Stainless Steel 904L
Stainless Steel 904L (UNS N08904) is a high-alloy austenitic grade with elevated levels of nickel, chromium, molybdenum, and copper. It resists sulfuric acid across a wide concentration range, handles chloride pitting well, and maintains a fully austenitic structure that is free from sensitisation in the as-welded condition.
1. Chemical Composition
The table below shows the controlled elemental composition of 904L. The multi-element alloying approach is what allows this grade to perform in aggressive acid and chloride environments.
| GRADE | UNS Designation |
C | Mn | P | S | Si | Cr | Ni | Mo | Ti | Cu | Al | Other |
| 904L | N08904 | ≤ 0.020 | 2.00 | ≤ 0.040 | ≤ 0.030 | ≤ 1.00 | 19.0-23.0 | 23.0-28.0 | 4.0-5.0 | - | 1.0-2.0 | - | N≤0.1 |
2. Mechanical Properties
These minimum mechanical values apply to 904L in the solution-annealed condition. They represent the performance levels for tubes and pipes in pressure service specifications.
| Grade | Condition & Size | Standard | Heat Treating Temp. : min | Tensile Strength Min. MPa |
Yield Strength Min. MPa |
Elongation min. % |
| N08904 | A312 | ≥ 1100°C | ≥ 490 | ≥ 215 | ≥ 35 % |
3. Equivalent Grade
This table maps 904L across major international designation systems. It is useful for cross-border procurement, third-party inspection, material traceability, and specification alignment.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| 904L | N08904 | S31782 | 015Cr21Ni26Mo5Cu2 | SUS890L | X1NiCrMoCu25-20-5 | 1.4539 | - |
3. Key Technical Advantages
- Broad Sulfuric Acid Resistance with Copper Synergy: The 4.0–5.0% molybdenum combined with 1.0–2.0% copper suppresses active dissolution in both dilute and moderately concentrated sulfuric acid. This extends service life in acid plant piping, absorption towers, and heat exchanger circuits well beyond what standard 316L or 317L can handle.
- High PREN for Chloride Environments: With a Pitting Resistance Equivalent Number (PREN) typically above 34, 904L resists pitting and crevice corrosion in warm seawater and chloride-bearing process streams. Conventional 300-series grades tend to fail within months in the same conditions.
- Resistance to Sensitisation: The ultra-low carbon ceiling of 0.020% prevents chromium carbide precipitation at grain boundaries during welding. Corrosion resistance is fully maintained in weld heat-affected zones without post-weld heat treatment.
Technical Note: 904L is not classified as a superaustenitic steel but sits at the upper boundary of standard austenitic alloys. If 904L does not provide enough pitting resistance (PREN) for the environment (for example, in seawater above 35°C or in concentrated chloride brines) use 254 SMO (UNS S31254) or 654 SMO (UNS S32654) instead.
4. Common Manufacturing Standards
- ASTM A312: Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes. UNS N08904 is covered under supplementary alloy listings.
- ASTM A213: Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes. N08904 is listed for heat exchanger service.
- ASTM A269: Standard specification for seamless and welded austenitic stainless steel tubing for general service, applicable to 904L.
- ASTM A249: Standard specification for welded austenitic steel boiler, superheater, heat-exchanger, and condenser tubes. N08904 is applicable.
- ASTM B677: Specification for seamless pipe and tube.
- DIN EN 10216-5: Seamless steel tubes for pressure purposes, stainless steel (Grade 1.4539 equivalent).
- JIS G3459: Stainless steel pipes, applicable to SUS890L designation.
- GB/T 14976: Seamless stainless steel tubes for fluid transport, 015Cr19Ni26Mo5Cu2 designation.
Manufacturing Standards Comparison Table:
| Standard | ASTM | EN/DIN | JIS | GB/T | GOST |
| Seamless Pipe | A312 N08904 | EN 10216-5 (1.4539) | G3459 SUS890L | GB/T 14976 | — |
| Seamless Tube | A213 N08904 | EN 10216-5 (1.4539) | G3463 SUS890L | GB/T 13296 | — |
| Welded Tube | A249 N08904 | EN 10217-7 (1.4539) | G3463 SUS890L | GB/T 12771 | — |
5. Primary Applications
- Sulfuric Acid Production Plants: Heat exchangers, absorption towers, and acid cooler tubing in sulfuric acid manufacturing. Both dilute and intermediate acid concentrations attack conventional stainless steels, making 904L a common choice here.
- Phosphoric Acid Processing: Process piping and evaporator tubes in wet-process phosphoric acid plants that handle aggressive fluoride-contaminated acid solutions.
- Offshore Oil and Gas: 904L is used for seawater injection piping, topside process equipment, and subsea instrumentation tubing. These applications need materials that can resist warm, oxygen-rich seawater without having to move up to more expensive superaustenitic grades.
- Bleach Plant Piping: 904L is used for chlorine dioxide and hypochlorite piping in pulp and paper bleaching. Its high molybdenum and low carbon content help prevent stress corrosion cracking and pitting.
- Flue Gas Desulfurisation: 904L is mainly used in pump casings, spray headers, and absorber liners in FGD systems that handle hot sulfur dioxide condensates and chloride-contaminated scrubber liquors.