904L
UNS N08904
High-alloy, low-carbon super-austenitic stainless steel · ASTM A240/A240M — plate, sheet and strip (current ASTM coverage) · ASTM A480/A480M — general requirements for flat-rolled stainless products · ASTM A312/A312M — seamless, welded and heavily cold-worked pipe (current ASTM coverage) · ASTM A269/A269M — seamless and welded tubing for general service (current ASTM coverage) · ASTM B625 — historical flat-product specification for N08904; current editions no longer cover this grade · ASTM B677 — historical seamless pipe and tube specification for N08904; ASTM B677-26 no longer covers this grade
904L (UNS N08904) is a highly alloyed, fully austenitic stainless steel containing about 20% Cr, 25% Ni, 4.5% Mo and 1.5% Cu. It was developed for corrosive chemical service where 316L or 317L is inadequate, particularly reducing acids and chloride-bearing environments. Its value lies in the combined effects of high nickel, molybdenum, copper and very low carbon—not in high mechanical strength. ASTM has transferred N08904 from the B-series nickel-alloy specifications cited in the input to stainless-steel product specifications, so the edition and product form must be stated explicitly when purchasing or reviewing legacy documentation.
- Stainless steel under ASTM Committee A01Current ASTM classification · N08904 has been removed from current ASTM B625 and, from B677-26, ASTM B677.
- 19–23% Cr, 23–28% Ni, 4–5% Mo, 1–2% CuDefining alloy system · The combination provides broad corrosion resistance; copper is especially relevant in reducing acid service.
- 490 MPa tensile; 220 MPa 0.2% yieldFlat-product minimum strength · ASTM A240/A240M requirements for annealed plate, sheet and strip; other product specifications must be checked separately.
- Fully austenitic and not hardenable by heat treatmentMetallurgical condition · Strength can increase through cold work, but cold-worked properties are not interchangeable with annealed specification values.
- Better than 316L/317L, but not seawater-proofChloride performance · Pitting, crevice corrosion and chloride stress-corrosion cracking remain possible, especially at elevated temperature or in stagnant crevices.
- Low carbon supports welded-condition corrosion resistanceWelding characteristic · Filler selection and weld cleanliness remain critical; over-alloyed filler is often selected for severe corrosion service.
Overview
- Designation system
- Common ASTM type designation with UNS composition identifier
- Product forms
- Plate, Sheet, Strip, Seamless pipe, Welded pipe, Seamless tube, Welded tube, Bar, forgings and fittings under their applicable product specifications
- Condition
- Solution-annealed or otherwise heat-treated as required by the governing product specification, Cold-worked or hot-finished tubular product followed by the specified final heat treatment, Welded fabrication, normally used without post-weld solution annealing when the qualified procedure and service permit
- Density
- 7.95 g/cm³ (Representative room-temperature value; a physical property rather than an ASTM acceptance requirement.)
What the designation means—and why B625/B677 now require caution
904L is the established common grade name and N08904 is the UNS identifier. The UNS number identifies an alloy composition; it does not by itself define product form, dimensions, delivery condition, testing, mechanical properties or certification. Those requirements come from the ordered product specification.
| Product form | Legacy specification in the input | Current ASTM route | Practical consequence |
|---|---|---|---|
| Plate, sheet and strip | ASTM B625 | ASTM A240/A240M with ASTM A480/A480M | B625-21 states that N08904 products previously covered by B625 are now covered by A240/A240M and A480/A480M. |
| Seamless pipe | ASTM B677 | ASTM A312/A312M | B677-26 states that N08904 products previously covered by B677 are now covered by A312/A312M. |
| General-service tubing | ASTM B677 | ASTM A269/A269M | B677-26 directs N08904 tubing to A269/A269M. |
| Legacy or code-controlled purchase | B625, B677 or ASME SB equivalents specified by project documents | Use the exact contract or code edition | Do not silently replace an older specification. Confirm acceptance with the purchaser, design authority and applicable construction code. |
A certificate marked only “904L” or “N08904” is incomplete for procurement purposes. The applicable product specification, edition, form, dimensions, condition and required tests must also be identified.
Composition and metallurgical character
| Element | Requirement, mass % | Metallurgical significance |
|---|---|---|
| Carbon | 0.020 max | Limits chromium-carbide sensitization and supports use of welded fabrications without routine post-weld solution annealing. |
| Chromium | 19.0–23.0 | Provides passivity and contributes to resistance to general and localized corrosion. |
| Nickel | 23.0–28.0 | Stabilizes the austenitic structure and materially improves resistance to chloride stress-corrosion cracking relative to ordinary 300-series grades. |
| Molybdenum | 4.00–5.00 | Improves pitting, crevice-corrosion and reducing-acid resistance. |
| Copper | 1.00–2.00 | Improves performance in reducing environments, notably certain sulfuric- and phosphoric-acid conditions. |
| Manganese | 2.00 max | Residual/deoxidation control. |
| Silicon | 1.00 max | Residual/deoxidation control. |
| Phosphorus | 0.045 max | Controlled residual element for flat product. |
| Sulfur | 0.035 max | Controlled residual element for flat product. |
| Nitrogen | 0.10 max | May contribute to localized-corrosion resistance and strength, but N08904 does not specify the substantial minimum nitrogen characteristic of 6Mo super-austenitic grades. |
| Iron | Balance | Iron is the largest constituent, which is the basis for ASTM's present treatment of N08904 as stainless steel. |
Tubular specifications can impose different limits on residual elements. For example, current A269/A269M and A312/A312M use tighter phosphorus and sulfur maxima for N08904 than the A240 flat-product limits. Chemistry must therefore be checked against the actual product specification rather than a generic 904L table.
