304 vs 310S – Composition, Heat Treatment, Properties, and Applications

Introduction

304 and 310S are two of the most frequently specified austenitic stainless steels in industry. Engineers and procurement professionals commonly weigh trade-offs between corrosion performance, high-temperature stability, weldability, and material cost when choosing between them. Typical decision contexts include food and pharmaceutical equipment (where 304 is often specified for cost and corrosion balance) versus furnace hardware and high-temperature process equipment (where 310S is preferred for oxidation and creep resistance).

The principal difference that drives the comparison is alloy chemistry: 310S contains substantially higher chromium and nickel than 304, which gives 310S much better high-temperature oxidation and strength retention but also a higher purchase cost and different fabrication characteristics. Because both grades are austenitic stainless steels with similar base metallurgy, they are often considered alternatives in design, with the final choice driven by operating temperature, corrosion environment, fabrication needs, and budget.

1. Standards and Designations

• Common standards and designations:
• ASTM / ASME: 304 (UNS S30400), 310S (UNS S31008)
• EN: 1.4301 (approx. 304), 1.4845 (approx. 310S)
• JIS: SUS304, SUS310S
• GB (China): 06Cr19Ni10 (304 equivalents), 25Cr20Ni (310/310S equivalents)
• Classification:
• Both 304 and 310S are austenitic stainless steels (stainless family).
• They are not carbon steels, alloy steels, tool steels, or HSLA classes.

2. Chemical Composition and Alloying Strategy

Table: typical composition ranges (wt.%). Actual limits depend on specification and mill; listed values reflect commonly used ASTM/EN ranges for general guidance.

Notes: - “—” indicates not intentionally added; only trace residual quantities are present. - 310S is the low-carbon variant of alloy 310; low carbon reduces carbide precipitation during high-temperature exposure. - The higher chromium and nickel in 310S are deliberate to stabilize austenite at elevated temperatures and form a more protective oxide scale during oxidation.

How the alloying affects performance: - Chromium contributes to corrosion resistance (passive film formation) and high-temperature oxidation resistance. Higher Cr in 310S improves scaling resistance at elevated temperatures. - Nickel stabilizes the austenitic microstructure and enhances toughness and high-temperature strength; significant Ni in 310S improves ductility retention and creep behavior at temperature. - Carbon promotes strength through solid solution and carbide formation but increases sensitization risk; controlling carbon (as in 310S) lowers carbide precipitation in service.

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• Both grades are fully austenitic (face-centered cubic) in the annealed condition at room temperature due to Ni content.
• Neither grade is heat-treatable by quench-and-temper methods used for ferritic or martensitic steels. Mechanical properties are modified by cold work and by solution annealing.
• Heat treatment response:
• Recommended solution annealing: typically in the range 1010–1150 °C followed by rapid cooling (water or air cooling per spec) to restore corrosion resistance and ductility.
• 304: solution anneal dissolves any carbides and restores ductility; prolonged exposure in the 425–850 °C range can cause sensitization and intergranular corrosion due to chromium carbide precipitation if carbon is not controlled.
• 310S: low carbon and high Ni reduces carbide precipitation and sensitization risk; however, long exposures in the sigma-phase precipitation range (approx. 600–1000 °C) can still promote intermetallics (sigma phase) in highly Cr alloys under some conditions. Proper solution anneal and controlled service exposure minimize this.
• Thermo-mechanical processing:
• Cold work increases strength by strain hardening but reduces formability and can increase susceptibility to stress corrosion cracking in certain environments.
• For high-temperature creep-critical applications, 310S is preferred because alloying gives better creep resistance; neither 304 nor 310S can be precipitation hardened.

4. Mechanical Properties

Table: typical annealed room-temperature values (indicative ranges; actual values depend on product form and specification).

Interpretation: - At room temperature, mechanical properties of 304 and 310S are broadly similar; both are ductile and tough. - Under elevated temperatures, 310S exhibits superior strength retention and creep resistance because of higher Ni and Cr contents. - Neither grade should be relied upon for high-strength structural applications without design calculations accounting for temperature-dependent creep and relaxation.

