317L vs 904L – Composition, Heat Treatment, Properties, and Applications

Introduction

317L and 904L are both austenitic stainless steels widely specified for corrosion-resistant equipment, but they occupy different places on the performance–cost spectrum. Engineers and procurement teams often weigh corrosion resistance (particularly to chlorides and reducing acids), weldability and formability, mechanical requirements, and lifecycle cost when choosing between them. The primary metallurgical distinction lies in their alloying strategy: 317L is a molybdenum-bearing austenitic designed to improve pitting and crevice resistance relative to 304/316 families, while 904L is a higher-alloy austenitic that combines elevated nickel, molybdenum and copper to provide superior resistance in aggressive reducing and chloride-bearing environments.

Because both are used for corrosion-critical applications, they are frequently compared during material selection for chemical processing, offshore, and high-chloride service. The remainder of this article compares standards, chemistry, microstructure and heat-treatment response, mechanical behavior, weldability, corrosion indices, fabrication attributes, application spaces, cost and availability, and concludes with recommendations.

1. Standards and Designations

• Common standards and specification families that cover these grades:
• ASTM / ASME: typical product standards include ASTM A240 / ASME SA-240 for plate and sheet, ASTM A276 for bars and stainless shapes, ASTM A312 for tubing.
• EN / ISO: covered within the EN 10088 series (stainless steels) and related product standards.
• JIS (Japanese Industrial Standards) and GB (Chinese National Standards) provide equivalent product specifications for austenitic stainless steels; consult conversion tables for exact mappings.
• UNS designations: 317L is commonly referenced as UNS S31703; 904L is commonly referenced as UNS N08904.
• Classification:
• 317L: stainless (austenitic alloy, low-carbon "L" variant for improved weldability/intergranular corrosion resistance).
• 904L: stainless (austenitic, high-alloy, low-carbon variant designed for elevated corrosion resistance in reducing and chloride-containing media).

2. Chemical Composition and Alloying Strategy

Table: typical chemical composition (wt%) — representative annealed ranges; Fe is balance.

Discussion of alloying strategy: - Chromium provides the primary passivity and general corrosion resistance for both grades. Higher Cr tends to improve oxidation resistance and base-level corrosion resistance. - Nickel stabilizes the austenitic microstructure, improves toughness and ductility, and increases resistance to chloride stress corrosion cracking (SCC) when present in higher amounts; 904L’s elevated Ni content enhances ductility and resistance to SCC in many environments. - Molybdenum is a key element for pitting and crevice resistance in chloride-bearing media; both alloys contain Mo, but 904L commonly contains somewhat higher Mo than 317L and complements it with higher Ni and added copper. - Copper in 904L enhances resistance to reducing acids such as sulfuric acid and helps performance in certain chloride-bearing reducing environments. - Low carbon (“L”) variants reduce the risk of intergranular attack after welding and permit extensive welding without post-weld stabilization in many service situations.

3. Microstructure and Heat Treatment Response

• Typical microstructure: Both 317L and 904L are fully austenitic in the annealed condition (face-centered cubic crystal structure). There is no hardenable martensitic transformation by quenching; strength is controlled primarily by solid-solution strengthening and cold work.
• Response to thermal processing:
• Annealing: Full anneal at typical temperatures (around 1000–1150 °C for austenitic stainless alloys) restores ductility and solution-solubilizes carbides and intermetallics.
• Stabilization: Because both are low-carbon grades, stabilization with Ti or Nb is generally unnecessary for avoiding sensitization, though some products may include stabilizers.
• Work-hardening: Both alloys work-harden under cold forming; 904L’s higher Ni and alloy content can lead to stronger work-hardening behavior than 317L, impacting formability and required force.
• Heat treatments aimed at precipitation hardening are not applicable; extended exposure between approximately 400–900 °C can promote carbide or intermetallic precipitation (which may affect corrosion resistance). Normalizing/quenching tempering processes used for ferritic/low-alloy steels are not relevant for fully austenitic grades.

4. Mechanical Properties

Table: typical mechanical properties (annealed condition, indicative ranges)

Interpretation: - Both alloys exhibit good ductility and toughness in the annealed condition. 904L often shows modestly higher yield strength due to higher solution strengthening from Ni and Mo; however, differences in UTS are usually small in the annealed state. - 317L may show slightly greater elongation in some vendor conditions, which can aid forming operations. - Impact toughness for both is generally excellent at ambient temperatures; neither displays a ductile-to-brittle transition typical of ferritic steels.

