2.4681 forgings UNS R31233 · CoCr26Ni9Mo5W — forged rings, bars, discs, shafts and valve parts

2.4681 is a cobalt-base alloy, not a nickel alloy. The confusion comes from the numbering system: the German 2.4xxx Werkstoff range covers both nickel and cobalt alloys, so 2.4681 is often filed alongside Inconel and Hastelloy grades, including in supplier catalogues.

The balance element is cobalt at approximately 54 wt %. Nickel is present only as a nominal 9 % addition that raises ductility and helps stabilise the face-centred-cubic matrix. Buyers normally have to choose between two families here. Cobalt wear alloys of the Stellite® type resist galling and abrasion well, but their high carbon content leaves them hard, brittle and prone to weld cracking. Corrosion-resistant nickel alloys of the C-type resist chlorides well, but gall against themselves and erode under cavitation.

2.4681 keeps carbon low, at 0.06 % maximum, and takes its strength from molybdenum and tungsten in solid solution. Wear resistance comes instead from a strain-induced transformation of the surface layer under sliding contact. That gives one alloy with cobalt-alloy wear behaviour, C-type corrosion behaviour, 38 % elongation and good weldability.

Because the strength comes from solid solution rather than precipitation, there is no ageing step to design around and no risk of over-ageing in service. Section size does not limit through-hardening either: a forged 2.4681 ring 400 mm thick is in the same condition at its core as at its surface once solution annealed. Any strength above the annealed values has to come from cold work, and the alloy work-hardens fast enough that this needs allowing for in machining.

Where 2.4681 sits against neighbouring grades

If the failure mode is purely corrosion, a nickel alloy is usually cheaper and performs at least as well. If it is purely abrasion, with no requirement for ductility, toughness or welding, a high-carbon cobalt alloy or a weld overlay costs less. 2.4681 is worth its cobalt content when both mechanisms act at once, or when a wear-resistant cobalt alloy has to be forged, machined and welded like an ordinary structural alloy.

This chemistry appears under several names depending on the specifying body. All of the designations below refer to the same generic alloy, and Jiangyin Jiangnan Metal Co., Ltd. accepts purchase orders under any of them, issuing a multi-designation material certificate on request.

Because no forging specification exists, the mechanical and NDT acceptance criteria have to be stated on the order. This is where most 2.4681 enquiries are under-specified. See the drawing callout in Testing, inspection and certification.

Cobalt is the balance element. Nitrogen and carbon are maximum values; the remaining figures are nominal aim points, with the actual heat analysis reported on the material certificate.

Melting route: forging stock is procured as EAF + AOD/VOD refined material, with electroslag remelting available where the customer requires improved cleanliness, reduced segregation and tighter inclusion ratings for critical rotating or pressure-containing parts.

Bar data are the relevant reference for forgings. Values are typical for solution-annealed material and are not guaranteed minima until agreed on the order.

Up to about 430 °C the tensile strength falls only slightly while elongation rises sharply, so the alloy becomes more ductile with temperature rather than more brittle.

Strength retention at temperature

Plate values are included because pressure-equipment designers usually work from the ASME-listed plate data rather than bar data.

Design ceiling: although 2.4681 retains useful strength well above 800 °C, ASME Section VIII Division 1 acceptance under Code Case 2121 stops at 427 °C (800 °F). For code-stamped pressure equipment treat 427 °C as the design limit regardless of the higher-temperature data.

Impact toughness, including cryogenic

2.4681 retains more than half of its room-temperature impact energy at liquid-nitrogen temperature. High-carbon cobalt wear alloys do not come close to this, which is the main reason 2.4681 is used where a cobalt alloy has to take shock loading.

Hardness and work-hardening rate

Cold work does not generally harm resistance to general corrosion or chloride pitting, but it does degrade stress-corrosion-cracking resistance. Any part destined for NACE MR0175 / ISO 15156 service must be supplied annealed, and re-annealed after any cold forming producing 7 % or more outer-fibre elongation.

The density of 8.47 g/cm³ is roughly 9 % higher than a typical stainless steel, so a like-for-like substitution raises part mass and raw-material cost in proportion. Thermal conductivity is low, at roughly one third that of carbon steel, so heat generated at a cutting edge stays at the edge instead of passing into the workpiece. This is part of why cutting speeds have to be kept down.

Solution annealing is the only heat treatment. The cycle below is applied to every 2.4681 forging we ship unless the order states otherwise, and is set to give the best combination of corrosion resistance and ductility.

Machining and welding guidance

The hardness table above explains why 2.4681 is hard to machine: 10 % cold work takes the surface from 30 to 40 HRC. If the tool stops cutting and starts rubbing, it hardens the material it is about to cut.

• Machine in the annealed condition, which is the delivery condition for our forgings
• Rigid setup: minimise overhang, use the shortest tool that reaches
• Sharp positive-rake carbide, replaced early rather than run to failure
• Low cutting speed with heavy positive feed, to stay under the work-hardened layer
• Never dwell or take light spring passes; they glaze the surface and destroy tools
• Flood coolant, generously applied and aimed into the cutting zone
• Weldable by GMAW (MIG), GTAW (TIG) and SMAW (stick) with matching filler metals
• Multiple weld layers can be deposited; overlay cracking is not the governing concern
• Re-anneal after welding where optimum corrosion resistance is required
• Budget machining time closer to a nickel superalloy than to an austenitic stainless

The figures below are the reason 2.4681 is specified in preference to cheaper alloys. They cover chloride pitting, seawater crevice corrosion, acid service, sour service and erosion.