5. Weldability

• Both 304 and 310S are generally highly weldable using standard fusion processes (TIG, MIG, SMAW). Austenitic stainless steels are not susceptible to the hydrogen-assisted cold cracking that can affect high-strength carbon steels.
• Carbon content and hardenability:
• Lower carbon reduces carbide precipitation and intergranular corrosion post-weld. 310S (low carbon) reduces sensitization risk compared to higher-carbon variants.
• Hardenability and cracking risk are low; however, work-hardening and thermal cycles can create distortion, and proper joint design and fixturing are important.
• Common weld metallurgy indices (for qualitative interpretation):
• Display carbon equivalent formula: $$ CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} $$
• Parisian Pcm formula: $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$
• Qualitative interpretation for these grades:
• Both grades have low carbon (especially 310S) so $CE_{IIW}$ and $P_{cm}$ are low compared with hardenable steels; this predicts low hardenability and low risk of martensite-induced cracking.
• Higher Ni in 310S slightly increases the numerical CE via the formula term, but nickel also improves ductility and reduces cold cracking susceptibility in practice.
• Welding practice:
• Use matching or appropriate filler metals selected for intended service temperature.
• For 304, avoid prolonged interpass times in the sensitization range without post-weld solution anneal if the application is susceptible to intergranular attack.
• 310S welds require attention to thermal expansion and distortion due to higher alloy content and may be less tolerant of rapid cooling stresses in thick sections.

6. Corrosion and Surface Protection

• General corrosion (room-temperature aqueous environments):
• 304 provides good general corrosion resistance for many service environments including air, mild acids, and food processing.
• 310S offers similar or slightly improved general corrosion resistance, but its primary advantage is high-temperature performance rather than improved pitting or crevice resistance in chloride-bearing aqueous environments.
• Pitting resistance:
• Pitting Resistance Equivalent Number (PREN) is useful when Mo and N are significant: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$
• Neither 304 nor 310S contains Mo; both have low N—so PREN is not a strong differentiator. For chloride pitting resistance, alloys with Mo (e.g., 316) perform better.
• High-temperature corrosion/oxidation:
• 310S has significantly better resistance to oxidation and scaling at elevated temperatures (for example, furnace atmospheres) because of higher Cr and Ni which stabilize protective oxide scales.
• If stainless grades are not suitable, typical surface protections for non-stainless steels (galvanizing, coatings, lining, painting) apply; for these two grades, protective considerations focus on passivity maintenance and avoiding sensitization.

7. Fabrication, Machinability, and Formability

• Formability:
• Both are highly formable in the annealed condition. 304 is commonly used for deep drawing and complex forming operations.
• 310S is less commonly used for extensive forming because the higher alloy content can increase work-hardening rate and springback; however, it remains workable with appropriate tooling and anneal cycles.
• Machinability:
• Austenitic stainless steels work-harden and are more difficult to machine than low-carbon steels.
• 304 is moderate in machinability for stainless grades; chip control, rigid setups, positive rake tooling, and carbide inserts help.
• 310S, owing to higher Ni content and tougher alloying, can be tougher on tools and may require slower cutting speeds and more robust tooling.
• Surface finish and polishing:
• Both polish well; 304 is widely used where bright finishes are required. 310S can also be finished to high standards but may present slightly more difficulty due to material toughness.

8. Typical Applications

Selection rationale: - Choose 304 when corrosion resistance at ambient temperatures, cost-effectiveness, and wide availability are primary drivers. - Choose 310S when sustained high-temperature strength, oxidation resistance, and low carbide precipitation at elevated temperatures are critical.

9. Cost and Availability

• Relative cost:
• 310S is more expensive than 304 because of the significantly higher Ni and Cr content.
• Price differentials fluctuate with global nickel and chromium markets; nickel content is the major cost driver.
• Availability:
• 304 is one of the most commonly stocked stainless steels worldwide in sheet, plate, bar, pipe, and tube forms.
• 310S is widely available but some product forms (e.g., very large plate or specialty cold-formed sections) can have longer lead times or limited suppliers compared with 304.

10. Summary and Recommendation

Table summarizing key trade-offs:

Recommendations: - Choose 304 if: - The application is primarily ambient- to moderately-elevated-temperature service. - You need broad availability, lower cost, and good overall corrosion resistance (food, architectural, general process plant). - Extensive forming or deep drawing operations are required. - Choose 310S if: - The primary requirement is sustained elevated-temperature performance, oxidation resistance, or improved creep strength (furnaces, high-temperature process hardware). - Sensitization risk must be minimized in a high-temperature cyclic environment. - Higher material cost is acceptable for improved service life and reduced scaling.

Final note: material selection should always consider the full service envelope (temperature, atmosphere, mechanical load, joint design, fabrication route, and total life-cycle cost). When in doubt for critical elevated-temperature or corrosive services, confirm selection with corrosion testing, consult long-term creep and oxidation data, and engage material suppliers or metallurgical consultants to verify suitability.