904L vs 316L – Composition, Heat Treatment, Properties, and Applications

Introduction

904L and 316L are two widely used austenitic stainless steels that frequently compete in material selection for chemical, marine, and pharmaceutical process equipment. Engineers and procurement managers often weigh corrosion resistance, weldability, and life‑cycle cost when choosing between them—balancing the need for superior performance in aggressive environments against budget and supply constraints. In broad terms, 904L is a higher‑alloyed, corrosion‑resistant austenitic grade engineered for aggressive chloride‑ and acid‑containing services, while 316L is a widely used “workhorse” low‑carbon austenitic stainless steel offering good general corrosion resistance, formability, and economical availability. These differences explain why they are commonly compared in design and manufacturing decisions.

1. Standards and Designations

Major standards and common designations for each grade:

• 316L
• ASTM/ASME: UNS S31603, ASTM A240 (plate/sheet), ASTM A276 (bars), ASTM A479 (forged/pipe), etc.
• EN: 1.4404 (also commonly referenced as X2CrNiMo17‑12‑2)
• JIS: SUS316L
• GB: 0Cr17Ni12Mo2 (approximate Chinese designation)

• Classification: Austenitic stainless steel

• 904L

• ASTM/ASME: UNS N08904
• EN: 1.4539 (sometimes referenced)
• JIS: Not commonly used as a JIS specific grade; often specified by UNS
• GB: Equivalent families available under different designations
• Classification: Super‑austenitic stainless steel (high alloy austenitic)

Both are stainless (austenitic) steels; neither is considered carbon, tool, or HSLA in the traditional sense.

2. Chemical Composition and Alloying Strategy

The table below lists typical composition ranges (wt%) for common commercial 316L and 904L. Values are representative ranges used in industry specifications; final selection should reference the specific standard or mill certificate.

How alloying affects performance - Chromium provides general corrosion resistance and oxide film stability (higher Cr raises baseline pitting resistance). - Nickel stabilizes the austenitic phase, improves toughness and ductility, and enhances resistance to chloride stress‑corrosion cracking in many contexts. - Molybdenum increases resistance to pitting and crevice corrosion in chloride environments. - Copper in 904L improves resistance to reducing acids (e.g., sulfuric acid) and enhances stability in certain corrosive media. - Low carbon (the “L” designation) reduces sensitization during welding and limits intergranular corrosion.

3. Microstructure and Heat Treatment Response

Both 316L and 904L are essentially fully austenitic (face‑centered cubic) in the annealed condition. Key microstructural and heat treatment behaviors:

• Typical microstructure
• 316L: Stable austenite with possible delta ferrite in welds or cold worked zones if not properly controlled. Grain size and twin formation occur depending on thermo‑mechanical history.

• 904L: Also fully austenitic but with higher Ni and Mo/Cu content that stabilizes austenite and reduces tendency toward ferrite or martensite formation. 904L shows robust resistance to carbide precipitation (sensitization).

• Heat treatment and processing

• Both grades are non‑hardening by conventional quench & temper routes. Annealing (solution treatment) is the standard process to restore ductility and corrosion resistance: typically solution anneal at approximately 1,040–1,120 °C followed by rapid cooling (water quench) to retain austenitic structure and dissolve carbides.
• Thermo‑mechanical processing (cold working) increases strength by strain hardening. Neither grade can be strengthened significantly by conventional heat treatment; strengthening is achieved by work hardening or by cold forming followed by possible stabilization (e.g., for 316Ti).
• Prolonged exposure in the 450–870 °C range can promote sigma phase or chromium carbide precipitation in highly alloyed austenitics; 904L's elevated Ni and Cu reduce but do not eliminate risks at extreme service exposures.

4. Mechanical Properties

Typical mechanical properties in the annealed condition depend on product form (sheet, plate, bar) and specific standard. The table gives representative annealed values commonly used for design comparison; consult material certificates for project data.

Interpretation - Strength: Both grades show broadly similar tensile properties in the annealed state; cold work raises strength for either. 904L can exhibit slightly higher strength in some product forms due to alloying, but differences are often small relative to design margins. - Toughness and ductility: Both are highly ductile and tough at ambient temperature due to fully austenitic structure. 904L's high nickel content generally improves low‑temperature toughness and reduces susceptibility to embrittlement. - The mechanical properties are primarily controlled by work hardening and cold‑forming rather than heat treatment; design should reference the specific material certificate.

5. Weldability

Weldability considerations include carbon content, alloying, and hardenability indices.

