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

Introduction

Engineers, procurement managers, and manufacturing planners frequently face the choice between 316 and 310S stainless steels when specifying parts for corrosive or elevated-temperature environments. The decision typically balances corrosion resistance and weldability against high-temperature strength and oxidation resistance, as well as first-cost and supply considerations. While both are austenitic stainless steels and share many fabrication characteristics, 316 is generally selected for chloride-containing environments and weldability-sensitive applications, whereas 310S is chosen when sustained high-temperature oxidation resistance and creep strength are required.

The primary practical distinction is that 316 emphasizes improved corrosion (especially pitting) resistance through molybdenum alloying, while 310S emphasizes high-temperature stability and oxidation resistance through elevated chromium and nickel content. These different alloying strategies drive differences in mechanical behavior, fabrication response, and application suitability.

1. Standards and Designations

• Common standards and specifications:
• ASTM/ASME: 316 (A240/A312/A403 family, UNS S31600 / S31603 for low-carbon 316L) and 310S (A240, UNS S31008).
• EN: 1.4401 / 1.4404 equivalents for 316 / 316L; 1.4845 for 310S.
• JIS: SUS316 / SUS316L and SUS310S.
• GB: 06Cr19Ni10 (approx. 316), 0Cr25Ni20 (approx. 310S) nomenclature may be encountered.
• Classification: both are stainless steels (austenitic). They are not carbon steels, tool steels, or HSLA grades.

2. Chemical Composition and Alloying Strategy

The following table shows typical compositional ranges for commercial annealed grades. Ranges vary by standard and producer; values below are representative.

Alloying strategy and effects: - Chromium (Cr) establishes the passive oxide film and high-temperature oxidation resistance. 310S has substantially higher Cr for better scaling resistance at elevated temperatures. - Nickel (Ni) stabilizes the austenitic phase and improves toughness and ductility; 310S has higher Ni to retain ductility at high temperatures and resist creep. - Molybdenum (Mo) in 316 enhances resistance to pitting and crevice corrosion in chloride-containing environments and increases localized corrosion resistance (not present in 310S). - Carbon content influences carbide precipitation and sensitization; low-carbon variants (316L / 310S) reduce intergranular attack after welding. - Minor additions (Nb, Ti) are used in stabilized grades to tie up carbon and prevent sensitization.

3. Microstructure and Heat Treatment Response

• Microstructure: Both 316 and 310S are fully austenitic (face-centered cubic) in typical annealed condition at room temperature. They do not transform martensitically on cooling.
• Heat treatment response:
• Solution annealing (e.g., 1,040–1,100°C) followed by rapid cooling is the standard treatment to dissolve precipitates and restore corrosion resistance for both alloys.
• Neither grade can be significantly hardened by conventional quench-and-temper treatments; strengthening is achieved by cold work or by high-temperature solid solution strengthening (e.g., for 310S).
• At intermediate temperatures (roughly 450–900°C), both alloys are susceptible to precipitation of sigma phase or carbides if held long enough; 310S’s higher Ni content improves phase stability at some temperatures, while 316’s Mo can encourage sigma formation under certain conditions. Proper heat treatment and selecting low-carbon or stabilized variants mitigate sensitization.
• Thermo-mechanical processing (rolling, forging) followed by proper anneal will produce fine-grained austenite in both alloys; cold work increases strength but also work-hardening and reduces ductility.

4. Mechanical Properties

Typical room-temperature, annealed-property ranges (note: actual values depend on product form, thickness, and supplier):

Interpretation: - 310S typically exhibits higher tensile and yield strengths in the annealed condition due to heavier alloying and greater solution strengthening (higher Cr and Ni). - 316 generally shows higher ductility and tends to be tougher at room and subambient temperatures, aided by its Ni content and lower solid-solution strengthening contribution from Cr. - Neither grade is designed for high hardness in the annealed state; cold work increases strength at the expense of ductility.

5. Weldability

Austenitic stainless steels are among the most weldable stainless families, but differences matter in practice.

