ALLOY 309, 309S, AND 309H STAINLESS STEELS — TECHNICAL DATASHEET AND METALLURGICAL REPORT
Alloy 309 (UNS S30900), along with its low-carbon modification Alloy 309S (UNS S30908) and high-carbon creep-resistant variant Alloy 309H (UNS S30909), represents a family of highly alloyed austenitic chromium-nickel stainless steels engineered primarily for high-temperature service. These alloys are characterized by outstanding resistance to thermal oxidation, high-temperature sulfidation, and environmental attack, retaining a higher fraction of mechanical strength at elevated temperatures compared to standard austenitic grades like Alloy 304.
The metallurgical design of the Alloy 309 family occupies a critical position between the lower-alloyed 18-8 stainless steels and the higher-alloyed, more expensive heat-resistant grades such as Alloy 310 or nickel-based superalloys. By balancing high chromium levels with moderate nickel additions, these alloys provide a cost-effective solution for structural components operating in highly oxidizing, nitriding, and mildly sulfur-bearing environments up to 1038°C (1900°F) under non-cyclic conditions.
■ Chemical Metallurgy and Grade Differentiation
The operational capabilities of the Alloy 309 family are directly linked to a highly controlled chemistry balanced between ferrite-promoting chromium and austenite-stabilizing nickel. The high chromium content promotes the rapid formation and continuous repair of a dense, adherent chromium oxide ($Cr_2O_3$) passive scale. The differences among standard 309, 309S, and 309H are dictated by the carbon concentration, balancing fabrication requirements against long-term creep parameters.
| Element (wt %) | Alloy 309 (UNS S30900) | Alloy 309S (UNS S30908) | Alloy 309H (UNS S30909) | EN 1.4833 (X12CrNi23-13) | EN 1.4828 (X15CrNiSi20-12) |
|---|---|---|---|---|---|
| Carbon (C) | ≤ 0.20 | ≤ 0.08 | 0.04 - 0.10 | ≤ 0.15 | ≤ 0.20 |
| Chromium (Cr) | 22.0 - 24.0 | 22.0 - 24.0 | 22.0 - 24.0 | 22.0 - 24.0 | 19.0 - 21.0 |
| Nickel (Ni) | 12.0 - 15.0 | 12.0 - 15.0 | 12.0 - 15.0 | 12.0 - 14.0 | 11.0 - 13.0 |
| Manganese (Mn) | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 |
| Silicon (Si) | ≤ 0.75 | ≤ 0.75 | ≤ 0.75 | ≤ 1.00 | 1.50 - 2.50 |
| Phosphorus (P) max | 0.045 | 0.045 | 0.045 | 0.045 | 0.045 |
| Sulfur (S) max | 0.030 | 0.030 | 0.030 | 0.015 | 0.015 |
| Nitrogen (N) max | 0.11 | 0.11 | — | 0.11 | 0.11 |
■ Proprietary Datasheet Download
For design engineers, metallurgists, and project managers requiring granular stress simulation criteria, international certifications, and advanced scaling charts, the complete technical datasheet can be accessed.
Alloy 309 / 309S / 309H — Comprehensive High-Temperature Technical Datasheet
Contains empirical metrics for finite element modeling, long-term stress-rupture curves, and certified welding procedures. Engineering credentials required.
⬇ DOWNLOAD DATASHEET■ Physical and Thermophysical Properties
Due to its face-centered cubic crystal lattice, Alloy 309 exhibits high thermal expansion coefficients and lower thermal conductivity compared to ferritic or martensitic steel grades. Thermal design requires careful accounting of these parameters to avoid scale spallation or joint warping.
