Inconel 602CA (UNS N06025): Chemical Composition, Properties and Radiant Tube Selection Guide

Inconel 602CA (UNS N06025 / W. Nr. 2.4633) is a wrought nickel-chromium-iron heat-resistant alloy containing 24.0–26.0% chromium, 1.80–2.40% aluminum and 0.15–0.25% carbon, with yttrium and zirconium added to improve oxide-scale adhesion. Its density is 7.93 g/cm³ and its melting range 1,340–1,400 °C. This guide sets out the composition limits of DIN EN 10302, the physical and short-time mechanical data, and the 10⁴ h / 10⁵ h creep and stress-rupture values published for the grade, then compares it with 601, 600, 617 and 625 to define where each alloy actually belongs.

The 1,200 °C figure needs one qualification. It is the oxidation-resistance ceiling in an oxidizing atmosphere, not a pressure-design temperature. Allowable stresses for pressure parts are capped by the code: 1,150 °C under VdTÜV 540, 982 °C under ASME Section VIII Div. 1, and 899 °C under Section I. Treating the two as interchangeable is one of the most common causes of premature radiant-tube failure.

1. What Is Inconel 602CA? Grade Definition and Alloy System

Inconel 602CA was developed for high-temperature oxidation, carburization and long-term thermal exposure. Its design logic differs from conventional nickel-chromium heat-resistant grades: on a 24–26% chromium base it raises aluminum to 1.80–2.40%, pairs that with 0.15–0.25% carbon, and adds reactive elements, so that a dense alumina scale and a substantial carbide population are obtained in the same material.

Against established grades such as 601, the difference is not simply more chromium. Three design threads run in parallel.

First, the scale shifts from chromia-forming to alumina-forming. In oxidizing atmospheres the surface develops a dense, tightly adherent Al₂O₃ layer whose spallation resistance is markedly better than a Cr₂O₃-based scale, so protection survives repeated heating and cooling. This is the metallurgical reason the grade outperforms 601 in oxidation resistance across its service range up to 1,200 °C.

Second, carbon acts as an alloying element, not an impurity. At 0.15–0.25%, carbon forms primary M₂₃C₆ and M₇C₃ carbides during solidification and solution treatment, and these constitute the main source of creep strength. That is what qualifies the grade for pressure and load-bearing components rather than refractory liners alone.

Third, reactive elements control the scale/metal interface. Yttrium and zirconium segregate to the interface, suppressing void formation and spallation, and thereby extending the effective wall-thickness life of components under thermal cycling.

Typical uses include radiant tubes, retorts and muffles, rotary and shaft furnace components, furnace rolls, and high-temperature parts in reforming, methanol and ammonia plants.

2. Inconel 602CA Chemical Composition: Chromium, Aluminum and Reactive Elements

Inconel 602CA is defined by the balance of its elements rather than by any single addition. The table gives the DIN EN 10302 specification, with differences under UNS N06025 flagged.

Source: DIN EN 10302, Table 2 of the VDM Alloy 602 CA data sheet. Manganese is the only element where the two systems diverge materially — DIN EN 10302 permits 0.50%, while UNS N06025 restricts it to 0.15%. For export orders quoting both, acceptance must follow the tighter limit, otherwise third-party testing will produce a discrepancy.

2.1 Chromium: the Basis of High-Temperature Oxidation Resistance

Chromium forms a chromium-rich oxide that lowers the inward diffusion rate of oxygen and, critically, establishes the conditions under which aluminum can oxidize selectively. The two elements act in sequence rather than in parallel: chromium first builds a continuous scale and drops the oxygen partial pressure at the metal surface, and only then can a dense Al₂O₃ layer grow beneath it.

High chromium does not, however, mean equal resistance in every high-temperature atmosphere. Oxidation, carburization, nitriding and sulfur-bearing service follow different reaction paths. In a low-oxygen-potential carburizing atmosphere a protective oxide cannot be sustained at all; the alloy then relies on its nickel-rich surface and low carbon activity to resist carbon ingress. Oxidation data cannot be transferred to that case.

