits, and a heat bought to one does not automatically satisfy the other. If your project is CE-marked, or the drawing cites 2.4606 rather than N06686, say so at enquiry stage so we procure to the tighter band from the start.

What are the mechanical properties of NiCr21Mo16W forgings?

There is one condition, so there is one set of numbers. A certificate must beat the specification minima; a well-processed forging clears them comfortably.

Typical values are representative published results for annealed wrought product and for our own forgings; they are not guaranteed minima. The only guaranteed figures are the specification minima in column two, unless your order sets higher acceptance values.

What are the physical properties of NiCr21Mo16W?

How corrosion resistant is NiCr21Mo16W, and where are its limits?

The pitting resistance equivalent number for tungsten-bearing alloys is PREN = %Cr + 3.3(%Mo + 0.5×%W). At mid-range chemistry of 21% Cr, 16% Mo and 3.7% W, NiCr21Mo16W reaches PREN ≈ 80. For context, 316L sits near 26, super-duplex near 42, Alloy 625 near 51 and C-276 near 74. That number is a ranking tool, not a design allowable, but it explains why this grade holds where others pit.

Sour service and code approvals

UNS N06686 appears in ISO 15156-3 / NACE MR0175 among the highly alloyed austenitic nickel-based materials, subject to the environmental limits, condition and hardness restrictions of the current edition. Solution annealed with hardness at or below 35 HRC is the normal supply condition for sour service. On the pressure side, the alloy holds VdTÜV Werkstoffblatt 515 approval to 400 °C, and ASME Section VIII Division 1 allowable stresses are published in Section II Part D Table 1B (originating in Code Case 2198) for service to 800 °F. We supply with hardness mapping and a contractual hardness cap where the order requires it, but a hardness cap on its own is not a compliance statement, and we will not issue one as if it were.

NiCr21Mo16W against C-276, Alloy 22, Alloy 59 and Alloy 625

This is the question we are asked most on this grade. NiCr21Mo16W is the grade to specify when the environment swings between oxidising and reducing, or when chlorides are extreme. Where the service is purely reducing, C-276 usually does the job for less money, and where it is purely oxidising, 625 or a high-chromium grade is cheaper still.

Relative cost is indicative for forging stock at comparable order size and moves with nickel, molybdenum and tungsten prices. Ask for a current figure at enquiry.

Three questions to settle first

1. Does the environment change state? A scrubber that runs reducing at the quench and oxidising after an oxidant dose, or a plant that acid-cleans on a shutdown, is a mixed-acid duty. That is where NiCr21Mo16W earns its premium and where C-276 disappoints.
2. How much chloride, and how hot? Above roughly 50,000 ppm at elevated temperature, the PREN gap between 80 and 71 stops being academic and starts showing up as crevice attack under gaskets and at flange faces.
3. Will it be welded and put straight into service? With 0.010% carbon and low iron, the as-welded heat-affected zone of this alloy keeps its corrosion resistance without a post-weld solution anneal. On a large fabricated vessel that can be the whole argument.

How is NiCr21Mo16W heat treated?

There is one operation, and the specification leaves no room to vary it.

1. Solution anneal at 1177 °C (2150 °F) minimum, holding roughly 30 minutes per 25 mm of section with a one-hour minimum. This dissolves the mu phase, P phase and carbides that hot working leaves behind and restores a clean, single-phase austenitic structure.
2. Water quench, or cool rapidly by other means. ASTM B564 states this as a requirement for the grade, not a recommendation. On sections above roughly 100 mm, forced-air cooling is not fast enough through the 1000–600 °C window and will sensitise the core while the surface looks acceptable.

There is no ageing step, no stress-relief step and normally no post-weld heat treatment. If a stress relief is imposed by a client specification, it has to be a full re-solution anneal plus quench. An intermediate-temperature soak will damage the part.

How is NiCr21Mo16W forged, welded and machined?

Forging

This alloy is difficult to forge. Flow stress at temperature is roughly two to three times that of carbon steel, the workable temperature window is narrow, and the alloy work-hardens quickly once it falls out of that window. Soak and forge between 1180 °C and 950 °C, finishing above 950 °C; below that, surface cracking risk rises sharply and the press simply stops moving metal. Expect frequent reheats, five or six on a heavy ring, and plan the reductions in incremental passes rather than heavy single blows.

Three things decide whether a NiCr21Mo16W forging comes out right.

• Sulphur discipline. Nickel alloys are hot-short in the presence of sulphur. Furnace fuel, marking compounds, lubricants and even fingerprints matter. We fire on clean fuel and control marking materials on this grade.
• Reduction ratio. Aim for at least 4:1 from the ingot or billet to break down the cast structure and develop grain flow. Under-worked material passes a tensile test and then fails an ultrasonic or corrosion test.
• Finish temperature control. Pyrometer readings on the workpiece, not the furnace, and a rule that says stop and reheat rather than press one more blow.

