What Is Alloy 602CA (2.4633 / NiCr25FeAlY)?
A Complete Metallurgical Guide

Most high-temperature nickel alloys rely on chromium alone to build their protective oxide barrier. At elevated temperatures, chromium reacts with atmospheric oxygen to form chromia (Cr₂O₃) — a dense, adherent scale that shields the underlying metal. This mechanism underpins the corrosion resistance of workhorse alloys such as Inconel® 600 (2.4816) and Alloy 800H (1.4958), performing reliably to roughly 950 °C.

Above 950 °C, chromia encounters two compounding failure modes: it volatilises as CrO₃ gas in oxidising atmospheres with any meaningful gas velocity, and it spalls away from the metal during rapid thermal cycling. Each spallation event exposes fresh metal, oxidation restarts, and — progressively — the alloy is consumed from the outside in.

Alloy 602CA — designated EN 2.4633, NiCr25FeAlY, or UNS N06025 — was engineered specifically to overcome this limitation. By incorporating aluminium (Al) at 1.8–2.4 wt% and yttrium (Y) at 0.05–0.12 wt% alongside 24–26 % chromium, it forms two simultaneous protective oxide layers: an outer Cr₂O₃ and an inner α-Al₂O₃ sublayer. This dual-oxide architecture allows 2.4633 to operate continuously at 1,100 °C — well beyond what chromia-only alloys can sustain.

At Jiangsu Liangyi, we have manufactured custom 2.4633 / NiCr25FeAlY forged components since 1997, exporting to customers in more than 50 countries. Every insight in this guide reflects real production experience from our Jiangyin manufacturing facility.

Engineers worldwide encounter this alloy under different designation systems. Every name below refers to the identical alloy composition and property requirement:

Every element in 2.4633 serves a defined metallurgical purpose. Understanding why each is present — and in what precise quantity — is the starting point for predicting alloy behaviour at temperature.

After solution annealing at 1,150–1,200 °C and rapid water quench, the microstructure consists of equiaxed austenite grains — typically ASTM grain size 2–4 (average diameter 90–225 μm). A controlled population of undissolved primary TiC particles and M₂₃C₆ carbides remain at grain boundaries by design, helping pin them against coarsening during high-temperature service.

Additional M₂₃C₆ carbides precipitate at grain boundaries during service — the intended microstructural evolution. Independent accelerated ageing studies at 1,000 °C/10,000 h confirm no harmful sigma (σ), mu (μ), or Laves phase formation in 2.4633 — distinguishing it from cheaper high-Cr alloys prone to phase instability over time.

At the alloy surface, a layered microstructure develops: the outer 5–30 μm is the dual oxide scale (Cr₂O₃ + α-Al₂O₃), below which is an internal oxidation zone of 5–15 μm. The aluminium reservoir in the bulk ensures self-repair capacity for tens of thousands of service hours.

This mechanism fundamentally differentiates 2.4633 from all chromia-forming nickel alloys and explains why it performs where others progressively fail.

On first exposure, Cr₂O₃ dominates the early surface scale as chromium diffuses faster at lower temperatures. Al₂O₃ forms as a discontinuous sub-scale — present but not yet a complete barrier.

Above 800 °C, aluminium diffusion accelerates and the discontinuous Al₂O₃ particles coalesce. By approximately 900 °C, a continuous α-Al₂O₃ sublayer has formed, creating the two-layer protective structure shown in Fig. 2.

Yttrium (0.05–0.12 wt%) and zirconium (0.01–0.1 wt%) segregate to oxide grain boundaries through the reactive element effect (REE), blocking outward diffusion of Cr and Al cations. Without REE, cation diffusion grows the oxide from inside outward, generating compressive stresses that crack and spall the scale during thermal cycling. With Y and Zr, the oxide grows by slow inward oxygen diffusion — producing a slow-growing, adherent bilayer that survives thousands of thermal cycles. Yttrium reduces cyclic oxidation mass loss at 1,100 °C by a factor of five to ten versus the same base composition without Y.

If localised spallation occurs, the exposed metal regenerates the α-Al₂O₃ sublayer from the aluminium reservoir in the bulk alloy. This self-repair capacity persists for the full design lifetime of typical industrial forgings.

For standard industrial applications, 2.4633 billet is produced by EAF primary melting + AOD — the AOD step is critical for the narrow carbon specification of 0.15–0.25 wt%. For nuclear-grade or aerospace-critical components, Jiangsu Liangyi uses VIM + ESR (double melt), bringing oxygen below 10 ppm and nitrogen below 30 ppm while eliminating macro-segregation that would cause inconsistent oxide-scale behaviour in service.

The working temperature window is 1,050–1,200 °C. Above 1,200 °C: incipient grain boundary melting risk from Al-rich films. Below 1,050 °C: adiabatic shear bands and cracking risk. Because 2.4633 has thermal conductivity roughly one-fifth that of carbon steel, large-section forgings dissipate heat slowly from the core. We use embedded reference thermocouples alongside every production forging above 400 mm section diameter on our 6,300-tonne hydraulic press. Minimum forging reduction ratio: 4:1.

Every 2.4633 component is solution annealed at 1,150–1,200 °C, held minimum 1 hour per 25 mm section thickness, then rapidly water quenched within 60 seconds of furnace exit. Slow cooling through 700–1,000 °C produces continuous grain boundary carbide films that severely reduce ductility.

For the full range of custom 2.4633 / NiCr25FeAlY open die forgings and seamless rolled rings — including rings to 6,000 mm OD, bars to 2,000 mm, and hollow forgings — see full specifications and request a quote on our product page.

2.4633 applications cluster around one requirement: sustained reliable performance above 900 °C in oxidising, carburising, or thermally cyclic environments where chromia-only alloys progressively fail. The six sectors below represent the highest-volume applications for forged 2.4633 components globally.

The table below reflects material selection decisions we have helped customers navigate across hundreds of real projects. It honestly identifies where 2.4633 is the correct choice — and where a different alloy would serve better.

The most common enquiries received by our engineering team from global procurement professionals and design engineers working with EN 2.4633 / Alloy 602CA.