2.2 Aluminum: the Conditions for an Alumina Scale

Aluminum is what separates 602CA from most nickel-chromium heat-resistant grades. At 1.80–2.40% it is sufficient to form a continuous Al₂O₃ layer, but formation has both a temperature and an oxygen-potential threshold: below a certain temperature aluminum diffusion is too slow, and at too low an oxygen potential a chromia scale never establishes itself first. In practice the alumina-forming advantage becomes decisive in high-temperature oxidizing service, roughly above 900 °C — an engineering guideline rather than a specification limit — and the alloy offers no corresponding benefit in low-temperature or strongly reducing media.

2.3 Carbon: Source and Limits of Creep Strength

At 0.15–0.25%, Inconel 602CA carbon is substantially higher than in conventional nickel-chromium heat-resistant alloys. Carbon combines with chromium to form primary M₂₃C₆ and M₇C₃ carbides that still provide skeletal reinforcement above 1,000 °C, which is why the grade retains load capacity in the ultra-high-temperature range.

Carbon content alone is nevertheless not a sufficient criterion for creep performance. Carbide size, distribution and stability depend on solution-treatment temperature, cooling rate and service history. Plate and heavy forgings of identical composition develop different carbide morphologies because of their different cooling rates, and their stress-rupture strength diverges accordingly.

2.4 Zirconium and Yttrium: Controlling the Scale/Metal Interface

Zirconium and yttrium total less than 0.25% yet directly govern scale life. The reactive elements segregate to the scale/metal interface, suppress void nucleation and spallation, and keep the scale intact through repeated thermal cycling.

That statement needs a boundary. Reactive elements improve scale adhesion and spallation resistance; they do not imply the scale will never detach. When thermal cycling is severe, when sulfur is present in the atmosphere, or when molten salt deposits form, the scale will still fail.

3. Inconel 602CA Microstructure and Strengthening Mechanisms

The Inconel 602CA matrix is face-centered cubic austenite. High-temperature performance follows from solid-solution strengthening, primary carbides and the protective scale together, with the carbide contribution becoming relatively more important as temperature rises.

3.1 Solid-Solution Strengthening: the Baseline

Nickel, chromium, iron and aluminum form a solid solution in the austenitic matrix; atomic size mismatch and stacking-fault energy change modify matrix strength and structural stability, providing the baseline from room temperature to the mid-temperature range.

One common simplification should be corrected: 602CA is not strong because "high chromium and high aluminum produce stronger solid-solution strengthening." Chromium and aluminum matter mainly through oxidation behavior and carbide-formation kinetics, not through the magnitude of solid-solution strengthening. For long-term high-temperature loading, precipitate type, grain size and heat-treatment condition outweigh solute content.

3.2 Carbides and Gamma Prime: Two Precipitation Paths

Primary M₂₃C₆ and M₇C₃ carbides form during solidification and solution treatment, sit on grain boundaries and within grains, obstruct dislocation climb and boundary sliding, and form the backbone of stress-rupture strength above 1,000 °C.

Below 800 °C, long-term exposure can additionally precipitate γ′ (Ni₃Al type), raising mid-temperature strength. The grade therefore cannot be classified as "free of precipitation strengthening." The accurate statement is that it is supplied in the solution-annealed condition and does not rely on artificial aging as its strengthening route, while carbide and γ′ precipitation continue to shape properties in service.

There is a practical consequence: in the 600–800 °C band, microstructure evolves with time, and properties will not remain at as-supplied values.

3.3 Oxide Scale: Resistance to Continuous Oxidation and Cycling Damage

In high-temperature oxidizing service the alloy develops a protective layer that is predominantly Al₂O₃ with Cr₂O₃ as a transitional zone. The scale is dense and well bonded; in cyclic oxidation testing the specific mass loss is among the lowest of comparable heat-resistant alloys, which is where the life advantage in frequently cycled furnace components originates.