In the solution-annealed condition, 904L is essentially fully austenitic. It is not precipitation- or transformation-hardenable by heat treatment. Its high nickel content also gives low magnetic permeability, including after substantial cold deformation, although magnetic response should not be used as a definitive grade-identification method.
Specified properties and delivery condition
| Property | Requirement | Qualification |
|---|---|---|
| Tensile strength | 490 MPa minimum | Room-temperature tensile test on conforming flat product. |
| 0.2% offset yield strength | 220 MPa minimum | ASTM A240/A240M SI requirement; do not substitute rounded values from another unit system or product specification. |
| Elongation in 50 mm | 35% minimum | Applicability and specimen provisions are governed by the standard. |
| Hardness | 90 HRBW maximum | Specified maximum for flat product. |
| Heat-treatment condition | Solution-treated/annealed condition required by the product standard | The final condition is essential to corrosion resistance, ductility and compliance. |
These values are not a universal property set for every 904L product. Pipe, tubing, bar, forgings and fittings must meet their own governing specifications.
Representative annealed material commonly exceeds the minimum tensile and yield values while retaining high ductility. Such typical values are useful for process planning but are not guaranteed unless incorporated into the purchase specification. Cold work raises strength and hardness, changes forming behavior and may leave residual stress; it does not create a new heat-treated grade.
| Property | Representative value | Status |
|---|---|---|
| Density | 7.95 g/cm³ | Typical manufacturer data |
| Elastic modulus | About 190 GPa | Typical; temperature-dependent |
| Thermal conductivity at 20°C | About 11.5 W/m·K | Typical; lower than carbon steel |
| Mean thermal expansion, 20–100°C | About 15.3 × 10⁻⁶/K | Typical; relevant to distortion and mixed-material joints |
| Magnetic permeability | Typically below 1.02 | Typical annealed-product behavior |
Physical-property values are representative rather than ASTM grade acceptance limits.
Corrosion behavior: where 904L earns its cost
904L occupies the corrosion-resistance range above conventional 316L and 317L but generally below modern 6% Mo, high-nitrogen super-austenitic grades in severe chloride service. Its defining feature is not simply a higher chromium level: the combination of high nickel, 4–5% molybdenum and 1–2% copper provides useful resistance across both chloride-bearing and reducing-acid environments.
| Mechanism or environment | Expected behavior | Engineering limitation |
|---|---|---|
| Sulfuric and phosphoric acids | Copper, nickel and molybdenum make 904L useful in many reducing-acid conditions where 316L is inadequate. | Suitability varies sharply with concentration, temperature, aeration, velocity and contaminants. Use environment-specific corrosion data or testing. |
| Pitting and crevice corrosion | Substantially better resistance than 316L in many chloride environments. | Not immune. Warm, aerated, stagnant or highly concentrated chloride solutions and tight crevices can cause attack. |
| Chloride stress-corrosion cracking | High nickel gives markedly better resistance than ordinary 300-series stainless steels. | Resistance is not absolute; severe hot chloride environments can still crack the alloy. |
| Intergranular corrosion after welding | Very low carbon reduces sensitization risk and supports welded-condition service. | Heat input, repeated thermal cycles, contamination and unsuitable filler metal can still degrade a joint. |
| Aerated natural seawater | May be considered where some attack can be tolerated or under controlled conditions. | 904L should not be treated as fully resistant to seawater crevice corrosion. More highly alloyed materials may be required. |
| Galvanic coupling | 904L is relatively noble in conductive waters. | It can accelerate corrosion of electrically connected, less noble materials; joint design and isolation require attention. |
A material-selection decision should be based on the complete process stream—not solely pH or chloride concentration. Temperature, oxidizing potential, acid concentration, chlorides, fluorides, solids, deposits, shutdown conditions, weld geometry and cleaning chemistry can control actual performance.
