Project Description

904L (UNS N08904), also designated as 1.4539, X1NiCrMoCu25-20-5, SUS904L, 015Cr21Ni26Mo5Cu2, and Z2NCDU 25-20, is a high-alloy fully austenitic stainless steel originally developed for exceptional resistance to sulfuric acid across a wide range of concentrations and temperatures. With 4.0–5.0% molybdenum, 23.0–28.0% nickel, and a deliberate 1.0–2.0% copper addition, 904L occupies the performance tier between standard austenitic grades such as 316L and super-austenitic grades such as 254SMO, achieving a pitting resistance equivalent number (PREN) of approximately 35 while offering outstanding resistance to stress corrosion cracking in chloride-bearing acidic environments. SHUNFU METAL, certified to ISO 9001:2015 and API Q1, produces 904L bars through controlled electric arc furnace (EAF) melting with AOD refining, precisely calibrating the copper and molybdenum additions to ensure consistent acid corrosion resistance verified by in-house OES on every heat.

904L Stainless Steel Bar — Overview

The defining performance characteristic of 904L is its broad-spectrum resistance to reducing acids, particularly sulfuric acid (H₂SO₄) across concentrations from dilute to approximately 80% at ambient temperatures, and phosphoric and acetic acid environments commonly encountered in petrochemical processing. The combination of high nickel (reducing the susceptibility to chloride stress corrosion cracking) and copper (passivating the surface in reducing acid conditions) makes 904L the industry-standard material for alkylation unit components, acid coolers, sulfuric acid storage and transfer systems, heat exchangers in refinery process units, instrumentation in sour gas service, and chemical injection hardware throughout the oil and gas value chain. Unlike precipitation hardening grades, 904L is supplied in the solution-annealed condition (1070–1150°C, water quenched), delivering a fully austenitic microstructure with low strength but excellent ductility (35% minimum elongation) and formability. SHUNFU METAL supplies 904L bars with full MTC documentation per EN 10204 Type 3.1, including intergranular corrosion testing per ASTM A262 Practice C when specified for critical service applications.

904L Equivalent Grades — International Standards

Standard Designation
ASTM A479 / A276 / A240 / A182 UNS N08904 (F904L forging grade)
EN 10088-3 / DIN 1.4539 / X1NiCrMoCu25-20-5
JIS G4303 SUS904L
GOST 5632 01Х21Н26М5Д
GB/T 1220 015Cr21Ni26Mo5Cu2 (new) / 00Cr20Ni25Mo4.5Cu (old)
AFNOR (France) Z2NCDU 25-20
BS (UK) 904S13, 904S14

904L enjoys the broadest international standardization of any high-alloy austenitic stainless steel. SHUNFU METAL supplies bars certified to ASTM A479 UNS N08904, EN 10088-3 Grade 1.4539, JIS G4303 SUS904L, and GB/T 1220, with full heat traceability and API Q1 documentation as standard.

904L high-alloy austenitic stainless steel round bar by SHUNFU METAL
904L (UNS N08904) Stainless Steel Bar
SHUNFU METAL

904L Applications — Oil & Gas Industry

904L is specified wherever sulfuric acid, phosphoric acid, or mixed reducing-acid environments preclude the use of standard 316L, yet the extreme conditions requiring 254SMO or nickel-based alloys are not present. SHUNFU METAL supplies 904L bars to manufacturers serving the following petroleum industry applications, with every production lot verified against ASTM A479 and related specifications:

Alkylation Unit ComponentsAcid cooler tubesheets, reactor internals, and acid transfer pump shafts in sulfuric acid alkylation units.
Sulfuric Acid Heat ExchangersShell-and-tube and plate exchanger components handling concentrated H₂SO₄ at refineries and gas plants.
Acid Storage & TransferTank nozzle flanges, valve bodies, and pump shafts in sulfuric and phosphoric acid storage and offloading systems.
Sour Gas InstrumentationValve stems, orifice plates, and pressure transmitter parts in wet H₂S and CO₂-containing gas streams.
Chemical Injection HardwareInjection quills, nozzles, and check valves for concentrated acid-based stimulation and well treatment chemicals.
Refinery Process PipingHigh-nickel-barrier piping components, flanges, and fittings in crude distillation and hydroprocessing units.
Pickling & Cleaning EquipmentTank heater coils, agitator shafts, and pump components for acid-based cleaning and passivation systems.
Phosphoric Acid ServiceConcentration evaporator components, slurry pump parts, and filter press hardware in phosphoric acid plants.
FPSO Process EquipmentTopside process vessel nozzles, agitator components, and instrument standoffs in floating production units.