5. Weldability

• General: Both 317L and 904L are considered weldable austenitic stainless steels. Their low carbon content reduces susceptibility to intergranular carbide precipitation during welding and reduces the need for post-weld heat treatment.
• Factors to consider:
• Austenitic stainless steels are prone to hot cracking (solidification cracking) in welds; proper filler selection and welding parameters are important.
• High alloy content (Ni, Mo, Cu) affects solidification behavior and may require matched or overmatching filler metals.
• Useful weldability indices (no numeric calculation required here):
• Carbon Equivalent for austenitics (IIW form) can be used qualitatively: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ Higher $CE_{IIW}$ indicates greater propensity for weldability concerns in some contexts.
• Pitting/crevice/more conservative index for weldability relevant to carbon-manganese and other strengthening elements: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ $P_{cm}$ is sometimes used as a qualitative predictor of weld cracking risk.
• Practical interpretation:
• 317L: generally straightforward to weld with common austenitic stainless fillers (e.g., 316L-type consumables). Preheat is usually not required. Post-weld anneal seldom necessary.
• 904L: weldable but requires attention to filler selection (often 904L-matching weld wire or specially selected nickel-base fillers) to avoid galvanic or corrosion performance differences. Higher alloy content can increase susceptibility to hot cracking if welding procedures are not optimized. Fabricators should consult supplier welding guidelines.
• For both grades, control of heat input, interpass temperature, and adequate post-weld cleaning are important for preserving corrosion performance.

6. Corrosion and Surface Protection

• Non-stainless steels: not applicable here; both alloys are stainless.
• Pitting Resistance Equivalent Number (PREN) is a common way to indicate relative resistance to localized attack in chloride-bearing environments: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Using this formula qualitatively, an alloy with higher Mo and N will show a higher PREN and therefore improved pitting resistance.
• Qualitative comparison:
• 317L: Mo content substantially improves pitting resistance versus 304/316 families; widely used where enhanced resistance to chlorides and localised attack is needed.
• 904L: typically achieves higher overall localized corrosion resistance than 317L because it combines higher Mo with significantly higher Ni and added Cu; the net effect raises the PREN and improves resistance to both oxidizing and reducing chloride-bearing media and some sulfuric acid concentrations.
• When indices are not sufficient: actual service conditions (temperature, chloride concentration, flow, crevices, oxidizing or reducing chemistry) must be assessed experimentally or via corrosion databases and testing; PREN is only an indicator and does not guarantee performance in all environments.

7. Fabrication, Machinability, and Formability

• Forming:
• 317L: good formability in the annealed condition; typical austenitic ductility allows moderate to severe forming processes.
• 904L: also formable, but higher alloy content and stronger work-hardening make deep drawing or tight-radius forming more demanding; slower forming speeds or intermediate anneals may be required.
• Machinability:
• Both are more difficult to machine than ferritic or low-alloy steels due to toughness and work-hardening. 904L is typically more challenging than 317L because of higher Ni and Mo content and increased toughness; expect lower tool life and need for robust tooling, higher power, and controlled cutting parameters.
• Surface finishing:
• Both polish and electrochemical finishing are straightforward, but 904L may require adjusted polishing procedures to remove work-hardened layers and to achieve desired surface metallurgy.
• Recommendations:
• Use sharp tools, rigid setups, heavy-duty tooling and lower cutting speeds for 904L. For 317L, standard austenitic stainless machining procedures generally suffice.

8. Typical Applications

Table: Typical uses by grade

9. Cost and Availability

• Relative cost: 904L is materially more expensive than 317L because of its higher nickel, molybdenum and copper contents. Expect a significant premium for raw material, and higher machining and fabrication costs.
• Availability: 317L is widely stocked globally in plate, sheet, bar, tubing and forgings. 904L is available in many product forms but is less commonly stocked and often requires longer lead times or special-order production runs; availability in large-diameter or custom shapes may be limited compared to 317L.
• Procurement implications: specification of 904L should consider lifecycle cost benefits vs. initial material and processing premiums; buy-to-plan strategies and supplier qualification are important.

10. Summary and Recommendation

Table: quick-summary comparison (qualitative)

Concluding recommendations: - Choose 317L if you need a cost-effective, broadly available austenitic stainless with enhanced pitting resistance over common 300-series grades, straightforward welding and forming, and service in moderately aggressive chloride environments. - Choose 904L if the application faces aggressive chloride exposure, reducing acids (e.g., certain sulfuric acid concentrations), or service conditions where the highest practical corrosion resistance (and associated long-term reliability) justify the higher material and fabrication cost. 904L is the preferred option when maintenance access is difficult and extended service life in aggressive chemistries is critical.

Final note: PREN and alloy indices are useful screening tools, but final material selection should be based on service-specific corrosion testing, lifecycle cost analysis, welding procedure qualification, and consultation with material suppliers and corrosion specialists.