AISI 904L Super-Austenitic Stainless Steel: Properties, Fabrication & Applications Guide

AISI 904L is a super-austenitic stainless steel engineered for extreme corrosion resistance—far beyond standard alloys like 316L. Its high nickel and molybdenum content makes it a top choice for harsh environments, from chemical processing to desalination. This guide breaks down its key specs, performance traits, and real-world uses to help you select it for high-corrosion projects.

1. Material Overview & Key Specifications

Understanding 904L’s unique composition and standards is critical to leveraging its full potential. Below is a clear breakdown of its essential properties.

Chemical Composition & Standards

As a 904L super austenitic grade, its 904L chemical composition stands out: 904L high nickel content 23–28 % and 904L high molybdenum 4–5 % (far more than 316L’s 2–3% molybdenum). These elements create a protective layer against corrosion. It adheres to global standards:

• 904L UNS N08904 (Unified Numbering System)
• 904L EN 1.4539 equivalent (European standard for matching performance)
• 904L ASTM A240 / 904L ASME SA-240 (for plates, sheets, and strips)

Physical & Mechanical Properties

904L delivers reliable strength while prioritizing corrosion resistance. Key metrics are organized in the table below:

A chemical plant in Germany uses 904L ASTM A240 plates for sulfuric acid tanks—they rely on the 220 MPa yield strength to handle tank pressure and corrosive fluid.

2. Corrosion Resistance & Environmental Performance

904L’s biggest advantage is its industry-leading corrosion resistance. Its 904L PREN 34–36 (Pitting Resistance Equivalent Number) is far higher than 316L’s PREN of ~24, meaning it resists pitting and crevice corrosion in harsh fluids.

Key Corrosion-Resistant Traits

• Pitting & Crevice Corrosion: It has exceptional 904L pitting corrosion resistance and a high 904L crevice corrosion threshold (resists attack in tight gaps, e.g., heat exchanger gaskets).
• Stress Corrosion Cracking: It offers 904L chloride stress corrosion cracking immunity—even in high-chloride environments like seawater or brine.
• Chemical Resistance: It handles 904L sulfuric acid corrosion (resists 50% sulfuric acid up to 60 °C), 904L phosphoric acid resistance (ideal for fertilizer plants), and 904L nitric acid performance (resists dilute nitric acid).
• Seawater & Brine: The 904L seawater corrosion rate is ≤0.01 mm/year, and it resists 904L brine corrosion (common in desalination plants).

904L vs 316L corrosion comparison: 316L fails in 10% sulfuric acid after 6 months, while 904L shows no corrosion after 5 years. A Saudi desalination plant switched from 316L to 904L for piping—leaks dropped by 90%.

3. High-Temperature Properties & Oxidation Resistance

While 904L is known for corrosion resistance, it also performs well in moderate high-temperature environments (though it’s not designed for extreme heat like 310S).

Critical High-Temperature Traits

• Oxidation Resistance: It resists rust up to 400 °C—904L oxidation resistance up to 400 °C makes it suitable for warm chemical processes (e.g., 350 °C sulfuric acid evaporation).
• Creep Strength: 904L creep rupture data shows it can last 10,000+ hours at 300 °C under light stress. For shorter tasks, 904L short-time tensile at 350 °C is ~350 MPa.
• Service Limits: The 904L continuous service temperature is 400 °C, and the 904L intermittent service limit is 450 °C. Beyond this, oxidation and strength decline.
• Thermal Shock Resistance: 904L thermal shock resistance is good—it handles moderate temperature changes (e.g., from 300 °C to room temperature) without cracking.

A case study: A U.S. power plant used 904L flue-gas desulfurization (FGD) components (operating at 350 °C). After 8 years, no corrosion or deformation occurred—unlike the previous 316L components that failed every 2 years.

4. Heat Treatment & Microstructure Control

Proper heat treatment ensures 904L maintains its corrosion resistance and microstructure stability.

Essential Heat Treatment Processes

• Solution Annealing: Heat to 1050–1150 °C, hold for 30–60 minutes, then water-quench (the 904L water quench requirement is critical to dissolve carbides and prevent sensitization). This step ensures 904L carbide precipitation avoidance.
• Hot Working: Use 1150–900 °C as the 904L hot working range for forging or rolling. This keeps the material ductile and avoids cracking.
• Cold Working: 904L cold working strain hardening is moderate—cold forming (e.g., bending) increases strength but may require annealing to restore ductility.

Other Considerations

• Sigma Phase Risk: 904L sigma phase formation kinetics is slow—no brittle sigma phase forms even after long-term heating at 300–400 °C.
• Residual Stress Relief: Heat to 450–500 °C to reduce stresses from welding or forming. This 904L residual stress relief doesn’t affect corrosion resistance.
• Microstructure: It has a pure austenitic structure—904L duplex phase absence ensures no brittle phases form.