Welding

Weldability is one of the reasons to choose this alloy. Use matching consumables: AWS A5.14 ERNiCrMo-14 bare filler for GTAW and GMAW, AWS A5.11 ENiCrMo-14 covered electrode for SMAW. No preheat is required. Keep heat input low and interpass temperature below about 150 °C, use stringer beads rather than wide weave, and clean between passes. Because carbon is capped at 0.010% and iron is controlled, the heat-affected zone keeps its corrosion resistance without a post-weld solution anneal in most applications, which is the practical advantage over older grades on large fabrications. The same consumables are widely used overmatched on 6Mo super-austenitic and super-duplex stainless steels, where matching filler would under-alloy the weld metal.

Machining

Machining practice follows C-276 rather than stainless steel. The alloy is tough, it work-hardens rapidly, and it pushes heat into the tool because thermal conductivity is only about 11.9 W/m·K. Use a rigid setup, sharp positive-rake tooling, a heavy constant feed, low surface speed and flood coolant, and do not allow the tool to dwell or rub. A tool that stops cutting for a moment glazes the surface, and the next pass then has to cut through work-hardened material. Take depths of cut below any previously work-hardened layer rather than skimming it.

How does NiCr21Mo16W fail, and how do you prevent it?

What can Jiangyin Jiangnan Metal forge in NiCr21Mo16W?

UNS N06686 is a made-to-order grade for us: the heat is bought against your specification rather than pulled from stock, which is why the drawing, the standard and the testing requirement all matter at enquiry stage rather than after the order.

Rolled ring OD

200–2,500mm

Ring wall min

30mm

Disc diameter

≤1,800mm

Shaft length

≤8,000mm

Bar diameter

25–500mm

Single piece

≤8,000kg

Certification

3.1 / 3.2EN 10204

Lead time

10–16weeks typical

Envelope figures are our standard open-die limits. On this grade the practical maximum for a single piece is often set by the available billet size and the quench tank rather than by press capacity, so confirm large sections at enquiry.

Which standards and certificates apply?

Quality management is certified to ISO 9001:2015. Third-party witness certificates are issued through the inspection body you nominate: Lloyd's Register, DNV, Bureau Veritas, ABS, TÜV or SGS. Customers keep an unrestricted right to witness any production stage, including chemistry, forging, heat treatment, quenching and mechanical testing.

How do you specify a NiCr21Mo16W forging order?

1. Name the material generically. Write UNS N06686 / W.Nr. 2.4606 / NiCr21Mo16W plus the product specification, ASTM B564 for forgings or B462 for flanges and fittings. A purchase order that only says "Inconel 686" names a trademark.
2. State the heat treatment as a cycle. "Solution annealed at 1177 °C minimum, water quenched" removes any argument about what was actually done, and lets us put the chart record against it.
3. Say which chemistry band governs. ASTM limits, or the tighter European band with Fe ≤ 2.0%. This decides what we buy, so it cannot be settled later.
4. Send the drawing with test location and direction. Above 50 mm section, state whether tensile tests are longitudinal or transverse and where the prolongation is taken.
5. Specify corrosion testing explicitly. "ASTM G28 Method A, maximum corrosion rate as agreed" or "ASTM G48 Method C at 85 °C, no pitting at 20×". An unqualified "corrosion test" is not a specification.
6. Define NDE and acceptance class. For example "UT per EN 10228-3, quality class 3" plus "PT per ASTM E165, Type I Method C".
7. Choose the certificate, then give quantity, date, Incoterm and destination. Quantity decides the melt route: below roughly 500 kg we consolidate onto a larger heat, which affects both price and lead time.

Drawing callout you can copy

MATERIAL:      UNS N06686 / W.Nr. 2.4606 / NiCr21Mo16W
               to ASTM B564 (ASME SB-564)
               [alt. ASTM B462 for flanges and fittings]
CHEMISTRY:     ASTM limits, with Fe 2.0% max, P 0.025% max, S 0.015% max
               (intersection of ASTM and European practice)
CONDITION:     Solution annealed 1177 deg C min, hold 30 min per 25 mm,
               water quenched. Furnace chart record to accompany certificate.
               No post-weld or intermediate heat treatment permitted.
MECHANICAL:    Rm 690 MPa min, Rp0.2 310 MPa min, A 45% min
               Hardness 35 HRC max where ISO 15156-3 applies
TEST DIRECTION: Longitudinal, from integral prolongation; state location on drawing
CORROSION:     ASTM G28 Method A, rate as agreed
               ASTM G48 Method C at 85 deg C, no pitting at 20x
GRAIN SIZE:    ASTM E112, report actual
NDE:           UT per EN 10228-3 quality class 3
               PT per ASTM E165 Type I Method C (no MT - material is non-magnetic)
PMI:           XRF or OES verification on each piece
CERTIFICATE:   EN 10204 3.1 (3.2 with third-party witness if stated on the PO)
MARKING:       Heat number, UNS number and drawing number, low-stress stamped,
               marking compound to be sulphur-free and chloride-free