For radiant tubes and furnace rolls, spallation directly reduces effective wall thickness. Selection should therefore check spallation behavior under cyclic conditions in addition to steady-state oxidation rate — the two have different engineering consequences.

3.4 Room-Temperature Strength and High-Temperature Design Must Be Assessed Separately

The grade shows roughly 675 MPa tensile and 270 MPa yield at room temperature — unremarkable among heat-resistant alloys. Those figures cannot be used to infer load capacity above 1,000 °C.

Long-term high-temperature service requires five checks: creep limit, stress-rupture strength, permissible deformation, oxidation loss and design life. Meeting a room-temperature tensile requirement says nothing about suitability for a high-temperature pressure application, and a modest room-temperature strength does not indicate poor high-temperature performance.

4. Inconel 602CA Properties: Physical Data, Short-Time Strength and Stress Rupture

4.1 Physical Properties

Source: Table 3 of the VDM Alloy 602 CA data sheet. Between 20 °C and 1,000 °C the modulus falls by about 45% while conductivity rises by a factor of 2.7 — the two move in opposite directions, so a thermal-stress calculation that applies room-temperature values throughout will simultaneously overestimate stiffness and underestimate the temperature gradient.

4.2 Room-Temperature and Short-Time Elevated-Temperature Tensile Properties

The values below are minimum short-time properties for material solution annealed at 1,220 °C with grain size ≥70 µm, within the following size range: sheet ≤50 mm, strip ≤3 mm, bar and forgings ≤100 mm, wire ≤12 mm.

Source: Table 4 of the VDM Alloy 602 CA data sheet, per VdTÜV 540. Impact toughness at 20 °C for the same condition is specified as >56 J/cm² transverse and >69 J/cm² longitudinal (ISO V-notch).

Note the behavior between 600 °C and 700 °C: tensile strength drops from 520 MPa to 420 MPa, nearly 20%, while proof strength falls only 5 MPa. A steep tensile drop combined with a yield plateau means the plastic reserve is being consumed rapidly; once overloaded in this band, a component moves straight into plastic deformation rather than recovering elastically. Design stresses established below 600 °C should not be carried across this threshold.

Products outside the size range above require separately agreed properties and cannot quote this table.

4.3 Creep Strength and Stress-Rupture Strength

Long-term high-temperature performance is the decisive selection criterion. The table gives both creep strength (Rp1.0, the stress producing 1% creep strain) and stress-rupture strength (Rm, the stress at fracture); the two are not interchangeable.

Condition: solution annealed at 1,220 °C, grain size ≥70 µm, per VdTÜV 540 (Table 5 of the VDM Alloy 602 CA data sheet). Rm here is rupture strength at the stated duration and has nothing to do with room-temperature tensile strength.

Two cautions. First, the gap between 10⁴ h and 10⁵ h rupture strength is about 35% at 650 °C but widens to 50–64% above 1,100 °C; using 10⁴ h data as a long-term design value overstates capacity at 1,100 °C by more than a factor of two. Second, Rp1.0 and Rm differ by 40–50% at the same temperature, so substituting rupture strength for creep limit in a deformation-controlled design allows a component to exceed its permissible strain long before fracture.

4.4 Design Values: the 625 °C Threshold

Applying the Rp1.0 creep limit with a safety factor of 1.5, long-term high-temperature strength values need only be used for design above approximately 625 °C. Below that temperature, short-time elevated-temperature strength data satisfy the design requirement.

The threshold has direct commercial consequences. Components rated below 625 °C need no creep-based sizing and can be made thinner; once the threshold is crossed, wall thickness must be back-calculated from rupture strength and design life, and it grows non-linearly with temperature — the 10⁵ h rupture strength at 1,100 °C is less than half that at 1,000 °C.