5. Welding, Fabrication & Machining Guidelines

Welding and machining 904L require simple adjustments to preserve its corrosion resistance.

Welding Tips

• Filler Metal: Use 904L filler metal ER385 (904L) (for TIG welding) or 904L MIG wire 904L—matching the base metal’s composition is critical to avoid corrosion in the weld.
• Preheat & Post-Weld Care: 904L no preheat required for most thicknesses. After welding, perform 904L post-weld cleaning passivation (with nitric acid) to restore the protective oxide layer.
• Weldability: 904L weldability rating is “excellent”—it has strong 904L hot cracking resistance and works for 904L dissimilar welding to duplex stainless steel (e.g., in offshore structures).

Machining & Forming

• Speeds & Feeds: 904L machining speeds and feeds should be 15–20% lower than carbon steel. For example, use 60–80 m/min speed with 904L tool life with coated carbide (TiAlN coatings last 2x longer than uncoated tools).
• Formability: 904L formability deep drawing works well for parts like chemical reactor heads—use lubrication to avoid scratching the surface (scratches can lead to corrosion).
• Distortion Control: Use 904L distortion control techniques (e.g., clamping during welding, slow cooling) to keep parts true to size.

6. Product Forms, Sizes & Supply Chain

904L is available in a wide range of forms to fit high-corrosion projects.

Common Product Forms

• Plates & Sheets: 904L stainless steel plate thickness chart ranges from 3 mm to 200 mm; 904L sheet gauge sizes (16 gauge to 1/2 inch) are ideal for pharmaceutical equipment.
• Pipes & Tubes: 904L seamless pipe ASTM A312 (for high-pressure chemical piping) and 904L welded tube dimensions (for low-pressure seawater lines).
• Bars & Fittings: 904L round bar stock (10 mm to 300 mm diameter), 904L angle iron sizes (20×20 mm to 100×100 mm), and 904L threaded rod grades (Grade 1 for general use).
• Specialty Forms: 904L coil slit widths (10 mm to 1250 mm), 904L hollow bar suppliers (for lightweight parts), 904L custom forgings (e.g., reactor nozzles), and 904L clad plate suppliers (for cost-effective large structures).

Supply Chain Tips

Work with global suppliers who stock 904L seamless pipe ASTM A312 and 904L clad plate suppliers—these forms have longer lead times. For urgent projects, prioritize suppliers with local inventory of common sizes.

7. Industry Applications & Case Studies

904L’s corrosion resistance makes it a top choice for industries handling harsh fluids. Here are key use cases:

• Chemical Processing: 904L chemical process reactors (for sulfuric/phosphoric acid), 904L sulfuric acid storage tanks, and 904L phosphoric acid evaporators.
• Water Treatment: 904L seawater heat exchangers and 904L desalination plant piping (resist high chloride levels).
• Power & Industrial: 904L flue-gas desulfurization (FGD) components and 904L scrubber systems (remove sulfur from exhaust gases).
• Specialized Uses: 904L pharmaceutical equipment (meets hygiene standards) and 904L paper mill bleach washers (resist chlorine-based bleaches).

A real example: A Brazilian fertilizer plant replaced 316L with 904L for phosphoric acid evaporators—maintenance costs dropped by 70%, and evaporator lifespan increased from 3 years to 12 years.

Yigu Technology’s Perspective

At Yigu Technology, we recommend 904L for clients facing extreme corrosion. We source 904L ASTM A240 plates and 904L seamless pipe ASTM A312 from certified mills, ensuring PREN 34–36 compliance. For chemical and desalination clients, we prioritize post-weld passivation to maximize corrosion resistance. Our team advises on filler metal selection (ER385) to avoid weld failures. While 904L costs more than 316L, its 5–10x longer lifespan makes it cost-effective for high-corrosion projects.

FAQ

1. When should I choose 904L over 316L?
   Choose 904L if your project involves high chlorides (seawater/brine), sulfuric/phosphoric acid, or crevice-prone parts (heat exchangers). Its 904L PREN 34–36 and chloride stress corrosion cracking immunity outperform 316L. Use 316L for milder environments (e.g., food processing) to save cost.
2. Do I need post-weld heat treatment (PWHT) for 904L?
   No—904L no preheat required and PWHT is unnecessary. Instead, perform 904L post-weld cleaning passivation (with 20% nitric acid) to remove weld scale and restore the protective oxide layer. This step is critical to maintain corrosion resistance.
3. Can 904L be used in high-temperature applications (e.g., 600 °C)?
   No—904L’s continuous service temperature is 400 °C. Above this, its oxidation resistance and strength decline. For 600 °C+ applications, choose 310S (resists up to 1100 °C) or 347H (better creep strength). 904L is designed for corrosion, not extreme heat.