• Factors:
• Carbon level, residual elements, and alloy content influence hot cracking susceptibility, solidification mode, and post-weld corrosion resistance.
• Because both are austenitic, the risk of martensitic transformation and hardenability-related cracking is low.
• Useful indices:
• Carbon equivalent for general weldability:
  $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ Higher values indicate greater hardenability and potential for cracking in ferritic steels; for austenitics this formula is used qualitatively to compare alloying effects.
• Welding parameter for stainless steels:
  $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ Larger $P_{cm}$ suggests increased propensity toward intergranular cracking phenomena during welding.
• Qualitative interpretation:
• 316 benefits from Mo for corrosion resistance but has overall moderate CE/Pcm; it welds readily with standard filler metals (e.g., 316/316L or 309 for dissimilar joins) and shows good ductility in weld HAZs. Use of low-carbon or stabilized grades reduces sensitization.
• 310S welds well in many conditions, but its higher alloy content can increase solidification cracking susceptibility and makes filler selection and pre/post-weld practice more critical, especially for heavy sections and multipass welds. 310/310L fillers or 309 for bridged dissimilar joints are common.
• Preheating is rarely required for either grade for metallurgical reasons, but attention to heat input, interpass temperature, and filler chemistry is important for controlling distortion and avoiding sigma-phase precipitation.

6. Corrosion and Surface Protection

• Stainless behavior:
• Use of PREN (Pitting Resistance Equivalent Number) helps compare localized corrosion resistance:
  $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
  • 316’s Mo content elevates PREN relative to 310S, giving 316 better pitting and crevice corrosion resistance in chloride-containing environments.
  • 310S, lacking Mo, has a PREN driven mainly by higher Cr — good general corrosion and oxidation resistance but less pitting resistance than Mo-bearing grades.
• High-temperature oxidation:
• 310S excels in oxidation resistance and scaling resistance at elevated temperatures (e.g., furnace work, exhaust components) because of higher Cr and Ni.
• When stainless protection is not used:
• For non-stainless steels, protection is via galvanizing, painting, or coatings. Not applicable when comparing these two stainless alloys, except where surface treatments (passivation, pickling) are used to restore passive film after fabrication.
• Clarification: PREN is meaningful for comparing pitting resistance among stainless steels; it is not applicable to general corrosion mechanisms like uniform acid attack or high-temperature oxidation.

7. Fabrication, Machinability, and Formability

• Forming and bending:
• 316 is generally easier to form and bend because of slightly better ductility and lower work hardening rate.
• 310S is more resistant to forming and springback due to higher strength and stronger work-hardening tendency.
• Machinability:
• Both grades machine worse than carbon steels. 316 is typically slightly easier to machine than 310S; both require rigid setups, sharp tooling, and frequent tool changes. Use of high-feed, positive-rake carbide tooling and flood coolant is common.
• Surface finishing:
• Both polish and finish well; 316’s lower scaling at moderate temps makes it easier to achieve a consistent finish after welding.
• Fabrication note:
• For high-temperature components (310S), machining allowances and heat-treatment plans should account for distortion and scaling.

8. Typical Applications

Selection rationale: - Choose 316 for environments where chloride-induced pitting, crevice corrosion, and weldability with retained corrosion resistance are primary concerns. - Choose 310S when operating temperature, oxidation scaling, or creep resistance at elevated temperatures dominate the design requirements.

9. Cost and Availability

• Cost:
• 310S is generally more expensive than 316 on a per-kilogram basis because of significantly higher nickel content and higher chromium content.
• 316 is typically less costly and often available in a wider range of commodity product forms.
• Availability by product form:
• 316: wide availability in plates, sheets, pipe, tube, bar, fasteners, and a wide selection of cold-worked and low-carbon variants (316L).
• 310S: available in sheets, plates, and high-temperature specialty forms, but less common in commodity fasteners or low-cost forms.
• Procurement guidance: for large-volume projects where high-temperature resistance is not required, 316 may offer better total cost and supply security. For high-temperature specialty components, 310S availability is usually sufficient but lead times and cost premiums should be expected.

10. Summary and Recommendation

Recommendation: - Choose 316 (or 316L) if your primary needs are resistance to chloride-induced pitting, excellent weldability with retained corrosion performance, good ductility and toughness, and lower material cost or broad supply. Typical use cases: marine, chemical processing, food/pharma tanks and piping. - Choose 310S if your primary needs are long-term high-temperature oxidation resistance, scaling resistance, or service at elevated temperatures where creep and thermal stability are critical. Typical use cases: furnace components, radiant tubes, kiln hardware, and high-temperature ducting.

Final note: both alloys are best specified with explicit product form, finish, and any post-weld heat treatment or stabilization requirements. Consult material suppliers and welding procedure specifications (WPS) for filler alloy selection, pre/post-weld practice, and application-specific temperature limits to avoid sigma-phase formation, sensitization, and premature failures.