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

Introduction

Selecting between 316L and 904L is a common materials decision for engineers, procurement managers, and manufacturing planners who must balance corrosion performance, fabrication cost, and service requirements. Typical decision contexts include chemical processing, marine and offshore environments, and equipment that requires high fabricability and reliable corrosion resistance.

The fundamental difference between the two alloys lies in their alloying strategy: 316L is a molybdenum-bearing, low-carbon austenitic stainless optimized for general-purpose corrosion resistance and weldability; 904L is a high‑nickel, high‑molybdenum, copper‑bearing austenitic stainless designed for superior resistance to highly corrosive acid and chloride environments. This divergence in chemistry drives the differences in corrosion performance, cost, and fabrication considerations.

1. Standards and Designations

• 316L
• Common designations: UNS S31603, EN 1.4404 / 1.4435, JIS SUS316L, ASTM A240 (for plate), ASTM A276 (bar)
• Classification: Austenitic stainless steel
• 904L
• Common designations: UNS N08904, EN (sometimes referenced as 1.4539 variants), ASTM B702/B574 for some product forms
• Classification: Austenitic stainless steel with high Ni and Mo, sometimes called "super austenitic"

Both are stainless steels (not carbon steels, tool steels, or HSLA), widely specified by ASTM/ASME, EN, JIS, and national standards for various product forms (plate, sheet, bar, pipe).

2. Chemical Composition and Alloying Strategy

The following table gives typical composition ranges for wrought, solution‑annealed material used in industry. Values are shown as weight percent and are representative ranges found in common specifications.

How alloying affects properties: - Chromium provides the passive oxide film that imparts basic stainless behavior; more Cr generally improves general corrosion and oxidation resistance. - Nickel stabilizes the austenitic phase, increases toughness and ductility, and enhances chloride stress‑corrosion cracking resistance when combined with other elements. - Molybdenum significantly improves pitting and crevice corrosion resistance in chloride-containing environments. - Copper in 904L improves resistance to reducing acids (e.g., sulfuric acid) and enhances crevice corrosion resistance in certain media. - Low carbon minimizes carbide precipitation during welding, preserving intergranular corrosion resistance.

3. Microstructure and Heat Treatment Response

• Microstructure:
• Both 316L and 904L are fully austenitic in the solution‑annealed condition. They do not transform to ferrite or martensite on cooling under normal processing.
• Heat treatment and response:
• Typical treatment: solution anneal at approximately 1010–1120 °C (depending on standard), followed by water quench to restore a homogeneous austenitic microstructure and dissolve precipitates.
• Neither grade is hardenable by quenching and tempering—strength changes are achieved primarily by cold work.
• 316L: Care in weld thermal cycles is required to avoid sensitization in higher‑carbon variants, but the L-grade (low carbon) minimizes carbide precipitation. Stabilizing alloys (e.g., 316Ti) are used where service includes extended exposure to sensitizing temperatures.
• 904L: Also solution-annealed; higher alloy content (Ni, Mo, Cu) means it is more resistant to sensitization and has higher resistance to intergranular attack after welding, but proper filler selection and heat‑input control are still important to retain corrosion performance.
• Thermo‑mechanical processing:
• Cold work increases yield and tensile strength in both grades but reduces corrosion resistance in some environments if the passive film is damaged and not restored.

4. Mechanical Properties

Typical mechanical properties for annealed, wrought material (values approximate; check product standards for exact guarantees):

Interpretation: - 904L typically exhibits higher nominal strength than 316L in the annealed condition because of higher alloy content and solid solution strengthening from Ni and Mo. - Both are tough and ductile at ambient temperatures; 316L commonly shows slightly higher elongation in some product forms. - Neither grade is selected primarily for high hardness or wear resistance; cold work increases strength and hardness but can reduce formability.

5. Weldability

Weldability for both grades is generally good compared with ferritic or martensitic stainless steels, but there are practical differences.

• 316L: Excellent weldability due to low carbon; low risk of sensitization and intergranular corrosion. Common fillers: ER316/ER316L. Post‑weld annealing is usually not required for general service.
• 904L: Weldable but requires care: higher alloy content (Ni, Mo, Cu) changes solidification behavior and can affect filler selection and hot‑cracking tendency. Matching filler metals produced for 904L or other high‑Ni fillers are often used. Control of heat input and pre/post‑weld practices preserves corrosion resistance.