| Physical Property | Value in Metric Units | Value in Imperial Units |
|---|---|---|
| Density (Ambient) | 7.89 – 8.00 g/cm³ | 0.285 – 0.289 lb/in³ |
| Melting Point / Range | 1480 – 1530°C | 2500 – 2590°F |
| Specific Heat Capacity (20 - 100°C) | 500 – 502 J/kg·K | 0.12 BTU/lb·°F |
| Modulus of Elasticity (Tension, 20°C) | 193 – 200 GPa | 28.0 × 10⁶ – 29.0 × 10⁶ psi |
| Shear Modulus | 77 GPa | 11.2 × 10⁶ psi |
| Poisson's Ratio | 0.27 – 0.30 | 0.27 – 0.30 |
| Electrical Resistivity (20°C) | 0.78 µΩ·m | 30.7 µΩ·in |
| Magnetic Permeability (Annealed) | ≤ 1.02 at H = 200 Oe | ≤ 1.02 at H = 200 Oe |
Thermal expansion and thermal conductivity vary continuously across the active high-temperature operating envelope:
| Temperature Range / Threshold | Thermal Conductivity (λ) | Mean Coefficient of Thermal Expansion (αm) |
|---|---|---|
| 20 - 100°C (68 - 212°F) | 14.2 - 15.6 W/m·K | 14.9 × 10⁻⁶ K⁻¹ (8.3 × 10⁻⁶ /°F) |
| 20 - 315°C (68 - 600°F) | 17.1 W/m·K | 16.7 × 10⁻⁶ K⁻¹ (9.3 × 10⁻⁶ /°F) |
| 20 - 500°C (68 - 932°F) | 18.7 - 21.0 W/m·K | 17.3 × 10⁻⁶ K⁻¹ (9.6 × 10⁻⁶ /°F) |
| 20 - 649°C (68 - 1200°F) | 20.0 W/m·K | 18.0 × 10⁻⁶ K⁻¹ (10.0 × 10⁻⁶ /°F) |
| 20 - 800°C (68 - 1472°F) | 22.5 W/m·K | 18.5 × 10⁻⁶ K⁻¹ |
| 20 - 1000°C (68 - 1832°F) | 24.0 W/m·K | 19.5 × 10⁻⁶ K⁻¹ |
■ Room-Temperature Mechanical Properties
Alloy 309 exhibits excellent ductility and strength in the solution-annealed state. Note that cold-worked property enhancements are unstable in the creep range and can accelerate long-term creep rate patterns.
| Grade Specification / Product Form | Ultimate Tensile Strength (Rm) | 0.2% Offset Yield Strength (Rp0.2) | Elongation in 50 mm (A5) | Hardness Limit (max) |
|---|---|---|---|---|
| Alloy 309 / 309S Plate (ASTM A240) | 515 MPa (75 Simple ksi) | 205 MPa (30 Simple ksi) | 40% | 217 HBW / 95 HRB |
| Alloy 309H Plate (ASTM A240) | 515 MPa (75 Simple ksi) | 205 MPa (30 Simple ksi) | 40% | 217 HBW / 95 HRB |
| F309 Forgings (ASTM A182) | 515 MPa (70 Simple ksi) | 205 MPa (30 Simple ksi) | 40% | 217 HBW |
| F309H Forgings (ASTM A182) | 515 MPa (75 Simple ksi) | 205 MPa (30 Simple ksi) | 30% | 217 HBW |
| EN 1.4828 Bar (+AT / EN 10088-3) | 550 - 750 MPa | 230 MPa | 28 - 30% | 223 HB |
| EN 1.4833 Bar (+AT / EN 10088-3) | 500 - 700 MPa | 210 MPa | 33 - 35% | 192 HB |
| ASTM A193 Grade B8S Fasteners | 690 MPa (100 Simple ksi) | 380 MPa (55 Simple ksi) | 35% | 271 HBW (28 HRC) |
■ Elevated-Temperature and Short-Term Tensile Data
At high temperatures, standard Alloy 309 retains a significant fraction of its ambient mechanical characteristics, resisting the rapid plastic collapse typical of mild structural steels.
| Temperature | Ultimate Tensile Strength (Rm) | 0.2% Offset Yield Strength (Rp0.2) | Elongation (A5) |
|---|---|---|---|
| 204°C (400°F) | 545 MPa (79.0 ksi) | 262 MPa (38.0 ksi) | 46% |
| 316°C (600°F) | 517 MPa (75.0 ksi) | 238 MPa (34.5 ksi) | 43% |
| 427°C (800°F) | 490 MPa (71.0 ksi) | 221 MPa (32.0 ksi) | 40% |
| 538°C (1000°F) | 441 MPa (64.0 ksi) | 200 MPa (29.0 ksi) | 38% |
| 649°C (1200°F) | 359 MPa (52.0 ksi) | 152 - 172 MPa (22.0 - 25.0 ksi) | 26 - 37% |
| 760°C (1400°F) | 241 MPa (35.0 ksi) | 148 MPa (21.5 ksi) | 39% |
| 871°C (1600°F) | 124 - 145 MPa (18.0 - 21.0 ksi) | 120 MPa (17.5 ksi) | 32 - 50% |
| 982°C (1800°F) | 72 MPa (10.5 ksi) | — | — |
■ Creep Limits and Stress-Rupture Strength
At service temperatures exceeding 550°C (1022°F), high-temperature structures undergo time-dependent plastic creep. Coarse-grained structural designs (ASTM grain size 5 or coarser) optimize creep-rupture performance by limiting thermally activated grain boundary sliding.
