2.4633 / Alloy 602 CA / UNS N06025 Forgings: NiCr25FeAlY Rings, Bars, Discs and Flanges

Published 18 June 2022. Last updated 8 August 2026. Data checked against VDM Metals Material Data Sheet No. 4137 Rev. 04 and VdTÜV material data sheet 540.

Alloy description

Three characteristics distinguish 2.4633 from general-purpose heat-resistant nickel alloys.

Specified high carbon content

Most corrosion-resistant nickel alloys limit carbon as far as the melting route allows. 2.4633 specifies 0.15% carbon minimum and 0.25% maximum. Coarse primary carbides form on solidification and remain stable at temperature, pinning grain boundaries and restricting dislocation climb. This is the main contributor to creep strength above 1,000 °C, where solid-solution and gamma-prime strengthening have largely faded. Datasheets that list 2.4633 with a carbon maximum of 0.15% have transposed the limit: 0.15% is the lower bound.

Aluminium and yttrium for scale adhesion

At 25% chromium the alloy would form a chromia scale on its own. The 1.8–2.4% aluminium addition instead produces a continuous, slow-growing alumina (Al₂O₃) layer, and 0.05–0.12% yttrium improves the adhesion of that layer during thermal cycling through the reactive-element effect. Under cyclic oxidation testing 2.4633 shows the lowest mass loss among the common high-temperature alloys, which is the condition that limits the life of most furnace hardware. Zirconium at 0.01–0.10% has a similar effect.

Coarse grain size by design

The solution anneal is carried out at 1,220 °C to develop a grain size of at least 70 µm. Coarse grains reduce the grain-boundary area available for diffusional creep and cavitation. For this reason 2.4633 should not be normalised or annealed at a lower temperature to improve machinability, since the resulting finer grain reduces creep life.

Equivalent designations and standards for 2.4633

This grade is specified under at least eight names. All entries below the trademark row refer to the same chemistry. Jiangyin Jiangnan Metal Co., Ltd. accepts orders under any of them and supplies UNS N06025 / 2.4633 forgings certified to the equivalent specification.

Chemical composition of 2.4633

The composition below is per DIN EN 10302. Chromium provides the base oxidation resistance, aluminium converts the scale to alumina, yttrium and zirconium maintain scale adhesion through thermal cycles, titanium contributes minor carbide and nitride formation, and the specified carbon forms the M₂₃C₆ and M₇C₃ carbides that carry the creep load.

Code approvals and temperature limits

The approved envelope for 2.4633 is wide for a wrought alloy, but the applicable ceiling depends on whether the part retains pressure and which code governs it. The oxidation limit and the pressure-code limit are different figures and are not interchangeable.

Mechanical properties of 2.4633

Values apply to material solution annealed at 1,220 °C with grain size of at least 70 µm, per VdTÜV material data sheet 540, for rod and forgings up to 100 mm section. Properties for heavier sections are agreed per order. For large forged rings and discs we take test coupons from prolongations at the actual section thickness.

Further room-temperature minima commonly specified on forging orders are Rp_1.0 of at least 310 MPa and ISO V-notch impact toughness of at least 55 J longitudinal and 45 J transverse. For sheet to 50 mm, VdTÜV 540 states impact energy on an area basis as above 69 J/cm² longitudinal and above 56 J/cm² transverse. State which basis applies on the order, since the two figures are not interchangeable.

Creep and stress-rupture strength

Physical properties of 2.4633

Two of these values have direct consequences for design. The coefficient of thermal expansion rises from 14.2 to 17.9 × 10⁻⁶/K across the working range, which is higher than the carbon or stainless steel that a 2.4633 part is usually bolted to, so differential expansion at flanged joints and roller journals has to be allowed for. The modulus falls from 215 GPa at room temperature to 102 GPa at 1,100 °C, so a furnace roller that is stiff when cold will deflect more than twice as much at operating temperature under the same load.

Oxidation and corrosion behaviour

Behaviour under the main high-temperature damage mechanisms is summarised below.

Comparison with other high-temperature alloys

Heat treatment of 2.4633

Solution annealing

Charge the part into a furnace already at temperature, hold at 1,220 °C (2,228 °F) to develop a grain size of at least 70 µm, then cool rapidly by water quench, or compressed air for thin sections. Soak time is measured from the point at which the material reaches temperature and is calculated from section thickness.

Excess soak time is less damaging than insufficient soak time. If further processing follows the anneal, cool rapidly. If the anneal is the final operation before service, a slower cool is acceptable and reduces distortion on large rings and discs.

Stabilisation annealing

Furnace cleanliness

Sulphur, phosphorus, lead and other low-melting metals cause damage during heat treatment of this alloy. Contamination commonly originates from marking paints and temperature-indicating crayons, grease, oil and fuel residues, so parts must be clean before charging. For directly fired furnaces, use natural gas below 0.1% sulphur or fuel oil below 0.5%, set a slightly oxidising atmosphere, avoid reducing or fluctuating conditions, and keep flames off the workpiece.

Forging and hot working of 2.4633

The usable temperature band is only 300 °C wide, the alloy work-hardens faster than austenitic stainless steel, and each reheat cycle adds cost and scale. Our production practice for this grade is as follows.