5. Inconel 602CA vs 601, 600, 617 and 625

Each of these grades was designed against a different target, and Inconel 602CA is no exception. Comparison should be made item by item — high-temperature oxidation, carburization resistance, long-term creep, room-temperature strength, corrosion resistance — rather than compressed into a single ranking of "strength."

Note: the 602CA column follows DIN EN 10302 and the published data sheet; the 601, 600, 617 and 625 columns give typical specification ranges and must be verified against the standard governing each grade. The table supports initial screening, not item-by-item performance substitution.

5.1 602CA vs 601: a Generational Difference in Cycling and Carburization

Both alloys serve high-temperature industrial equipment, but Inconel 602CA raises chromium, aluminum and carbon simultaneously and adds reactive elements. Per its published data, oxidation resistance exceeds that of 601 across the service range, cyclic mass loss is lower, and carburization and metal-dusting resistance are substantially better.

For duty with repeated heating and cooling plus a carburization risk, 602CA should therefore be evaluated first. Upgrading still needs to be justified against equipment temperature, expected life, atmosphere, component size and material cost — for low-load furnace hardware with modest temperature swings, 601 may well be the more economical choice.

5.2 602CA vs 600: Different Design Targets

600 is a widely applied nickel-chromium-iron alloy aimed at a broad range of corrosive media and moderate-temperature heat resistance; 602CA concentrates on high-temperature oxidation, carburization and long-term loading. Where temperature is low and creep does not govern, 600 holds the cost advantage. Where surface temperature exceeds 1,000 °C or cycling is frequent, scale stability is the variable that determines life.

5.3 602CA vs 617: Surface Performance versus Structural Duty

617 is a nickel-chromium-cobalt-molybdenum alloy whose cobalt and molybdenum provide high structural strength at temperature, making it a natural choice for pressure-retaining and load-bearing hot structures. 602CA concentrates its advantage at the surface — scale life under oxidation, carburization and spallation.

The two cannot be ranked by designation or by room-temperature tensile strength. Structural parts should be compared on stress-rupture strength and allowable stress at design temperature; furnace internals and radiant-tube walls should be compared on cyclic oxidation mass loss and carburization depth. Different metrics produce opposite conclusions, legitimately.

5.4 602CA vs 625: High-Temperature Oxidation versus Media Corrosion

625 is built on a nickel-chromium-molybdenum-niobium system; molybdenum delivers excellent pitting resistance and resistance to reducing acids, which is why it dominates in marine, chemical and chloride-bearing service. 602CA contains no molybdenum, offers no advantage in chloride-bearing aqueous media, and is clearly superior in oxidation and carburization service above 1,000 °C.

Selection should start from the dominant failure mode: oxidative wall loss or carburization embrittlement points to 602CA; wet chloride corrosion or reducing-acid service points to 625. Neither is a substitute for the other.

6. Inconel 602CA Applications: Radiant Tubes, Retorts and High-Temperature Equipment

6.1 Heat-Treatment Industry: Radiant Tubes and Furnace Hardware

Radiant tubes, retorts, furnace rolls and internal supports operate at high temperature with frequent stops and starts. Oxidation resistance, cyclic-oxidation resistance and creep strength make 602CA a primary candidate for this duty.

Radiant-tube design must resolve wall temperature, internal pressure, support spacing, thermal expansion and oxidation loss together. Sizing the wall from the alloy's maximum service temperature is a leading cause of early bulging and cracking: design temperature should be the tube-wall metal temperature, not the furnace-chamber temperature, and the two may differ by 50–100 °C in a high-temperature furnace depending on furnace design and firing arrangement.

6.2 High-Temperature Carburizing Service

In carbon-bearing atmospheres at temperature, Inconel 602CA and comparable grades can carburize, undergo carbide transformation and suffer metal dusting. 602CA resists carburization and metal dusting well, but actual performance depends on carbon activity, gas composition, temperature and time.