• Both 316L and 904L have low carbon levels (the “L” grades), which reduces risk of sensitization and intergranular corrosion after welding and improves weldability.
• Use of standard austenitic filler metals (e.g., matching or higher nickel, Ni‑based where required) and appropriate welding procedures is customary. Post‑weld solution anneal is rarely required for standard applications but may be used for critical corrosion resistance.
• Weldability indices (to guide qualitative comparison):
• Carbon equivalent (IIW form): $$ CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} $$
• Pitting corrosion‑resistance equivalent for weldability sensitivity: $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$
• Qualitative interpretation
• 316L: Good weldability, widely used and well understood. Low carbon minimizes sensitization; filler metal selection is straightforward.
• 904L: Also weldable but requires attention. Higher Mo and Cr increase CE and Pcm terms relative to 316L; higher Ni mitigates hardenability but can affect hot cracking susceptibility if improper filler or parameters are used. 904L often requires matching filler with sufficient nickel and may be more sensitive to heat input and dilution control. Preheating is generally not required; control of interpass temperatures and cleaning is important.

6. Corrosion and Surface Protection

• For non‑stainless steels: standard protections are galvanizing, painting, cathodic protection, and coatings. (Not applicable to these two stainless grades which rely on passive films.)

• Stainless corrosion indices

• Pitting Resistance Equivalent Number (PREN) is commonly used to compare pitting resistance: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$
• Approximate PREN (using representative mid‑range compositions):
  • 316L: with Cr ≈ 17% and Mo ≈ 2.5% → PREN ≈ 17 + 3.3×2.5 ≈ 25.25
  • 904L: with Cr ≈ 20% and Mo ≈ 4.5% → PREN ≈ 20 + 3.3×4.5 ≈ 34.85

• Interpretation: Higher PREN indicates greater resistance to localized attack (pitting/crevice) in chloride environments. 904L’s significantly higher PREN and the addition of copper confer superior resistance in many aggressive chloride and reducing acid environments (e.g., sulfuric acid), while 316L provides good general pitting resistance for moderate chloride exposures.

• When PREN is not applicable

• PREN is a simplified index focused on pitting; it does not capture behavior in strongly oxidizing environments, general corrosion in sulfuric acid, or stress‑corrosion cracking susceptibility fully. Real application assessment requires testing or experienced corrosion engineering judgment.

7. Fabrication, Machinability, and Formability

• Machinability
• 316L: Fair to poor machinability relative to carbon steels; work hardening during cutting requires rigid tooling and appropriate feeds. Performance improves with coated carbide tooling and optimized parameters.
• 904L: Generally more difficult to machine than 316L due to higher nickel content and increased work hardening; tooling wear and cutting forces are higher. Nickel‑rich alloys often require lower cutting speeds and robust tooling.
• Formability and bending
• Both grades have good formability in annealed condition; 316L is widely used for deep drawing and complex shapes. 904L can be formed but may require increased forming force and attention to springback; annealing after heavy forming is commonly used.
• Surface finishing
• Polishing and passivation are effective for both; 904L may require slightly different pickling/cleaning chemistry due to Cu and Mo content. Proper pickling and passivation restore the passive film and optimize corrosion performance.

8. Typical Applications

Selection rationale - 316L is selected for cost‑sensitive projects requiring reliable general corrosion resistance, good formability, and broad availability. - 904L is chosen when the service includes aggressive chloride environments, high concentrations of reducing acids, or when extended life with minimal corrosion maintenance is critical despite higher first‑cost.

9. Cost and Availability

• Cost
• 904L typically commands a significant premium over 316L due to higher nickel and molybdenum content and the inclusion of copper. Price differentials can be substantial and fluctuate with alloying metal markets.
• Availability
• 316L: Extremely common in mills worldwide in many product forms (sheet, plate, pipe, tube, bar, forgings).
• 904L: Readily available from specialty suppliers and larger mills, but range of product forms and lead times may be more limited than 316L; custom fabrication and procurement planning are often necessary for large volumes.

10. Summary and Recommendation

Summary table (qualitative)

Recommendations - Choose 316L if: - The application requires reliable general corrosion resistance in moderate chloride or atmospheric environments, combined with good formability and economical cost. - Supply chain simplicity and broad material and fabrication options are priorities. - The system does not face prolonged exposure to high chloride levels, strong reducing acids, or environments that demand very high PREN.

• Choose 904L if:
• The service environment includes aggressive chlorides, sulfuric or other reducing acids, or conditions prone to severe pitting and crevice corrosion.
• Long service life with minimal corrosion maintenance is a priority and the project can justify higher material cost.
• Welding and fabrication expertise are available to control procedures and specify appropriate filler metals.

Final note: Material selection must consider the whole system—temperature, chloride concentration, flow, crevices, stress state, fabrication method, and life‑cycle cost. Laboratory immersion tests, electrochemical data, or field experience with the specific process fluid are recommended for critical services.