ASTM A268 UNS S44600 High chromium ferritic tube
ASTM A268 standard UNS S44600 High chromium ferritic tube
ASTM A268 UNS S44600 steel tube — high-chromium ferritic grade
The ASTM A268 UNS S44600 Tube (also known as TP446-1 / TP446-2) is manufactured from AISI 446 stainless steel, a high‑chromium ferritic grade widely recognised for its outstanding resistance in high‑temperature aggressive environments. What makes this alloy exceptional? It resists oxide scale peeling up to 1082 °C, offers remarkable oxidation resistance, and retains good plasticity and ductility even after prolonged service.
What defines ASTM A268 UNS S44600 tube chemical composition?
The precise chemistry of UNS S44600 is the foundation of its performance. It typically contains 26–29 % chromium, with small additions of nickel, molybdenum, aluminium and nitrogen to enhance high‑temperature strength and corrosion resistance. The nitrogen also contributes to finer grain structure. This composition meets ASTM A268 (TP446‑1, TP446‑2) and aligns with EN 1.4749, DIN X18CrN28, and the national grade 16Cr25N (2Cr25N).
| element | composition range (%) | role in alloy |
|---|---|---|
| Chromium (Cr) | 26.0 – 29.0 | forms protective Cr₂O₃ scale |
| Nickel (Ni) | ≤ 0.75 | stabilises ferritic matrix |
| Manganese (Mn) | ≤ 1.0 | deoxidiser, improves hot workability |
| Silicon (Si) | ≤ 1.0 | enhances oxidation resistance |
| Nitrogen (N) | 0.10 – 0.25 | solid‑solution strengthener |
| Carbon (C) | ≤ 0.20 | kept low to avoid sensitisation |
How does this composition influence fabrication? The low carbon and controlled nitrogen make the tube stable in welded and annealed conditions.
ASTM A268 UNS S44600 tube mechanical properties (annealed)
In the annealed state, the tube delivers a tensile strength of at least 410 MPa and yield strength above 205 MPa, with elongation around 20 % (depending on wall thickness). Which heat treatment gives the best combination? For most applications, annealing at 780–830 °C followed by air cooling produces an optimum ferritic structure without embrittlement.
| property | value (min) | typical |
|---|---|---|
| Tensile strength Rₘ | 410 MPa | 480‑560 MPa |
| Yield strength Rₚ₀.₂ | 205 MPa | 280‑350 MPa |
| Elongation A₅₀ | 18 % | 22‑28 % |
| Hardness (HB) | ≤ 207 | 180‑200 |
How is ASTM A268 UNS S44600 tube manufactured?
Both seamless and welded production routes are possible. Seamless tubes are usually cold drawn or cold rolled to ensure homogeneous density and close tolerances. Welded tubes are fabricated using TIG or plasma arc welding, then cold worked and fully annealed to meet ASTM A268 requirements. Which process suits your project? Seamless is preferred for high‑pressure systems, while welded offers economical larger diameters.
- Seamless (SMLS) : cold finished, no weld seam – ideal for furnace rolls, pyrolysis coils.
- Welded + cold worked : excellent for heat exchanger tubes, economiser coils.
ASTM A268 UNS S44600 tube corrosion resistance in chlorides
Thanks to its high chromium content, this tube offers remarkable resistance to chloride‑induced pitting and stress corrosion cracking. How does it compare to austenitic grades? In many marine and chemical environments, UNS S44600 outperforms 304/316 due to its resistance to chloride attack at elevated temperatures. It also withstands sulphurous gases.
High‑temperature behaviour of ASTM A268 UNS S44600 tube
The tube shows excellent oxidation resistance up to 1080 °C with minimal scaling. Its ferritic structure resists thermal fatigue, which makes it a prime candidate for furnace internals, heat recovery systems, and finned tube assemblies. For economiser and evaporator banks, finned tubes based on UNS S44600 extend service life dramatically.
In waste‑heat recovery, the Finned Tube for Economizer often uses UNS S44600 as base material; likewise, Finned Tube for Evaporator variants benefit from its high‑temperature strength. The link remains seamlessly integrated — no colour change, no underline.
Where is ASTM A268 UNS S44600 tube applied?
Typical applications include furnace tubes, annealing boxes, thermocouple sheaths, high‑temperature gas reactors, and heat exchangers in chemical and petrochemical plants. The alloy EN 1.4762 / DIN X10CrAlSi25 is also used for similar environments. Which industries rely on it? Power generation, marine scrubbers, and mineral processing all benefit from its unique corrosion/oxidation balance.
| industry | component example | operating condition |
|---|---|---|
| Petrochemical | Gas reaction furnace tubes | up to 1050 °C, sulfidising |
| Power generation | Economiser / evaporator finned tubes | flue gas, chloride dew point |
| Heat treatment | Annealing boxes, radiant tubes | cyclic oxidation |
| Instrumentation | Thermocouple protection | high temp, thermal shock |
UNS S44600 ferritic steel is non‑hardenable by heat treatment, which simplifies fabrication. It exhibits excellent corrosion and oxygen resistance at high temperatures, especially under low mechanical stress. That is why it performs reliably in furnace parts, furnace linings, and annealing boxes — components where durability is non‑negotiable. The steel’s track record confirms its stability in demanding roles.
Moreover, stainless steel seamless pipes manufactured to EN 1.4762 and DIN X10CrAlSi25 are widely used in high‑temperature gas reaction furnaces and thermocouple instruments within the petrochemical industry. This UNS S44600 grade remains a workhorse for equipment facing both heat and corrosion.
For engineers specifying heat‑transfer components, the combination of high chromium and stable ferritic structure makes ASTM A268 UNS S44600 tube a technically sound choice — whether as bare tube or as finned tube for economisers and evaporators.
Detailed datasheet: UNS S44600 steel tube properties (original reference).