The design stage should obtain an actual furnace-gas analysis and confirm that the material data being quoted covers the target atmosphere. Where carbon activity is very high or oxidizing and carburizing cycles alternate, even 602CA should be validated by coupon testing rather than by datasheet alone.

6.3 Petrochemical and Chemical Engineering: Reforming and Synthesis Plant

Inconel 602CA serves reformer tubes, high-temperature components in methanol and ammonia plants, hydrogen production equipment and synthesis-gas cooling systems. Such duty typically combines hydrogen partial pressure, carburization and thermal cycling, so selection must weigh medium composition, pressure, cycling and design life, and confirm compliance with the applicable equipment code.

6.4 Other Heat-Resistant Components

Depending on design, 602CA is also used for furnace rolls, rotary and shaft furnace parts, automotive exhaust components, diesel glow plugs and crucibles for vitrifying radioactive waste.

For high-temperature bolts, pressure fittings and other safety-critical parts, creep, rupture, jointing practice and standard requirements must be verified separately. Excellent oxidation resistance is not evidence of pressure-service suitability — the two are qualified by entirely different routes.

7. Fabrication, Welding and Heat Treatment

7.1 Hot Working

The hot-working range is 900–1,200 °C, followed immediately by rapid cooling in water or with air jets. Workpieces must be charged into a furnace already at temperature, equalized, then removed for deformation; if temperature falls below the lower limit, the part must be reheated rather than worked further. A solution anneal after hot working is recommended to restore optimum properties.

The heating atmosphere should be neither reducing nor alternating in character. Fuel sulfur must be controlled — natural gas below 0.1%, fuel oil at most 0.5% — and the workpiece must not be impinged by flame. Sulfur, phosphorus, lead and other low-melting-point metals damage this alloy; their sources include marking pens, temperature-indicating paints, greases and fuels.

7.2 Cold Working

Cold forming should be carried out in the solution-annealed condition. 602CA work-hardens faster than austenitic stainless steels, so forming equipment and sequences must be adjusted; intermediate anneals are required for heavy deformation, and any cold reduction above 7% should be followed by re-solution annealing. For bending and flanging, the inside radius should exceed three times the sheet thickness to avoid cracking.

7.3 Solution Annealing and Stabilization Annealing

For maximum stress-rupture strength, solution annealing is carried out at 1,220 °C to a target grain size of ≥70 µm. Holding time depends on thickness: 3 min/mm for thickness ≤10 mm; 30 min + (d − 10) × 2 min/mm for 10–20 mm; 50 min + (d − 20) × 1 min/mm above 20 mm. Holding begins once the workpiece has equalized; over-holding is far less critical than under-holding.

Stabilization annealing is the step most often omitted. Solution-annealed material is susceptible to stress-relaxation cracking in the 600–750 °C range; where a component will serve continuously in that band for more than 100 hours, stabilization annealing is required at 950 °C for at least 3 hours. Heating and cooling rates are not critical, though they should not be so fast as to cause distortion. The treatment may be applied before or after welding, and components that have already seen service should be stabilized before repair welding.

7.4 Welding

Matching filler metal is recommended: VDM FM 602 CA (W. Nr. 2.4649), ISO 18274 S Ni 6025, AWS A5.14 ERNiCrFe-12. Covered electrodes may also be used.

Process controls include low heat input, stringer beads, interpass temperature not exceeding 120 °C, acetone cleaning of the joint faces and filler, and thorough argon backing. Preheating is normally unnecessary. The welded zone should be brushed with a stainless-steel brush while still warm to remove temper colors; if pickling is required, keep it short to avoid intergranular attack. Where the weldment will serve between 600 and 750 °C, stabilization annealing per 7.3 applies.

8. Procurement and Application FAQ

Q1. How does the composition of 602CA differ from 601?