• Billets are charged into a furnace already at working temperature and held until fully equalised through-section.
• Reduction sequences are planned around the 900 °C floor. On heavy rings and discs this normally means three to five heats rather than two.
• Deformation is kept even across the section, since carbides pin the structure and uneven reduction produces uneven recrystallisation, which appears later as scattered grain size and inconsistent creep results on test coupons.
• Parts are cooled rapidly from the finish temperature, then given a full 1,220 °C solution anneal to develop the specified grain size of at least 70 µm.
• A lower anneal temperature is not substituted. An anneal at 1,050 °C improves machinability but reduces creep life, so it is not used on this grade.

Welding of 2.4633

Use a matching-composition filler: material number 2.4649, classified S Ni 6025 (NiCr25Fe10AlY) in DIN EN ISO 18274 and ERNiCrFe-12 in AWS A5.14. Coated electrodes in the same composition are available.

Nickel alloys pick up iron contamination readily, and embedded iron particles become corrosion sites in service. Keep a separate work area away from carbon-steel fabrication, use stainless brushes reserved for nickel alloys, protect shear and roller surfaces with felt or card, clean the joint area and filler rod with acetone, and avoid draughts during gas-shielded welding.

Machining of 2.4633

Machine in the solution-annealed condition. The alloy work-hardens strongly, so use a low cutting speed, a feed that is not too light, and a depth of cut sufficient to cut below the previously work-hardened layer. The tool should remain engaged, since dwelling in the cut creates a hardened skin that damages the following pass and the tool. Heat generation is high and thermal conductivity is low, so flood coolant with a water-based emulsion of the type used for stainless steels is required.

Production capability for 2.4633 forgings

Quality, testing and certification

• Chemistry: full elemental analysis by optical emission spectrometry, with C, Al, Y and Zr reported.
• Mechanical testing: room-temperature tensile to ISO 6892-1 or ASTM E8, elevated-temperature tensile on request, ISO V-notch impact.
• Grain size: metallographic verification against the 70 µm minimum to ASTM E112, reported on the certificate.
• Ultrasonic testing: to EN 10228-3, SEP 1921 or ASTM A388, acceptance class per order.
• Surface NDE: dye penetrant to EN ISO 3452 or ASTM E165. Magnetic particle testing does not apply to this non-magnetic alloy.
• Certification: EN 10204 3.1 as standard, EN 10204 3.2 with third-party witness (TÜV, DNV, BV, Lloyd's, ABS, SGS) on request.
• Traceability: heat number hard-stamped or vibro-etched on a non-functional surface, documentation retained for ten years.

Applications of 2.4633 forgings

Forged shapes most often supplied into these applications are roller journals and end caps, seamless rolled rings for tube-sheet and flange connections, forged discs and hubs, bushings and sleeves for furnace mechanisms, nozzles, and long forged shafts and rolls up to 8 m.

Service damage mechanisms

Ordering information for 2.4633 forgings

1. State the grade as 2.4633 / UNS N06025 / NiCr25FeAlY per DIN EN 10302. Ordering against a trade name alone is not recommended, since those names belong to their owners and imply their material.
2. State the product-form standard: ASTM B 564 or ASME SB-564 for forgings, B 166 or SB-166 for rod and bar, B 168 or SB-168 for plate. Add VdTÜV 540 where the part is pressure-retaining in Europe.
3. State the carbon range as 0.15 to 0.25%. This is the most frequent source of non-conforming material on this grade.
4. State the heat treatment and grain size: solution annealed at 1,220 °C, rapid cooled, grain size at least 70 µm to ASTM E112, measured value reported on the certificate.
5. State the service temperature. Where any part of the duty cycle sits between 600 and 750 °C for more than 100 hours, add a stabilisation anneal at 950 °C for 3 hours minimum.
6. Specify the correct NDE: ultrasonic to EN 10228-3, SEP 1921 or ASTM A388 with the acceptance class, and surface examination by dye penetrant rather than magnetic particle.
7. Set the certificate level: EN 10204 3.1 as standard, or 3.2 naming the inspection body where third-party witness is required.
8. Supply the drawing, including machining allowance, tolerances, surface roughness and the marking location for the heat number.

Drawing callout

Order checklist for 2.4633

Enquiries for 2.4633 / UNS N06025 forgings

Send a drawing, dimensions or the enquiry text generated above and we will reply within 24 hours with price, lead time and confirmation of the applicable standards. For this grade we also confirm the melt route, the anneal temperature and the grain size we will certify to.

Glossary

2.4633

German Werkstoff (material) number for the NiCr25FeAlY high-temperature nickel alloy described on this page.

UNS N06025

Unified Numbering System designation for the same chemistry in North American practice.

NiCr25FeAlY

EN chemical name: nickel-chromium 25% with iron, aluminium and yttrium additions. Written NiCr25Fe10AlY in filler-metal standards.

Alloy 602 CA

Common industry short name. CA refers to the controlled-aluminium chemistry that produces the alumina scale.

Reactive-element effect

Mechanism by which small additions of yttrium or zirconium improve the adhesion of a protective oxide scale during thermal cycling.

