Overview
AISI 310S (Russian designation 20Х23Н18, Chinese designation 16Cr25Ni20Si2, EN 1.4845) is an austenitic heat-resistant stainless steel with high chromium (~25%) and nickel (~20%) content. It is the industry standard material for furnace equipment and high-temperature components operating continuously to 1100°C in oxidizing and mildly oxidizing atmospheres. The low carbon variant (≤ 0.08%) significantly reduces susceptibility to carbide precipitation and intergranular corrosion during high-temperature service.
Chemical Composition (ASTM A240)
| Element | AISI 310S (UNS S31008) |
|---|---|
| Carbon (C) | ≤ 0.08% |
| Chromium (Cr) | 24.0–26.0% |
| Nickel (Ni) | 19.0–22.0% |
| Manganese (Mn) | ≤ 2.00% |
| Silicon (Si) | ≤ 1.50% |
| Phosphorus (P) | ≤ 0.045% |
| Sulfur (S) | ≤ 0.030% |
| Density | 7.98 g/cm³ |
The low carbon content (≤ 0.08%) versus standard AISI 310 (C ≤ 0.25%) is the critical differentiator, dramatically reducing chromium carbide precipitation during welding and high-temperature service.
High-Temperature Oxidation Resistance
In oxidizing air environments, AISI 310S forms a dense, adherent chromium oxide (Cr₂O₃) protective scale that provides excellent oxidation resistance:
- Continuous service: Up to 1100°C in oxidizing air
- Intermittent cycling: Recommended upper limit of 1050°C to avoid thermal fatigue spallation
- Sulfur-containing environments (H₂S, SO₂): Maximum temperature derated to ~1000°C due to nickel sulfide attack
- Carburizing atmospheres: Moderate resistance; for severe carburization, nickel-based alloys are preferred
Physical Properties
| Property | Value |
|---|---|
| Density | 7.98 g/cm³ |
| Melting Range | 1370–1400°C |
| Thermal Conductivity (100°C) | 14.2 W/m·K |
| Coefficient of Thermal Expansion (20–1000°C) | 18.5 × 10⁻⁶ /°C |
Heat Treatment
AISI 310S is a solid-solution alloy — it does not precipitation-harden. The standard heat treatment is solution annealing:
- Temperature: 1040–1150°C
- Holding time: Approximately 1 hour per 25 mm of thickness
- Cooling: Water quench or rapid air cool
Purpose: Dissolve chromium carbides (Cr₂₃C₆) formed during welding or service, restoring full corrosion resistance. For welded critical structures, a stabilizing anneal at 870–925°C can be applied.
International Standards
| Standard | Designation |
|---|---|
| GOST (Russia) | 20Х23Н18 (GOST 5632-2014) |
| GB (China) | 16Cr25Ni20Si2 |
| AISI (USA) | 310S |
| UNS | S31008 |
| EN/DIN | 1.4845 (X8CrNi25-21) |
| ASTM (Product) | A240 (sheet), A312 (tube), A276 (bar), A182 (forgings) |
Weldability
AISI 310S welds well using standard methods (TIG, MIG, SMAW, PAW). Recommended filler metals: E310-15/E310-16 electrodes (SMAW), ER310 wire (TIG/MIG). Key considerations:
- High Cr/Ni content causes greater thermal expansion — plan for distortion control
- HAZ carbide precipitation possible — post-weld anneal recommended for critical applications
- Not prone to hot cracking with proper filler selection
- Welding to carbon steel not recommended due to thermal expansion mismatch
Furnace & Industrial Applications
- Petrochemical furnace tubes — radiant and convection sections
- Heat exchanger components — high-temperature shell and tube
- Combustion chamber liners
- Conveyor belts and rollers for continuous furnaces
- Muffles and retorts for heat treatment furnaces
- Industrial oven components
Cost Comparison
Approximate pricing (2024–2025):
- Sheet 2–5 mm: $4.5–7.0/kg
- Seamless tube: $8–15/kg
- Bar: $5–9/kg
- Forgings: $10–18/kg
AISI 310S costs approximately 1.3–1.8 times more than AISI 304 due to elevated Cr (25%) and Ni (20%). Compared to nickel-based alternatives like Inconel 601 ($15–25/kg) or RA330 ($8–12/kg), AISI 310S is the most economical choice for oxidizing service to 1100°C.