STAINLESS STEEL GRADE 309S (UNS S30908 / EN 1.4833) — COMPREHENSIVE TECHNICAL DATASHEET
Stainless steel grade 309S (UNS S30908 / EN 1.4833) is a highly alloyed, heat-resistant austenitic stainless steel engineered to operate in structural and thermal environments where standard 18-8 grades, such as 304 and 316, fail due to oxidation, sulfidation, or mechanical degradation. Incorporating approximately 23% chromium and 14% nickel, the alloy provides a highly stable phase structure combined with superior scale integrity under continuous operating temperatures up to 1038°C (1900°F).
The primary metallurgical distinction of Grade 309S is its strictly restricted carbon threshold ($\le 0.08\%$), separating it from the standard carbon variant Alloy 309 and the high-carbon variant Alloy 309H. The low-carbon modification suppresses the thermodynamic driving force for chromium carbide ($Cr_{23}C_6$) precipitation along the grain boundaries. Consequently, Grade 309S is the preferred specification for welded structures, complex thermal assemblies, and parts subjected to cyclic sensitization temperatures where post-weld solution annealing is logistically or geometrically impractical.
■ Global Standard Specifications and Equivalents
Grade 309S is globally standardized across multiple manufacturing and safety codes, ensuring compliance in pressure-retaining equipment, general corrosion-resistant structures, and aerospace components.
| Standard Body | Standard / Specification | Alloy Code / Designation |
|---|---|---|
| ASTM / ASME Plate | ASTM A240 / ASME SA240 | UNS S30908 / Type 309S |
| ASTM Bars | ASTM A276 | UNS S30908 / Type 309S |
| ASTM / ASME Pipe | ASTM A312 / ASME SA312 | UNS S30908 / TP309S |
| ASTM / ASME Boiler | ASTM A479 / ASME SA479 | UNS S30908 / Type 309S |
| Aerospace | AMS 5523 | Type 309S thin sheet / strip |
| European Union | EN 10095 | 1.4833 / X12CrNi23-13 |
| Japan (JIS) | JIS G 4304 | SUS 309S |
■ Chemical Composition Matrix (Weight %)
The performance of Grade 309S depends on its precise balance of ferrite-stabilizing and austenite-stabilizing elements. Silicon is restricted to optimize high-temperature strength and weldability while maintaining long-term phase stability.
| Element | Type 309S (UNS S30908) | Type 309 (UNS S30900) | Type 309H (UNS S30909) | EN 1.4828 (309Si variant) |
|---|---|---|---|---|
| Chromium (Cr) | 22.00 – 24.00 | 22.00 – 24.00 | 22.00 – 24.00 | 19.00 – 21.00 |
| Nickel (Ni) | 12.00 – 15.00 | 12.00 – 15.00 | 12.00 – 15.00 | 11.00 – 13.00 |
| Carbon (C) max | 0.080 | 0.200 | 0.040 – 0.100 (range) | 0.200 |
| Manganese (Mn) max | 2.000 | 2.000 | 2.000 | 2.000 |
| Silicon (Si) max | 0.750 (1.000 for bars) | 0.750 | 0.750 | 1.500 – 2.500 (range) |
| Phosphorus (P) max | 0.045 | 0.045 | 0.045 | 0.045 |
| Sulfur (S) max | 0.030 | 0.030 | 0.030 | 0.015 – 0.030 |
| Nitrogen (N) max | — | — | — | 0.110 |
■ Proprietary Datasheet Download
For engineering specifications, full mechanical test documentation, and long-term creep curves of Alloy 309S high-temperature components, the comprehensive technical manual should be downloaded.
Alloy 309S — Complete Technical Yield & Creep Datasheet
Contains empirical data for finite element stress modeling, oxidation kinetics, and multi-code safety compliances. Corporate credentials required.
⬇ DOWNLOAD DATASHEET■ Physical and Thermophysical Properties
Due to its face-centered cubic crystal lattice, Alloy 309S exhibits high thermal expansion coefficients and lower thermal conductivity compared to ferritic steel grades.
| Physical Property | Metric Value | Imperial Value |
|---|---|---|
| Density (at 20°C) | 7.89 – 8.03 g/cm³ | 0.285 – 0.290 lb/in³ |
| Melting Range | 1399°C – 1530°C | 2500°F – 2650°F |
| Specific Heat Capacity (0 – 100°C) | 502 J/kg·K | 0.12 BTU/lb·°F |
| Elastic Modulus in Tension (E) | 193 – 200 GPa | 28.0 × 10⁶ – 29.0 × 10⁶ psi |
| Shear Modulus (G) | 77 GPa | 11.2 × 10³ ksi |
| Poisson's Ratio | 0.30 | 0.30 |
| Electrical Resistivity (at 20°C) | 78.0 µΩ·cm | 30.7 µΩ·in |
| Magnetic Permeability (Annealed) | ≤ 1.02 at H = 200 Oe | ≤ 1.02 at H = 200 Oe |
Temperature-dependent values illustrate the physical behavior of Alloy 309S across its active continuous operating envelope:
| Temperature | Thermal Conductivity (λ) | Mean Coefficient of Thermal Expansion (CTE) |
|---|---|---|
| 20°C (68°F) | 15.0 W/m·K (8.7 BTU/hr·ft·°F) | — |
| 100°C (212°F) | 15.6 W/m·K (9.0 BTU/hr·ft·°F) | 14.9 – 15.6 µm/m·K (8.3×10⁻⁶ – 8.7×10⁻⁶ /°F) |
| 315°C (600°F) | — | 16.7 µm/m·K (9.3 × 10⁻⁶ /°F) |
| 500°C (932°F) | 18.7 W/m·K (10.8 BTU/hr·ft·°F) | 17.3 – 17.6 µm/m·K (9.6×10⁻⁶ – 9.8×10⁻⁶ /°F) |
| 648°C (1200°F) | — | 18.0 µm/m·K (10.0 × 10⁻⁶ /°F) |
| 1000°C (1832°F) | 21.0 W/m·K (at 500°C+) | 19.4 – 19.5 µm/m·K (10.8 × 10⁻⁶ /°F) |
■ Mechanical Performance and Hardness Values
Grade 309S maintains high ductility and strength at room temperature, while preserving structural load capacity at elevated temperatures.