602CA contains 24.0–26.0% Cr, 1.80–2.40% Al and 0.15–0.25% C with Ti, Zr and Y; 601 contains 21.0–25.0% Cr, 1.0–1.7% Al and 0.10% C maximum, and no yttrium. Of the three differences, carbon has the largest effect on high-temperature creep performance, while aluminum determines which type of protective scale forms.

Incoming inspection should verify the full chemical report and the material identification (UNS N06025 / W. Nr. 2.4633). Chromium content alone cannot establish that a heat is 602CA.

Q2. Does 602CA require no precipitation strengthening at all?

No. The alloy is supplied solution annealed, but its high-temperature performance is closely tied to primary M₂₃C₆ and M₇C₃ carbides, and long-term exposure below 800 °C can additionally precipitate γ′. The accurate description is that 602CA does not use artificial aging as its strengthening route, while precipitation continues to shape properties throughout manufacture and service.

Q3. Room-temperature strength looks modest — why is it used for ultra-high-temperature parts?

Room-temperature mechanical properties and long-term high-temperature capacity are different assessment dimensions. The value of 602CA lies in scale life, carburization resistance and stress-rupture strength, not in room-temperature yield strength.

Selection should check Rp1.0 creep limit, Rm rupture strength and oxidation loss rate at the target temperature, then set wall thickness against design life. Pressure or safety-critical parts must additionally satisfy the allowable stresses and fabrication requirements of the governing code.

Q4. What information should a 602CA purchase order specify?

1. Grade and identification: UNS N06025 / W. Nr. 2.4633, with the governing product standard (ASTM B166 for bar and wire, ASTM B168 for plate and strip, DIN EN 10302).

2. Chemical composition: in particular Cr 24.0–26.0%, Al 1.80–2.40%, C 0.15–0.25%, and the manganese limit applicable to the named standard.

3. Delivery condition and grain size: solution-treatment temperature, grain size (≥70 µm for critical duty) and surface condition (oxidized or descaled).

4. Service conditions: design temperature, process medium and carbon activity, thermal cycling, design life.

5. High-temperature data: Rp1.0 and Rm at the target temperature plus cyclic oxidation data, matched to the product form, size and heat-treatment condition actually ordered.

6. Documentation: mill test certificate with heat-number traceability, and third-party testing or retest where required.

7. Welding and fabrication: filler metal designation, welding procedure qualification, and whether stabilization annealing is required.

One point deserves emphasis: reference values from general material handbooks cannot be used directly for high-temperature structural design. The difference in stress-rupture strength between plate and heavy forgings of the same grade is enough to change the calculated wall thickness.

Conclusion: Using 602CA Within Its Performance Envelope

Inconel 602CA is a nickel-base heat-resistant alloy developed for high-temperature oxidation, cyclic thermal exposure, carburization and long-term loading. Its advantages come from the high-chromium, high-aluminum composition, reactive-element control of the scale interface, and the microstructural skeleton provided by primary carbides.

For radiant tubes, retorts, furnace rolls and high-temperature chemical equipment, 602CA warrants first evaluation. Against 601, 600, 617 and 625, the comparison must be made item by item against actual temperature, atmosphere, load, thermal cycling and design life — not by ranking designations.

Reliable high-temperature selection rests on three verifiable foundations: a complete product standard, a traceable mill certificate, and property data at the actual design temperature. Judgments based only on grade designation, room-temperature strength or maximum service temperature tend to be expensive above 1,000 °C.

Data in this guide is compiled from DIN EN 10302, ASTM B166 / B168, VdTÜV material data sheet 540, and the published VDM Alloy 602 CA (Nicrofer 6025 HT) technical data sheet, Tables 2–5. Values are either specification limits or published typical values for the stated condition; formal design and acceptance must follow the governing standard, the purchase order and the mill test certificate. The 900 °C alumina-forming guideline in 2.2 and the 50–100 °C wall-to-chamber differential in 6.1 are engineering practice notes, not specification values.