M₂₃C₆ and M₇C₃

Chromium-rich carbide phases. In 2.4633 these are the primary strengthening constituents at high temperature, which is why carbon has a specified minimum.

Solution annealing

Heating to 1,220 °C to dissolve and redistribute phases and develop a coarse grain of at least 70 µm, followed by rapid cooling. The standard delivery condition for this alloy.

Stabilisation annealing

A 950 °C hold of at least 3 hours applied to prevent stress-relaxation cracking in components serving between 600 and 750 °C.

Stress-relaxation cracking

Intergranular cracking that occurs when residual stress relaxes by grain-boundary cavitation during service in an intermediate temperature range.

Metal dusting

Catastrophic carburisation in high-carbon-activity gas, disintegrating the alloy into metal particles and carbon. A common failure mode in syngas and reformer service.

Rp_1.0/10⁵ h

The stress producing 1.0% creep strain in 100,000 hours at a given temperature. The usual basis for long-term design of this alloy.

VdTÜV 540

German technical inspection association material data sheet approving this alloy for pressure-retaining service from −10 to 1,150 °C.

EN 10204 3.1 and 3.2

Inspection certificate types. 3.1 is issued by the manufacturer's independent inspection department, 3.2 is countersigned by an independent third party or the customer's representative.

Open-die forging

Hot deformation between flat or simple contoured dies that do not fully enclose the workpiece. Used for large shafts, discs, blocks and ring preforms.

Seamless rolled ring

A ring produced by piercing a forged billet and rolling it out radially and axially, giving circumferential grain flow and no weld seam.

Frequently asked questions

References

1. VDM Metals International GmbH, VDM® Alloy 602 CA / Nicrofer® 6025 HT, Material Data Sheet No. 4137, Revision 04, March 2022.
2. DIN EN 10302, Creep resisting steels, nickel and cobalt alloys, CEN.
3. VdTÜV Werkstoffblatt 540, Nickel-Chrom-Eisen-Legierung NiCr25FeAlY (2.4633), Verband der TÜV e.V.
4. ASTM B 166 and ASME SB-166, Standard Specification for Nickel-Chromium-Aluminum Alloy, Nickel-Chromium-Iron Alloys Rod, Bar, and Wire, ASTM International.
5. ASTM B 168 and ASME SB-168, Standard Specification for Nickel-Chromium-Aluminum Alloy, Nickel-Chromium-Iron Alloys Plate, Sheet, and Strip, ASTM International.
6. ASTM B 564 and ASME SB-564, Standard Specification for Nickel Alloy Forgings, ASTM International.
7. ASME Boiler and Pressure Vessel Code, Section I and Section VIII Division 1, latest edition, American Society of Mechanical Engineers.
8. DIN EN ISO 18274, Welding consumables. Wire and strip electrodes, wires and rods for fusion welding of nickel and nickel alloys, CEN.
9. AWS A5.14/A5.14M, Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods, American Welding Society.
10. U. Brill and D. C. Agarwal, "Alloy 602 CA, a new high-strength, high-temperature alloy for service temperatures up to 1200 °C", CORROSION '93, Paper No. 226, NACE International, Houston, Texas, 1993.
11. D. C. Agarwal, U. Brill and M. Metzler, "Practical Experience with the New Alloy 602 CA (NiCr25FeAlY). Applications in Heat Treat Industry", CORROSION '93, Paper No. 235, NACE International, 1993.
12. U. Brill, G. Giersbach and H.-W. Kettler, "Effizienzsteigerung kontinuierlicher Wärmebehandlungsanlagen durch den Einsatz ungekühlter Ofenrollen aus dem neuen Werkstoff Alloy 602 CA (2.4633)", VDI-Berichte Nr. 1151, 1995, pp. 65 to 88.
13. D. C. Agarwal, U. Brill and J. Klöwer, "Recent results on metal dusting of nickel-base alloys and some applications", CORROSION 2001, Paper No. 382, NACE International, 2001.
14. J. Wilson and D. C. Agarwal, "Case histories on successful applications of alloy 602 CA, UNS N06025 in high temperature environments", CORROSION 2005, Paper No. 5423, NACE International, 2005.
15. A. Chyrkin, R. Pillai, H. Ackermann, H. Hattendorf, S. Richter, W. Nowak, D. Grüner and W. J. Quadakkers, "Modeling carbide dissolution in alloy 602 CA during high temperature oxidation", Corrosion Science, 2015, pp. 32 to 41.
16. EN 10228-3, Non-destructive testing of steel forgings. Ultrasonic testing, CEN, with SEP 1921 and ASTM A388 as alternative UT acceptance practices.
17. EN 10204, Metallic products. Types of inspection documents, CEN.
18. ASTM E112, Standard Test Methods for Determining Average Grain Size, ASTM International.

Standards cited are the revisions known at the time of the last page review. For procurement, reference the revision in force at contract date. Property values are published minima or typical values for the solution-annealed condition and are not a guarantee of the properties of any individual delivery. The governing values for a specific order are those on its material test certificate. All trademarks are the property of their respective owners.

Related nickel alloy forging grades