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

Introduction

Engineers, procurement managers, and manufacturing planners routinely face a trade-off between corrosion resistance, mechanical performance, manufacturability, and cost when selecting austenitic stainless steels. 316L and 904L are both austenitic stainless grades used where corrosion resistance is required, but each occupies a different point on the performance–cost envelope: 316L is the economical, widely available “workhorse” for chloride-containing environments, while 904L is a highly alloyed, higher-cost option optimized for aggressive acidic and chloride-bearing chemistries.

The key practical difference is alloy strategy: 316L relies primarily on chromium, nickel, and molybdenum for general corrosion resistance and weldability; 904L increases nickel, molybdenum and adds copper to extend resistance to reducing acids and localized attack. This makes the two grades frequent comparators in design and procurement decisions where increased corrosion resistance must be justified against higher material and processing costs.

1. Standards and Designations

Major standards and common identifiers used to specify these grades include: - 316L - UNS: S31603 - Common ASTM/ASME specifications for sheet/plate and bar: e.g., ASTM A240 (plate/sheet), ASTM A276 (bars), and related piping specifications - EN: commonly referenced as EN 1.4404 - JIS: commonly referred to as SUS316L - GB: equivalents are available in Chinese standards (specified by chemical composition) - Classification: Austenitic stainless steel (stainless) - 904L - UNS: N08904 - Commonly supplied to ASTM/ASME documents where appropriate (e.g., A240 for plate when specified by UNS) - EN: commonly referenced as EN 1.4539 (used as a general industry equivalence) - JIS/GB: available as special alloys or equivalent compositions - Classification: Austenitic, high-alloy nickel-bearing stainless steel (stainless)

Both are stainless steels (austenitic family), not carbon steels, tool steels, or HSLA. They are specified across standards for different product forms (plate, pipe, bar, wire, fittings) according to application.

2. Chemical Composition and Alloying Strategy

The following table shows typical compositional ranges or limits found in common specifications. These are representative ranges used in selection and standards — always confirm with the specific material certificate for procurement.

How alloying affects performance: - Chromium (Cr): provides the passive oxide film and general corrosion resistance. - Nickel (Ni): stabilizes the austenitic microstructure and improves ductility and toughness; higher Ni also improves resistance to chloride stress corrosion cracking and reduces magnetic permeability. - Molybdenum (Mo): enhances resistance to pitting and crevice corrosion in chloride-containing environments. - Copper (Cu) in 904L: improves resistance to sulfuric and other reducing acids and enhances local corrosion resistance in some acidic chloride media. - Low carbon (L grades): limits sensitization and intergranular corrosion after welding.

3. Microstructure and Heat Treatment Response

Both 316L and 904L are fully austenitic in the annealed condition at ambient temperatures. Key microstructural and heat-treatment points: - Typical microstructure: single-phase face-centered cubic (FCC) austenite with possible small amounts of carbides or intermetallics if exposed to sensitizing temperatures. - Solution annealing / recrystallization: Both grades are commonly solution-annealed (typical solution anneal temperatures for austenitic stainless steels are in the range 1,040–1,120 °C depending on specification) and rapidly cooled to retain a homogeneous austenitic structure and dissolve chromium carbides. - Sensitization: Low carbon (316L, 904L) reduces susceptibility to chromium carbide precipitation at grain boundaries (sensitization) compared with higher-carbon variants; therefore both are preferred for welded structures requiring corrosion resistance. - Precipitation and sigma phase: Extended exposure in the 500–900 °C range can promote intermetallic phases (e.g., sigma) that embrittle the alloy and reduce corrosion resistance. High nickel in 904L tends to shift precipitation behavior, but anti-sigma processing windows and solution annealing practices should be followed. - Hardening: Neither grade is hardenable by conventional quench-and-temper treatment because they are austenitic; strength is adjusted primarily by cold work (work hardening) or by using heavier section thickness/product form.

4. Mechanical Properties

The following table provides qualitative comparative traits typically seen in the annealed condition (specific values depend on product form, heat treatment, and supplier certifications).

Explanation: 904L’s higher nickel and molybdenum (and copper) lead to higher solid-solution strengthening and often slightly higher yield and tensile strengths in the annealed condition compared with 316L. Both alloys retain substantial ductility and toughness; 316L is widely used where extensive forming or deep drawing is required due to its well-understood forming characteristics.