| Mechanical Property | Code Minimum (ASTM A240 / A276) | Typical Values (Representative Product) |
|---|---|---|
| Tensile Strength, Ultimate (UTS) | 515 MPa (75,000 psi) | 586 – 620 MPa (85,000 – 89,900 psi) |
| Yield Strength (0.2% Offset) | 205 MPa (30,000 psi) | 280 – 310 MPa (40,500 – 45,000 psi) |
| Elongation (in 50mm / 2 in.) | 40% | 45% – 54% |
| Reduction of Area (RA) | — (Bar ≥ 40-50%) | 50% – 65% |
| Hardness, Brinell (HBW) | 217 max. | 147 – 159 |
| Hardness, Rockwell B (HRB) | 95 max. | 85 |
■ Elevated-Temperature Tensile and Creep Behavior
For structural service temperatures above 540°C (1000°F), high-temperature creep and creep rupture limits are key structural design considerations.
| Temperature | Yield Strength (0.2% Offset) | Ultimate Tensile Strength (UTS) | Creep Limit (1% Strain / 10,000 h) | Creep Rupture Strength (10,000 h) |
|---|---|---|---|---|
| 600°C (1112°F) | 170 MPa (24.7 ksi) | 386 MPa (55.9 ksi) | 80 MPa | 120 MPa |
| 700°C (1292°F) | 153 MPa (22.2 ksi) | 248 MPa (36.0 ksi) | 25 MPa | 36 MPa |
| 800°C (1472°F) | 114 MPa (16.6 ksi) | 142 MPa (20.7 ksi) | 10 MPa | 18 MPa |
| 900°C (1652°F) | 90 MPa (13.1 ksi) | 106 MPa (15.4 ksi) | 4 MPa | 8.5 MPa |
| 1000°C (1832°F) | 32 MPa (4.6 ksi) | 46 MPa (6.6 ksi) | 2.5 MPa | 4.0 MPa |
■ ASME Section VIII, Division 1 Allowable Design Stresses
Standard allowable stresses for plate materials in pressure-retaining applications per ASME SA240 Type 309S (UNS S30908) vs common alternatives:
| Metal Temperature | Type 309S (UNS S30908) | Type 304 (UNS S30400) | Type 304L (UNS S30403) | Type 310 (UNS S31008) |
|---|---|---|---|---|
| Up to 100°F (38°C) | 20.0 ksi (138 MPa) | 20.0 ksi | 16.7 ksi | 20.0 ksi |
| 400°F (204°C) | 20.0 ksi (138 MPa) | 18.3 ksi | 15.8 ksi | 19.9 ksi |
| 600°F (316°C) | 18.8 ksi (130 MPa) | 16.6 ksi | 14.0 ksi | 18.5 ksi |
| 800°F (427°C) | 17.7 ksi (122 MPa) | 15.2 ksi | 13.0 ksi | 17.4 ksi |
■ Environmental Resistance and Media Compatibility
| Chemical / Media | Compatibility Rating | Industrial Context & Environmental Limits |
|---|---|---|
| Nitric Acid ($HNO_3$) | Good | Resistant to highly oxidizing acids; used in chemical process towers. |
| Sulfuric Acid ($H_2SO_4$) | Moderate | Prone to active attack in hot, concentrated reducing acids. |
| Sea Water | Restricted | Susceptible to localized crevice corrosion and pitting under stagnant conditions. |
| High-Temp Sulfidation | Good | Resistant in hot, sulfur-bearing gases up to 1000°C due to low nickel ($\le 15\%$). |
■ Fabrication, Heat Treatment, and Weldability Guidelines
- Solution Annealing (Austenitizing): Anneal between 1038°C and 1121°C (1900°F to 2050°F). Soaking time is typically 30 minutes per inch of thickness. It is critical to cool rapidly via water quench or forced air to keep all carbides in solid solution.
- Sigma Phase Embrittlement: Prolonged exposure within 560°C to 980°C (1050°F to 1800°F) triggers the precipitation of the brittle, intermetallic sigma phase ($\sigma$) along grain boundaries, causing severe room-temperature embrittlement. To dissolve the sigma phase and restore toughness, perform a remedial anneal at 1100°C to 1150°C, followed by a rapid water quench.
- Weld Zone Hot Cracking Prevention: Preheating is not necessary, and the interpass temperature should be kept below 150°C (300°F). Fully austenitic weld pools are susceptible to solidification cracking; to prevent hot cracking, the weld metal chemistry must be balanced to maintain a small volume of delta ferrite (3 to 10 FN) in the solidified weld structure. Specify low-carbon ER309L or matching ER309 filler metals.