| Temperature | Creep Limit (1% Strain) 1,000 h | Creep Limit (1% Strain) 10,000 h | Stress-Rupture Strength 1,000 h | Stress-Rupture Strength 10,000 h | Stress-Rupture Strength 100,000 h |
|---|---|---|---|---|---|
| 600°C (1112°F) | 120 MPa | 80 MPa | 190 MPa | 120 MPa | 65 MPa |
| 700°C (1292°F) | 50 MPa | 25 MPa | 75 MPa | 36 MPa | 16 MPa |
| 800°C (1472°F) | 20 MPa | 10 MPa | 35 MPa | 18 MPa | 7.5 MPa |
| 900°C (1652°F) | 8 MPa | 4 MPa | 15 MPa | 8.5 MPa | 3.0 - 7.5 MPa |
| 1000°C (1832°F) | 4 MPa | 2.5 MPa | 8 MPa | 4 MPa | 1.5 MPa |
■ Material Specifications and Product Forms
| Product Form | ASTM Standard | ASME Standard | UNS Designation Reference |
|---|---|---|---|
| Plate, Sheet, and Strip | ASTM A240 / A167 | ASME SA-240 | S30900 (309) / S30908 (309S) / S30909 (309H) |
| Seamless and Welded Pipe | ASTM A312 | ASME SA-312 | S30900 (309) / S30908 (309S) / S30909 (309H) |
| Seamless and Welded Tube | ASTM A213 / A249 / A269 | ASME SA-213 / SA-249 | S30908 (309S) / S30909 (309H) |
| Flanges, Forged Fittings, Valves | ASTM A182 | ASME SA-182 | S30900 (F309) / S30908 (F309S) / S30909 (F309H) |
| Bars, Rods, and Shapes | ASTM A276 / A479 | ASME SA-276 / SA-479 | S30900 (309) / S30908 (309S) / S30909 (309H) |
| Fasteners and Bolting | ASTM A193 / A194 | ASME SA-193 / SA-194 | S30900 (309) / S30908 (309S) |
■ Fabrication, Corrosion Mechanics, and Welding
- Oxidation & Spallation: Excellent up to 1038°C (1900°F) in continuous dry air. Cyclic thermal states cause mismatch stresses between the substrate and chromia passive scale, lowering the threshold to 1010°C (1850°F).
- Sulfidation Mastery: Possesses a high chromium-to-nickel ratio, preventing the formation of liquid nickel-sulfide eutectic phases (which melt at 635°C). Safe in oxidizing sulfur atmospheres ($SO_2$) up to 1000°C.
- Sigma Phase Embrittlement: Extended thermal exposure within 650 - 950°C decomposes metastable delta-ferrite into the hard, intermetallic sigma phase ($\sigma$), reducing impact toughness at room temperature. A full solution anneal at 1100 - 1150°C followed by a water quench dissolves this phase.
- Weld Zone Hot Cracking Prevention: No preheating or PWHT is required. The weld metal chemistry must be balanced to maintain a minor delta-ferrite content (4 - 10%) during solidification to dissolve phosphorus and sulfur impurities, thereby preventing micro-fissuring. Welders must restrict interpass temperatures below 150°C (302°F).
■ ASME Pressure Vessel Design Criteria (UG-27 cylindrical shell text limits)
Under ASME Section VIII Division 1, the required minimum wall thickness ($t$) under circumferential tension is given by:
Where $P$ is design internal pressure, $R$ is shell inside radius, $S$ is maximum design allowable stress from Section II Part D, and $E$ is joint efficiency (E=1.0 for full radiography, E=0.85 for spot check, E=0.70 for un-inspected setups).
Design Note (Note G5): High-strength allowable stress criteria are appended for specific 309 classifications, reaching up to 90% of yield limits at temperature. While beneficial for wall reduction, they can cause minor permanent micro-deformation under load and must be avoided in precision gasketed components.