Nickel alloy / open-die forging grade

NC21FeDU Forgings Alloy 825 / UNS N08825 / W.Nr. 2.4858 / NiCr21Mo / NA16

What NC21FeDU is

NC21FeDU is fully austenitic, and each of its main additions does a specific job. The nickel content puts it well outside the range where 300 series stainless steels crack in hot chlorides. Chromium covers oxidising media such as nitric acid. Molybdenum and copper together give the alloy its resistance to reducing acids, sulfuric and phosphoric in particular, and the copper is the reason NC21FeDU handles sulfuric acid better than most other nickel grades. Titanium ties up carbon so the alloy does not sensitise during welding or hot working, which keeps it resistant to intergranular attack without a post-weld heat treatment.

Strength comes from solid solution rather than from precipitation. Proof stress in the annealed condition is therefore modest, 241 MPa minimum, but ductility and toughness are high and the alloy takes heavy hot reduction without cracking. In a forging shop that matters, and it is why NC21FeDU turns up in heavy single-piece parts for acid plant and wellhead equipment rather than just in sheet and tube.

NC21FeDU equivalent designations and standards

If your drawing calls for NC21FeDU, any of the designations below describe the same material and can be cross-certified. Forgings are normally ordered to ASTM B564 or ASME SB-564.

Chemical composition of NC21FeDU

If your project specification calls for the tighter DIN carbon and sulfur limits, say so on the enquiry. The heat has to be picked before forging starts, not after.

Mechanical properties of NC21FeDU forgings

The values below are specification minimums for solution annealed material at room temperature. Actual figures for your parts are reported on the mill test certificate against the heat number.

Physical properties of NC21FeDU

Corrosion behaviour

• Sulfuric acid. Resists all concentrations up to moderate temperatures. The copper addition is what makes this possible.
• Phosphoric acid. A standard choice for wet process phosphoric acid evaporators and heat exchangers.
• Chloride stress-corrosion cracking. At around 42% nickel the alloy sits well clear of the range where 300 series stainless steels fail.
• Pitting and crevice corrosion. Molybdenum and chromium give it a clear margin over 316L in chloride bearing media.
• Intergranular corrosion. Titanium stabilisation prevents chromium carbide sensitisation, verified by ASTM A262 or ASTM G28 when the order calls for it.
• Seawater and brine. Used for propeller shafts, tank truck linings and marine heat exchange duty.
• Where it does not suit. Hot concentrated oxidising acids such as boiling nitric, and sustained service above about 540 °C.

Forging and heat treatment

Melting

Material is melted by electric arc furnace with AOD or VOD refining. An electroslag remelt is added where the specification calls for better cleanliness, tighter segregation control or a higher ultrasonic acceptance class. Sour service parts and rotating parts usually fall in that group.

Hot working

• Heat uniformly to 1150 to 1180 °C and soak long enough for the section to equalise.
• Work down through 1180 to 900 °C. Reheat rather than carry on below the finishing temperature.
• Do not finish below about 870 °C. Light reductions at low temperature produce duplex grain structure and ultrasonic noise, and both show up later at inspection.
• Put real reduction on the final heat. A high forging ratio is the only way to refine grain in an alloy that has no phase transformation to do it instead.

Heat treatment

NC21FeDU is not age hardenable. Its titanium is there to stabilise the alloy against carbide sensitisation, not to form gamma prime, so a solution plus ageing cycle of the sort used on Inconel 718 or Incoloy 925 will not raise its strength. A drawing that calls for one usually means the grade was substituted somewhere upstream, and it is worth checking back before anything goes under the hammer. The correct final condition is:

• Solution or stabilising anneal at 930 to 980 °C, hold according to section thickness, then water quench or rapid air cool.
• Fast cooling through the sensitisation range is what preserves corrosion resistance.
• Where the highest resistance to intergranular attack is required, anneal at the top of the range and verify by ASTM G28 method A.

If your design needs more strength in the same corrosion family, specify Incoloy 925 or Incoloy 945. Those two are genuinely precipitation hardenable.

NC21FeDU forged forms and size envelope

Supplied as forged, rough machined or finish machined to drawing. Tolerances, machining allowance and surface condition are agreed at quotation stage.

Where NC21FeDU forgings are used

Testing, NDT and certification

• Ultrasonic testing. EN 10228-3, SEP 1921 or ASTM A388, at the class stated on the order.
• Surface NDT. Liquid penetrant to ASTM E165 or EN 571. Magnetic particle does not apply here, the alloy is non-magnetic.
• Chemical analysis. Spectrometric analysis per heat, plus positive material identification on finished parts when required.
• Mechanical testing. Tensile and hardness as standard. Charpy impact and elevated temperature tensile on request.
• Corrosion testing. ASTM G28 method A or ASTM A262 practice C where intergranular resistance has to be demonstrated.
• Grain size and microstructure. ASTM E112, reported when specified.
• Certification. EN 10204 3.1 as standard. EN 10204 3.2 witnessed by TUV, BV, SGS, DNV or Lloyd's Register on request.

About the works

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging manufacturer at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, inside the Yangtze River Delta forging cluster and about two hours from the ports of Shanghai and Ningbo. We produce open-die forgings and seamless rolled rings in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys, for pressure vessel, valve, oil and gas, power generation, marine and heavy machinery customers.

NC21FeDU parts are made to drawing, not taken off a shelf. The forging route, the heat treatment and the inspection plan are all set against your governing specification before production starts, and written into the order.

Frequently asked questions about NC21FeDU

Reference standards

Data on this page is compiled from the standards and producer datasheets listed below. Treat the figures as guidance. The governing document for any order is the specification named on the purchase order, and the governing record for any delivered part is its mill test certificate.

• ASTM B564 and ASME SB-564, Nickel Alloy Forgings
• ASTM B425, B424 and B423, UNS N08825 rod and bar, plate sheet and strip, seamless tube
• DIN 17744, Wrought nickel alloys, W.Nr. 2.4858
• EN 10204, Types of inspection documents for metallic products
• EN 10228-3, SEP 1921 and ASTM A388, Ultrasonic testing of forgings
• ASTM G28 and ASTM A262, Intergranular corrosion test methods
• NACE MR0175 and ISO 15156-3, Materials for use in H₂S containing environments
• Producer datasheets for Alloy 825 published by Special Metals and VDM Metals

Reviewed by the technical sales team at Jiangyin Jiangnan Metal Co., Ltd., 22 August 2026. A plain text version of this page is at NC21FeDU.md.