Supply Range — 904L Bar Dimensions

SHUNFU METAL maintains comprehensive production capability for 904L bars across hot-rolled, cold-finished, and forged product forms, processed through ISO 9001:2015 controlled workflows. The high nickel and copper content of 904L requires careful control of solution annealing temperature (1070–1150°C) to avoid incipient melting of copper-rich phases while ensuring complete carbide dissolution. SHUNFU METAL’s metallurgical team verifies each heat treatment batch by microstructural examination and intergranular corrosion testing. The following dimensional ranges are offered as standard:

Product Form Diameter / Thickness Range Width Range Length (Standard) Surface Finish
Hot Rolled Round 10 mm – 300 mm — 3000 – 6000 mm Black / Pickled
Cold Finished / Peeled Round 3 mm – 100 mm — 2000 – 6000 mm Peeled / Polished
Hot Rolled Flat 10 mm – 120 mm 50 mm – 400 mm 2000 – 6000 mm Pickled
Forged Round 100 mm – 800 mm — 1000 – 6000 mm Rough Machined / Black
Forged Flat / Block 50 mm – 350 mm 100 mm – 800 mm 1000 – 5000 mm Rough Machined / Black

All bars are supplied with EN 10204 Type 3.1 Mill Test Certificates documenting heat number, full chemical analysis (including copper and molybdenum), mechanical test results, solution annealing verification, and dimensional compliance. Intergranular corrosion testing per ASTM A262 Practice C (Huey test) and Practice E (Streicher test) is available for acid service qualification requirements.

904L Chemical Composition per ASTM A479

The 904L chemistry is defined by its high nickel (23.0–28.0%), deliberate copper addition (1.0–2.0%), and elevated molybdenum (4.0–5.0%) — a combination engineered specifically for broad-spectrum acid resistance. The 1.0–2.0% copper content is the key differentiating alloy element that enhances the alloy’s passivation behavior in non-oxidizing (reducing) acids, particularly sulfuric acid, by catalyzing the cathodic reduction reaction that stabilizes the protective chromium oxide film. At SHUNFU METAL, every heat is verified by in-house optical emission spectrometry (OES) against ASTM A479 UNS N08904 limits:

Element C Mn Si P S Cr Ni Mo Cu
Wt % 0.020 max 2.00 max 1.00 max 0.045 max 0.030 max 19.00–23.00 23.00–28.00 4.00–5.00 1.00–2.00

Composition conforms to ASTM A479 UNS N08904 and EN 10088-3 Grade 1.4539. The PREN calculated as %Cr + 3.3(%Mo) = 21 + 3.3(4.5) ≈ 35.9 places 904L in the upper range of standard austenitic grades — substantially above 316L (PREN ~24) and 317L (PREN ~28), though below the super-austenitic category (PREN > 40). The alloy’s true performance advantage lies in its resistance to reducing-acid environments rather than neutral chloride pitting, for which 254SMO or 654SMO are preferred. SHUNFU METAL’s API Q1 certified traceability links every finished bar to its original heat analysis, with copper verification included on every MTC.

904L Mechanical Properties — Solution Annealed Condition

904L achieves its mechanical properties through solid solution strengthening from its high nickel, chromium, and molybdenum content. The alloy’s yield strength (220 MPa minimum) is modest compared to nitrogen-enhanced super-austenitic grades, reflecting its primary design objective as a corrosion-resistant rather than high-strength material. SHUNFU METAL performs in-house tensile testing (ASTM E8) and hardness testing (ASTM E18) on every production lot, with full results documented on the MTC. The following values represent minimum guaranteed properties per ASTM A479 for bar products in the solution-annealed condition:

Property 904L (N08904) 316L (S31603) Test Method
Tensile Strength (MPa / ksi) 490 min / 71 485 min / 70 ASTM E8
Yield Strength 0.2% (MPa / ksi) 220 min / 31 170 min / 25 ASTM E8
Elongation in 4D (%) 35 min 30 min ASTM E8
Reduction of Area (%) 50 min — ASTM E8
Hardness (HB / HRB) HB 212 max / HRB 90 max HB 217 max ASTM E10 / E18

Values per ASTM A479 for bar products in the solution-annealed condition (1070–1150°C, water quenched). 904L delivers approximately 30% higher yield strength than 316L (220 vs 170 MPa) while providing substantially better resistance to reducing-acid corrosion environments. Its mechanical properties are comparable to standard austenitic grades — 904L is specified for corrosion resistance, not structural strength — and design engineers should account for its relatively low yield strength when dimensioning pressure-containing components.

Service temperature range: 904L retains excellent toughness at cryogenic temperatures and resists sigma phase embrittlement up to approximately 400°C in continuous service, though prolonged exposure above 300°C is generally avoided in practice to maintain optimal corrosion resistance. SHUNFU METAL’s ISO 9001:2015 accredited laboratory validates all mechanical properties on representative test specimens from each heat treatment batch, with API Q1 ensuring full material traceability from melt to end user.