Fabrication, welding and heat treatment
| Operation | Guidance | Reason |
|---|---|---|
| Cold forming | Formable by conventional austenitic-stainless methods, but allow for higher forming loads and springback than familiar 304/316 products. | High alloy content and work hardening increase deformation forces. |
| Machining | Use rigid tooling, positive cutting action, adequate power and suitable feeds; avoid dwelling and repeated light passes. | 904L work-hardens and has low thermal conductivity, promoting tool wear and local heating. |
| Welding | Common fusion processes are applicable when procedures are qualified. Maintain low contamination, appropriate heat input and effective shielding/purging. | Weld-surface oxidation, iron contamination and segregation can reduce corrosion resistance. |
| Filler selection | Matching filler may be acceptable in mild service; over-alloyed nickel-base or highly alloyed austenitic filler is often selected for severe corrosion duty. | Over-alloying helps compensate for weld-metal segregation and dilution, but the exact filler must suit the environment, code and procedure. |
| Post-weld treatment | Remove heat tint and embedded contamination by an appropriate cleaning, pickling and passivation procedure. | Visible heat tint represents a chromium-depleted oxide region and can become the corrosion-limiting surface. |
| Solution annealing | Representative producer guidance is 1095–1150°C followed by rapid cooling. | Restores a uniform solution-treated structure after severe cold work or unsuitable thermal exposure. The governing product specification or qualified fabrication procedure controls acceptance. |
The low carbon content normally allows welded fabrications to enter service without post-weld solution annealing, which is important for large vessels and piping. This does not mean that welding has no metallurgical consequences. Weld-metal composition, dilution, segregation, oxide removal and crevice geometry frequently control the corrosion performance of the completed equipment.
Do not specify a filler solely by the words “for 904L.” The welding process, base-metal combination, service environment, required mechanical properties, construction code and applicable AWS/ASME classification must be reviewed together.
Specifying and purchasing 904L correctly
| Item | What should be stated or verified |
|---|---|
| Product specification | Use the correct current or contractually required ASTM/ASME/EN specification and edition. Do not combine B625 chemistry with A240 testing or B677 terminology with A312 dimensions. |
| Grade identification | State 904L and UNS N08904; for European procurement, state the relevant EN product standard as well as 1.4539/X1NiCrMoCu25-20-5. |
| Product form and route | Plate, sheet, strip, seamless pipe, welded pipe, tube, bar, forging or fitting; identify seamless, welded or heavily cold-worked construction where applicable. |
| Dimensions and tolerances | Thickness or wall, width, length, outside diameter/NPS, schedule, straightness, edge condition and applicable dimensional standard. |
| Delivery condition | Required heat-treated or solution-annealed condition, plus any restrictions on cold work. |
| Surface condition | Finish, scale removal, pickling/passivation, weld heat-tint acceptance and protection from carbon-steel contamination. |
| Testing | Required tensile, hardness, flattening/flaring, hydrostatic or nondestructive electric tests, corrosion tests and any supplementary requirements. |
| Certification | Material test report showing heat identity, chemistry, mechanical results, heat treatment, product specification and traceability. |
| Code status | For pressure equipment, verify the exact ASME or other construction-code listing, allowable stresses, temperature limits and weld qualification requirements. |
| Service qualification | Where corrosion consequences are high, establish acceptance from relevant operating history, published isocorrosion data or representative corrosion testing. |
Positive material identification is useful for distinguishing 904L from lower-alloy stainless steels because molybdenum, nickel and copper are readily discriminating elements. PMI does not replace full chemical analysis, mechanical testing, heat-treatment records or traceability, and handheld methods may not reliably verify carbon or nitrogen.
Legacy drawings often call for ASTM B625 or B677 because those specifications historically included N08904. A technically controlled substitution should document the old edition, proposed current specification, dimensional differences, mechanical requirements, testing, code implications and purchaser approval.
Selection context
| Material family | Relative position | Important distinction |
|---|---|---|
| 316L / 317L | Lower alloy content and generally lower cost | Often adequate in less aggressive service, but substantially less resistant to reducing acids and chloride localized corrosion. |
| 904L / N08904 | High-nickel, Cu-bearing super-austenitic grade | Good combination of formability, weldability, acid resistance and improved chloride performance, but moderate annealed strength. |
| 6Mo super-austenitic grades such as S31254 or N08367 | Higher localized-corrosion resistance | Higher Mo and controlled nitrogen generally make them preferable for more severe warm-chloride or seawater duties. |
| Duplex 2205 | Much higher strength and often lower nickel cost | Not a direct substitute: fabrication, temperature capability, acid behavior, toughness, weld metallurgy and code properties differ. |
| Nickel-base corrosion-resistant alloys | Potentially higher resistance in severe acids or mixed environments | Material choice is environment-specific and usually more costly; 904L should not be assumed equivalent. |
The table is a selection map, not a substitution schedule. Interchangeability requires comparison under the governing product and construction standards.
Sources
- ASTM A240/A240M-26 — Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and StripASTM International
- ASTM A480/A480M — General Requirements for Flat-Rolled Stainless and Heat-Resisting SteelASTM International
- ASTM A312/A312M-25 — Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel PipesASTM International
- ASTM A269/A269M-25 — Seamless and Welded Austenitic Stainless Steel Tubing for General ServiceASTM International
- ASTM B625-21 — Alloy Plate, Sheet, and StripASTM International
- ASTM B677-26 — Nickel-Iron-Chromium-Molybdenum-Nitrogen Seamless Pipe and TubeASTM International
- Composition/Properties — Stainless SteelSpecialty Steel Industry of North America
- ATI 904L Technical Data SheetATI
- 904L Super-Austenitic Stainless Steel Technical SheetAperam
- Atlas Stainless Steel Grade Datasheetsworldstainless
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