Chloride pitting

Critical pitting temperature determined in Green Death solution (11.5 % H₂SO₄ + 1.2 % HCl + 1 % FeCl₃ + 1 % CuCl₂), measured as the lowest temperature at which pitting occurred in a 24-hour test.

Seawater crevice corrosion

From a US Navy study at LaQue Laboratories, Wrightsville Beach, North Carolina, published by Aylor and co-workers at CORROSION 99. Crevice specimens were exposed for 180 days in both quiescent and flowing natural seawater at 29 ± 3 °C, with two crevice sites per sample.

Acid service

The boiling column is worth checking closely. 2.4681 handles dilute hydrochloric acid well below boiling but corrodes at 43.85 mm/y in boiling 2.5 % HCl. Small temperature excursions therefore matter a great deal in HCl service. Confirm with field coupon testing in the actual process stream before committing to a forged component.

Sour service

Tested to NACE TM0177 in 5 % NaCl plus 0.5 % glacial acetic acid saturated with H₂S, coupled to carbon steel and stressed to yield, 2.4681 withstood the conditions both annealed and 15 % cold reduced, indicating good resistance to hydrogen embrittlement. In elevated-temperature testing at 121 °C and 177 °C in 20 % NaCl plus 0.517 MPa H₂S plus 4.83 MPa CO₂, the alloy resisted H₂S-induced stress corrosion cracking in the annealed condition but was prone to cracking when cold reduced.

Wear, galling and erosion

Under cavitation 2.4681 erodes roughly 17 times less than C-276 type alloy and 12 times less than 625 type, while matching or exceeding both on chloride pitting. Few other wrought alloys give that combination in one material, which is why it is used for pump parts that see cavitation in seawater. It also performs well under unlubricated metal-to-metal sliding at high load and low speed, the conditions that cause galling.

2.4681 is hot workable but strong, and it loses ductility quickly once the finishing temperature drops. We forge it with frequent reheats and incremental reductions rather than heavy single blows, and re-anneal after hot forming to restore the solution-annealed condition.

Machining allowance, tolerance class and surface finish are agreed per drawing. Finish machined, semi-finished and rough machined deliveries are all available, as are near-net shapes that reduce the buy-to-fly ratio on an expensive alloy. Maximum practical sections in 2.4681 are smaller than in steel because the alloy is stronger and work-hardens faster. Send the drawing and we will confirm feasibility for your geometry before quoting.

• Pumps and rotating equipmentWear rings, sleeves, shafts, impeller hubs and plunger pump components, where cavitation resistance and chloride pitting resistance are needed in the same part.
• Valves and flow controlBodies, stems, seat rings, plugs and trim for ball, gate, globe, check and plug valves. Chosen for galling resistance on the stem and seat contact faces.
• Oil, gas and subseaWellhead and Christmas tree components, choke trim and subsea housings. Listed in NACE MR0175 / ISO 15156, supplied annealed with no cold work.
• Marine and offshoreSeawater pump parts, seals, bearing sleeves and deck machinery components. No crevice attack recorded over 180 days in quiescent or flowing seawater.
• Chemical processAgitator shafts, mixer blades, nozzles and reactor internals in slurry duty where acid resistance and abrasion resistance are required together.
• Steel plant and galvanisingElectrogalvanising rolls, guide rolls and conveyor components exposed to wear plus hot chloride and acid carry-over.
• Pulp and paperDoctor blades, screen components and refiner parts subject to slurry erosion in chloride-bearing process water.
• Power generationSteam-path wear components and feedwater pump parts where cavitation and liquid-droplet impingement limit component life.
• Cryogenic serviceValve stems and seats for LNG and industrial gas duty, where 92 J Charpy energy is retained at −196 °C.

Because no forging specification exists for R31233, more of the acceptance criteria have to be written into the order than for most grades. Everything below is issued as a single dossier tied to one heat number.

• Ladle and product chemical analysis by optical emission and combustion analysis
• Tensile, yield, elongation and reduction of area at room temperature
• Elevated-temperature tensile testing on request
• Charpy V-notch impact testing, including sub-zero and cryogenic temperatures
• Rockwell or Brinell hardness, multiple positions per piece
• Grain size and microstructure to ASTM E112
• Ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388
• Liquid penetrant testing to ASTM E165 / EN ISO 3452
• Dimensional report against the customer drawing
• Positive material identification by handheld XRF before packing
• NACE MR0175 / ISO 15156 compliance statement where sour service applies
• EN 10204 3.1 mill certificate as standard; 3.2 with TUV, SGS, BV, Lloyd's Register or DNV on request

Recommended drawing callout

Intergranular corrosion testing, customer or end-user witnessed inspection and additional hold points can all be built into the inspection and test plan. Send your ITP with the enquiry and we will mark it up before quoting.