Seven mistakes buyers make with NiCr21Mo16W

1. Confusing it with NiCr21Mo14W / Alloy 22. Different molybdenum, different alloy, different price. Check the UNS number, not the name.
2. Quoting UNS N07750. That is Inconel X-750, an age-hardenable alloy with no relationship to this grade. The error propagates from old datasheets and web pages.
3. Asking for an aged or hardened condition. No such condition exists for this alloy. It is solid-solution strengthened, and solution annealed plus quenched is the only correct condition.
4. Accepting air cooling to save money. It removes the corrosion resistance the order was placed for, and a tensile test will not detect the loss.
5. Specifying magnetic particle inspection. The material is non-magnetic. The surface method is liquid penetrant.
6. Leaving the corrosion test unspecified. "Corrosion tested" without a method, temperature and acceptance criterion means nothing at inspection.
7. Ordering 200 kg and expecting stock pricing. Small quantities are consolidated onto a larger heat. Tell us the quantity early and we will tell you honestly whether waiting for a consolidation window saves money.

Where is NiCr21Mo16W used?

Two worked examples

Example 1: choosing between C-276 and NiCr21Mo16W for a scrubber flange

Given. A DN 600 forged flange for an FGD absorber recirculation line. Process liquor at 65 °C, pH 4.5 in normal operation, dropping to pH 1.2 during upsets, chloride 45,000 ppm, fluoride present, and an oxidant dose that shifts the potential upward for part of each cycle.

Assessment. The steady-state duty is within C-276 territory. The upset condition is not: at pH 1.2 with 45,000 ppm chloride and a raised potential, C-276's 15.5% chromium is the weak term, and crevice attack at the gasket seat is the expected failure mode rather than general corrosion. Compare mid-range PREN: C-276 at about 74, NiCr21Mo16W at about 80. The difference is entirely chromium, and chromium is precisely what the oxidising excursion demands.

Decision. Specify UNS N06686 to ASTM B462, solution annealed and water quenched, with ASTM G48 Method D crevice testing at 85 °C on a production coupon. The material premium over C-276 is roughly 25–40% on a flange that is a small fraction of the line cost, against an unplanned outage to replace it. Where the same line runs a section that never sees the upset, C-276 remains the sensible choice. There is no need to alloy a whole system to its worst point.

Example 2: sizing a forged ring and reading the quotation

Given. A finished ring 620 mm OD × 430 mm ID × 180 mm high in UNS N06686, quantity 4.

Method. Net volume = π/4 × (0.620² − 0.430²) × 0.180 = 0.02821 m³. At 8,730 kg/m³ that is 246 kg net per ring, 985 kg for the set. Add 28% machining stock and the rough forging is about 315 kg each, 1,260 kg for the set, and the billet requirement is higher again because the ring is punched and rolled from an upset blank.

Reading the quotation. Roughly 1,400 to 1,500 kg of N06686 has to be bought in order to ship 985 kg of finished rings, and below about 500 kg an order is consolidated onto another heat. Two conclusions follow from that. Ask for the price on the rough weight rather than the finished weight, so that quotations from different suppliers can be compared like for like. And if you have other N06686 items coming in the next two quarters, place them together, because on this grade the melt quantity moves the price more than the machining does.

Glossary

NiCr21Mo16W frequently asked questions

References

Standards are cited by number; always work to the revision in force at your contract date. Test results on our certificates are independent and traceable to calibrated equipment.

About the manufacturer

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, operating since 2008 with 460 employees including 9 senior and 32 intermediate engineers. The plant runs 1 t to 9 t open-die hammers, 4,500 t and 5,000 t hydraulic presses and radial-axial ring rolling mills to 2,500 mm outside diameter, with in-house heat treatment, water quenching, machining, mechanical testing and non-destructive examination. Alongside NiCr21Mo16W / UNS N06686 we forge the full corrosion-resistant nickel-alloy range, including C-276, C-22, Alloy 59, Inconel 625 and Incoloy 825, plus carbon, alloy and tool steels, precipitation-hardening and duplex stainless grades. Quality management is certified to ISO 9001:2015 and material is supplied with EN 10204 3.1 certification as standard, 3.2 with third-party witness on request.

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