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Certifications: EN 10204 3.1 / 3.2 material test certificates, PMI verification, and complete export documentation packages.
Stainless 904L
(UNS N08904)
A world-class technical reference for EPC contractors, chemical process and marine engineers, procurement heads, and TPI inspection agencies specifying Stainless Steel 904L super- austenitic stainless steel — covering the copper-enhanced sulphuric acid resistance mechanism, the critical single-phase- austenitic vs. duplex microstructure trade-off, low-carbon “L” weldability, and the QC and documentation discipline required for critical acid process and marine equipment supply.
Copper Addition
& Selection Logic
Stainless Steel 904L is a “super-austenitic” stainless steel — a high-alloy austenitic grade with substantially higher nickel, molybdenum, and a deliberate copper addition compared to standard 316L, engineered specifically for broad-spectrum sulphuric and phosphoric acid resistance while retaining the single-phase austenitic structure’s excellent weldability and toughness.
1.1 — What “Stainless 904L (UNS N08904)” Means and 904L’s Position
904L is classified as a super-austenitic stainless steel — a category of high-alloy, fully austenitic (single-phase, not duplex) stainless grades with substantially elevated nickel (23–28%) and molybdenum (4–5%) content compared to standard austenitic grades like 316L (10–14% Ni, 2–3% Mo), positioning 904L’s overall corrosion resistance and cost between standard austenitic stainless steel and the fully nickel-based alloys discussed throughout RR Hydraulic’s other materials references (Incoloy 825, Hastelloy C-22/C-276). Unlike duplex and super duplex stainless steel (2205 and 2507, discussed in RR Hydraulic’s dedicated references), 904L retains a fully austenitic microstructure — this is a deliberate trade-off discussed in detail in Section 1.3, since the fully austenitic structure provides certain fabrication and toughness advantages at the cost of somewhat different corrosion resistance characteristics compared to a duplex alloy of similar general alloy content.
1.2 — The Role of Copper: Sulphuric Acid Resistance
1.3 — Critical Trade-Off: Single-Phase Austenitic vs. Duplex Microstructure
1.4 — Why Specify 904L Despite This Trade-Off
Superior Sulphuric/Phosphoric Acid Resistance vs. Duplex
904L’s copper addition and overall alloy balance provide better resistance to sulphuric and phosphoric acid environments than duplex 2205/2507 — where the process chemistry is acid-dominant rather than primarily chloride-driven, 904L is frequently the better-performing and more cost-effective choice despite its lower chloride SCC resistance relative to duplex.
Excellent Weldability Without Mandatory Complex Heat Treatment
904L’s fully austenitic structure avoids the intermetallic (sigma) phase precipitation risk discussed in detail for duplex/super duplex stainless steel in RR Hydraulic’s dedicated references — 904L can be welded using standard austenitic stainless welding practice without the specific rapid-quench solution annealing and tight heat-input control duplex grades require, a genuine practical fabrication advantage.
Excellent Toughness Across a Broad Temperature Range
The fully austenitic structure retains good toughness at low and cryogenic temperature without the ductile-to-brittle transition behaviour that can affect ferritic-containing alloys — relevant for cryogenic or low-temperature process equipment applications.
Product Forms
& Composition Reference
SS 904L is manufactured across tube, pipe, bar, and plate product forms, each governed by a specific ASTM/ASME standard. Full detail on related corrosion-resistant alloys is available across our standards reference library.
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2.1 — Governing Standards
ASTM A240 / ASME SA-240 — Plate, Sheet, and Strip
Governs flat-rolled 904L product — plate for pressure vessel fabrication, sheet and strip for general fabrication requiring the alloy’s acid resistance.
ASTM A312 / ASME SA-312 — Seamless and Welded Pipe
Governs seamless and welded 904L pipe — the primary specification for process piping applications in acid production and handling.
ASTM A182 (Grade F904L) — Forged Flanges and Fittings
Governs forged 904L flanges, fittings, and valve bodies — Grade F904L corresponds to UNS N08904, referenced alongside RR Hydraulic’s ANSI B16 flange dimensional reference for forged pressure-boundary components.
ASTM A249 / A269 — Welded and Seamless Tube
A249 governs welded austenitic tube for boiler/heat exchanger applications; A269 governs seamless and welded austenitic tube for general corrosion-resisting service — the specification family for 904L tube in heat transfer and instrumentation applications.
2.2 — Chemical Composition, PREN, and Mechanical Properties
| Element / Property | Value / Range |
|---|---|
| Chromium | 19.0–23.0% |
| Nickel | 23.0–28.0% |
| Molybdenum | 4.0–5.0% |
| Copper | 1.0–2.0% |
| Carbon (max.) | 0.02% (“L” low-carbon grade) |
| Tensile Strength | 490–620 MPa (min. 490 MPa per spec) |
| Yield Strength | 220–290 MPa (min. 220 MPa per spec) |
| Elongation | 35–45% |
PREN = 21 + (3.3 × 4.5) + (16 × 0.05) = 21 + 14.85 + 0.8 = ≈ 36.7
Comparable in numerical range to standard duplex 2205 (PREN ~33–38, per RR Hydraulic’s dedicated reference) — but, per the critical Section 1.3 discussion, comparable PREN does NOT imply comparable chloride SCC resistance given the differing austenitic vs. duplex microstructure.
