AISI 446 – UNS S44600
Oxidation-resistant 25% Cr ferritic stainless steel
Ferritic with 23–27% Cr and nitrogen, non-hardenable, with the highest oxidation and sulfidation resistance among ferritic stainless steels (up to ≈ 1,100 °C). It has low creep strength and limited toughness; it embrittles at 475 °C and by sigma phase on prolonged exposure.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | S44600 |
| W.Nr./EN | ≈ 1.4749 (X18CrN28; Cr 26–29%, approximate) |
| DIN | X18CrN28 |
| JIS | SUH446 |
| Trade names | Type 446 |
| Others | ISO X15CrN26; tubes A268 TP446-1 / TP446-2 |
Standards by product form
| Form | Standard |
|---|---|
| Bar | ASTM A276 (446) |
| Plate | ASTM A240 (446, on request); EN 10095 (1.4749) |
| Tube | ASTM A268 TP446-1 / TP446-2 |
| Forging | ASTM A473 (446) |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Seamless pipe | 7304.41.90 | 7304.41 | 7304.41.6045 | 7304 41 00 |
| Welded pipe | 7306.40.00 | 7306.40 | 7306.40.5085 | 7306 40 80 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM A276 (S44600)
| Element | Range (%) |
|---|---|
| C | ≤ 0.20 |
| Mn | ≤ 1.50 |
| Si | ≤ 1.00 |
| P | ≤ 0.040 |
| S | ≤ 0.030 |
| Cr | 23.0 – 27.0 |
| Ni | ≤ 0.75 |
| N | 0.10 – 0.25 |
| Fe | balance |
Mechanical properties
Annealed (ASTM A276)
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 275 MPa |
| Tensile strength | ≥ 485 MPa |
| Elongation | ≥ 20% |
| Hardness | ≈ 160–210 HB (typical) |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.50 g/cm³ |
| Melting range | 1,425 – 1,510 °C |
| Modulus of elasticity | 200 GPa |
| Thermal conductivity (100 °C) | 21.0 W/m·K |
| Thermal expansion (20–100 °C) | 10.4 µm/m·K |
| Electrical resistivity | 67 µΩ·cm |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Oxidation: up to ≈ 1,100 °C continuous / 1,150 °C intermittent in air and sulfurous atmospheres. Low hot strength; avoid structural loads. Embrittlement at 400–540 °C (475 °C) and 540–870 °C (sigma). |
| Heat treatment | Non-hardenable. Annealing 760–845 °C followed by rapid cooling (water or air) to avoid 475 °C embrittlement; stress relief after forming. |
| Welding | Limited weldability (grain growth and low toughness in the HAZ). Preheat 150–230 °C, low heat input; anneal after welding when possible. Consumables AWS ER309/ER310 (tougher) or matching (ER446) when the oxidation resistance of the joint is critical. |
| Machining | Fair; behavior similar to 430, with long chips and a tendency to smear. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Furnace components (supports, baskets, muffles)
- Burners and radiant tubes
- Recuperators and hot exchangers
- Nozzles and blowing lances
- Equipment in sulfurous atmospheres
- Thermocouples (protection sheath)
Alternatives: AISI 310/310S when there is hot loading and toughness is required; 253 MA for creep; 430 or 409 for lower temperatures; nickel alloys (600/601) for severe cycling.
AISI 446: complete guide
What AISI 446 stainless steel is
446 belongs to the family of martensitic and ferritic stainless steels, in the high-chromium ferritic group. With 23 to 27% chromium and at most 0.75% residual nickel, the structure is ferritic at all temperatures: the steel is magnetic, does not harden by quenching and owes its oxidation resistance to the chromium oxide layer that forms and regenerates up to about 1,100 °C.
Chemical composition and role of the elements
The high chromium ensures the protective layer in air, combustion gases and sulfur-bearing atmospheres; nitrogen, typically 0.10 to 0.25%, refines the grain and improves toughness; carbon up to 0.20% is the standard's ceiling, but commercial material usually has much less. There is no molybdenum, so pitting resistance in aqueous chlorides is limited despite the PREN of 25 to 31.
Applications
Burners and nozzles, thermowells, furnace supports, rails and baskets, heat recuperators, incinerator components, sulfurous gas ducts, heat treatment and foundry furnace parts in oxidizing atmospheres. In sulfur-bearing environments it is the alternative to high-nickel austenitics.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, heat treatment (quenching and tempering, annealing, solution annealing and aging) in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of AISI 446?
Per ASTM A276 (S44600): C ≤ 0.20; Mn ≤ 1.50; Si ≤ 1.00; P ≤ 0.040; S ≤ 0.030; Cr 23.0 – 27.0.
Which standards cover AISI 446?
Bar: ASTM A276 (446). Plate: ASTM A240 (446, on request); EN 10095 (1.4749). Tube: ASTM A268 TP446-1 / TP446-2. Forging: ASTM A473 (446).
Up to what temperature can AISI 446 be used?
Oxidation: up to ≈ 1,100 °C continuous / 1,150 °C intermittent in air and sulfurous atmospheres. Low hot strength; avoid structural loads. Embrittlement at 400–540 °C (475 °C) and 540–870 °C (sigma).
Which materials can replace AISI 446?
AISI 310/310S when there is hot loading and toughness is required; 253 MA for creep; 430 or 409 for lower temperatures; nickel alloys (600/601) for severe cycling.
Is 446 stainless steel magnetic?
Yes. It is ferritic at all temperatures. Like other ferritic stainless steels, it can be used in soft magnetic components that need corrosion resistance, such as solenoid housings, cores and pole pieces, although its magnetic properties are inferior to those of conventional magnetic alloys. The relatively high electrical resistivity favors use in alternating current.
Does 446 resist sulfur-bearing atmospheres?
Yes. It is the ferritic stainless with the highest sulfidation resistance, up to ≈ 1,100 °C continuous in air and sulfurous atmospheres. It is the alternative to high-nickel austenitics, which suffer from sulfur.