5. Weldability

Weldability considerations for austenitic stainless steels depend on carbon, nitrogen, and alloy content, plus thermal cycles. - Carbon level: Low carbon (316L, 904L) reduces risk of intergranular corrosion after welding; both are considered weldable without preheat in most applications. - Hardenability and cold cracking: Austenitic stainless steels do not harden like martensitic steels; hydrogen-induced cold cracking is not a primary concern. However, distortion control and residual stress management are important. - Alloying effects: High nickel and molybdenum in 904L increase hot cracking tendency in some filler-metal/process combinations and can make matching filler selection and procedure qualification more critical. - Common weldability equations used for interpretation (no numeric inputs here). Carbon equivalent (IIW) is often used qualitatively to assess weldability: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ And the more detailed Pcm index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ Interpretation: Higher alloying increases these indices, indicating the need for controlled welding procedures. Practically, 316L is easier to weld and qualifies with common filler metals (e.g., 316L weld filler), while 904L often requires qualified welding consumables, specialized procedures, and post-weld corrosion testing for demanding environments.

6. Corrosion and Surface Protection

• Stainless behavior: Both are stainless and rely on a passive chromium oxide film for general corrosion resistance.
• PREN for localized corrosion resistance: The pitting resistance equivalent number (PREN) is commonly used to compare localized corrosion resistance in chloride-bearing environments: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Interpretation: A higher PREN indicates greater resistance to pitting; 904L usually has a higher PREN than 316L due to greater Mo and Cr content and sometimes higher Ni facilitating resistance to localized attack.
• Specific corrosion behaviors:
• 316L: Good resistance to seawater splashes, moderate chloride environments, and many chemical process conditions.
• 904L: Superior resistance to reducing acids (e.g., sulfuric acid), better resistance to crevice and pitting corrosion in chloride-bearing environments, and improved performance in mixed acid/chloride service owing to Cu and higher alloy content.
• When stainless is not applicable (e.g., non-stainless carbon steels), surface protection options include galvanizing, painting, and polymer linings. Those are not relevant for 316L/904L comparisons as both are stainless.

7. Fabrication, Machinability, and Formability

• Machinability: Both grades are more difficult to machine than low-alloy steels. 316L machines reasonably well with the right tooling and speeds; 904L typically machines less easily due to high nickel and work hardening — it often requires lighter cuts, robust tooling, and more frequent tool changes.
• Formability/bendability: 316L has excellent formability and deep-drawing capability in the annealed condition. 904L forms well but can work-harden faster and may require intermediate annealing for severe forming operations.
• Surface finishing: Both accept common finishing techniques (polishing, passivation). 904L’s high alloying can require more aggressive pickling/cleaning procedures and careful passivation to restore the passive film after fabrication.

8. Typical Applications

Selection rationale: Use 316L when moderate chloride resistance, excellent weldability, and cost-effectiveness are priorities. Use 904L when specific process chemistries (strong reducing acids, mixed acid/chloride environments) or long-term localized corrosion resistance justify the higher material and fabrication costs.

9. Cost and Availability

• Relative cost: 904L is materially more expensive than 316L because of significantly higher nickel and molybdenum (and the addition of copper). The premium can be substantial and varies with commodity metal markets.
• Availability: 316L is globally ubiquitous in multiple product forms and grades; lead times are typically short. 904L is available but in fewer stock-keeping forms and often requires longer lead times or special-order production for certain product forms and thicknesses.
• Procurement implications: Total cost of ownership (material + fabrication + inspection + maintenance life) should be evaluated; in many cases 316L is the economical choice unless process conditions make 904L necessary to avoid failures or frequent maintenance.

10. Summary and Recommendation

Choose 316L if: - The environment involves moderate chloride exposure, general chemical service, marine atmospheres, or where cost and ease of fabrication are primary concerns. - You require excellent weldability and formability for high-volume manufacturing or extensive fabrication work.

Choose 904L if: - The service involves strong reducing acids (e.g., sulfuric acid) or aggressive chloride/acid mixtures where 316L shows pitting, crevice corrosion, or reduced life. - Long maintenance-free service life and resistance to localized attack justify higher material and processing expenditure, or when specified by process chemistry and corrosion engineering assessment.

Final note: Always specify exact alloy, product form, heat treatment, and testing requirements on purchase orders, and consult corrosion specialists or materials engineers when service conditions are ambiguous. Material certificates and site-specific corrosion testing are recommended when upgrading from 316L to 904L for critical process equipment.