2.3 — Comparison to 316L and Duplex 2205
| Property | 316L | 904L | Duplex 2205 |
|---|---|---|---|
| Microstructure | Austenitic | Austenitic (super-austenitic) | Duplex (~50/50 austenite-ferrite) |
| PREN | ~24 | ~34–37 | ~33–38 |
| Copper addition | No | Yes (1.0–2.0%) | No |
| Sulphuric/phosphoric acid resistance | Moderate | Excellent | Good, but not 904L’s specific strength |
| Chloride pitting/SCC resistance | Moderate | Good (but see Section 1.3 nuance) | Excellent — ferrite phase advantage |
| Weldability / heat treatment complexity | Simple | Simple | More demanding — sigma phase risk (per RR Hydraulic’s duplex references) |
Low-Carbon Weldability
& Fabrication Guidance
904L’s low-carbon “L” designation and fully austenitic structure provide genuine fabrication advantages compared to both standard austenitic stainless (sensitisation risk) and duplex stainless (intermetallic phase risk).
3.1 — Solution Annealing Heat Treatment
904L is supplied in the solution-annealed condition — heated to approximately 1050–1120°C and rapidly cooled (typically water quenched for tube and pipe) to dissolve secondary phases into solid solution and retain the fully austenitic microstructure that provides the alloy’s designed corrosion resistance. Unlike duplex and super duplex stainless steel (RR Hydraulic’s dedicated references), 904L does not require the same critically rapid quench through a specific intermetallic precipitation temperature range — its fully austenitic structure is considerably less susceptible to the sigma-phase embrittlement risk that governs duplex heat treatment process control, though standard, reasonably prompt cooling remains good general practice.
3.2 — Low-Carbon “L” Grade Weldability
904L’s low carbon content (0.02% maximum) provides the same sensitisation-resistance benefit discussed for standard 316L stainless steel — low carbon content minimises chromium carbide precipitation at grain boundaries during welding, allowing 904L to be welded and placed into service in the as-welded condition without mandatory post-weld solution annealing for the majority of applications, similar in principle to the low-carbon weldability advantage discussed for Hastelloy C-276/C-22 in RR Hydraulic’s dedicated references.
3.3 — Weld Practice and General Fabrication
Matching Filler Metal
Welding consumables matched to 904L (typically ER385/E385-type filler, per AWS A5.9/A5.4) are used to maintain corrosion resistance and mechanical property matching in the weld deposit — using a mismatched or lower-alloy filler metal compromises the weld’s specific copper-enhanced acid resistance.
Standard Austenitic Welding Practice
Standard GTAW/GMAW/SMAW austenitic stainless steel welding practice applies — controlled heat input and interpass temperature remain good general practice, but without the specific, tightly constrained parameters required for duplex/super duplex welding discussed in RR Hydraulic’s dedicated references.
Machining and Forming
904L machines and forms similarly to other high-alloy austenitic stainless steel — work-hardening during machining requires appropriate tooling and cutting parameters, and good ductility in the annealed condition supports cold forming operations for vessel and general fabrication applications.
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from certified super- austenitic stainless heat to finished, tested, and packed 904L component shipment. Chemical composition, mechanical, and corrosion resistance verification are standard on all project- grade supply.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 / Documentation Requirements
| Certificate | Content | EPC Requirement | When Mandatory |
|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Not acceptable for critical acid process/marine supply | Never for critical acid production or marine equipment supply |
| 3.1 (EN 10204) | Heat-traceable chemical + mechanical test report | Mandatory — all EPC supply | All chemical process and marine component supply |
| 3.2 (EN 10204) | 3.1 + TPI countersign | Critical / owner-specified critical items | High-consequence acid process pressure equipment |
4.3 — Applications by Industry
Sulphuric and Phosphoric Acid Process Equipment
SS 904L vessels, piping, and heat exchanger components for sulphuric acid production, concentration, and phosphoric acid fertiliser manufacturing — leveraging the alloy’s copper-enhanced acid resistance advantage (Section 1.2) as a lower-cost alternative to fully nickel-based Incoloy 825 or Hastelloy alloys where the specific service conditions are within 904L’s demonstrated performance envelope.
Flue Gas Desulphurisation and Pollution Control
904L components for FGD scrubber systems exposed to hot, acidic flue gas condensate — a demanding acid-dominant environment where 904L’s specific acid resistance strength and reliable weldability provide a cost-effective alternative to higher-alloy nickel-based options.
Pulp and Paper Bleach Plant Equipment
904L tanks and piping for pulp and paper bleaching stages involving acidic and mixed chemical process streams, where the alloy’s broad-spectrum acid resistance and straightforward weldability (Section 3.2) support reliable, cost-effective equipment fabrication.
4.4 — Export Packaging Specification
- Tube, pipe, and bar ends protected to prevent contamination and moisture ingress during transit
- Heat/lot number stamped or tagged on each item, cross-referenced to the accompanying material test certificate, with clear grade marking (904L vs. 316L vs. duplex) to prevent confusion at site receiving inspection
- Components segregated from carbon steel and other dissimilar materials during packing to avoid surface contamination affecting the alloy’s corrosion performance
- Documentation in a waterproof pocket: EN 10204 3.1/3.2 MTC, chemical composition report, mechanical properties report, corrosion test report (ASTM G48, where specified), PMI report, and packing list with form/size breakdown per item
- ISPM-15 timber or export cartons for international shipment, with country of origin and HS tariff code documentation matched to the stainless steel product category
Submit your form, size, and quantity to RR Hydraulic for a complete, certified commercial offer.