Useful weldability indices (no numeric inputs required here): - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Interpreting $CE_{IIW}$ qualitatively: higher values imply increased hardenability and potential for welding cracking in steels. For austenitic stainless steels, low carbon and high Ni lower cracking risk despite higher alloy content. - Pcm (Siewert) parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ - $P_{cm}$ can be used qualitatively to assess susceptibility to cold cracking in welds; for these austenitic alloys the dominant concerns are hot cracking and preserving corrosion resistance rather than martensitic cold-cracking.

Practical guidance: prequalification welding procedure specifications and matching filler metals should be used for 904L, especially for critical pressure‑containing or corrosive service.

6. Corrosion and Surface Protection

• Stainless behavior:
• Both grades rely on a Cr‑rich passive oxide for general corrosion resistance.
• Pitting and crevice resistance:
• The pitting resistance equivalent number (PREN) is often used to compare localized corrosion resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Using typical nominal values as an illustrative example (approximate):
  • 316L (Cr ≈ 17, Mo ≈ 2.2, N ≈ trace): PREN ≈ 17 + 3.3×2.2 + 16×0.02 ≈ 25
  • 904L (Cr ≈ 20.5, Mo ≈ 4.5, N ≈ trace): PREN ≈ 20.5 + 3.3×4.5 + 16×0.02 ≈ 36
• Interpretation: 904L’s higher Mo and Cr produce a substantially higher PREN and thus markedly better resistance to pitting and crevice corrosion in chloride-bearing and oxidizing environments.
• Specific media:
• 316L: Good general corrosion resistance; chosen for seawater splash zones, food processing, pharmaceuticals, and many chemical services with moderate halide concentrations.
• 904L: Superior resistance to strong oxidizing acids, chloride containing environments at higher temperatures, and environments containing sulfuric acid (where Cu is advantageous).
• Non-stainless alternatives:
• For non-stainless steels, corrosion protection is provided by coatings (galvanizing, painting, polymer linings, etc.). These are not directly relevant to 316L/904L comparisons except when considering cost or design substitution.

7. Fabrication, Machinability, and Formability

• Machinability:
• 316L is easier to machine than many higher‑alloy austenitics; it work‑hardens moderately and responds well to sharp tooling and rigid setups.
• 904L machines more slowly, tends to work‑harden, and requires more robust tooling and coolant management. The higher Ni and Mo increase tool wear and demand lower cutting speeds and higher feeds.
• Formability:
• Both grades form well in the annealed condition; 316L is commonly used for deep drawing, bending, and stamping operations.
• 904L is ductile and formable but its higher strength and work‑hardening rate increase forming loads and springback; more intermediate annealing or higher forming forces may be required.
• Surface finishing:
• Both polish and passivate well; 904L may require more aggressive surface preparation to achieve the same surface reflectivity due to alloying differences.

8. Typical Applications

Selection rationale: - Choose 316L when the service environment has moderate chloride content and the priority is good overall corrosion resistance and weldability at a lower cost. - Choose 904L when the environment involves strong oxidants, higher chloride levels, sulfuric or mixed acids, or when extended life in aggressive media offsets the higher alloy cost.

9. Cost and Availability

• Cost:
• 904L is significantly more expensive than 316L on a material cost basis because of the high nickel and molybdenum contents, and copper addition. Price sensitivity is driven primarily by Ni and Mo market prices.
• Availability:
• 316L is widely available in nearly all product forms (sheet, plate, pipe, tube, forgings, bar) and in many global markets.
• 904L is commercially available in plate, pipe, tube, and some bar and fittings, but certain product forms or custom dimensions may require lead time or special production runs. Procurement planning should allow for longer lead times and qualified suppliers for 904L.

10. Summary and Recommendation

Recommendation: - Choose 316L if your application demands a cost-effective, broadly weldable austenitic stainless with reliable general corrosion resistance, good formability, and wide availability (e.g., food, pharmaceutical, general chemical service, moderate marine exposure). - Choose 904L if the service includes aggressive chloride environments, oxidizing acids, or conditions that require exceptional pitting/crevice resistance and longer life despite higher material and fabrication cost (e.g., specialized chemical process pipelines, heat exchangers in severe chemistries).

Closing note: final grade selection should be driven by a holistic assessment: specific corrosive species and concentration, temperature, mechanical loading, fabrication route, weld procedure specifications, and total life‑cycle cost. For critical or unfamiliar services, perform corrosion testing, consult corrosion specialists, and qualify weld procedures before production.