Alloy 309 (UNS S30900) is an austenitic stainless steel developed
for use in high temperature corrosion resistance applications. The
alloy resists oxidation up to 1900°F (1038°C) under non-cyclic
conditions. Frequent thermal cycling reduces oxidation resistance
to approximately 1850°F (1010°C).
Because of its high chromium and low nickel content, Alloy 309 can
be utilized in sulfur containing atmospheres up to 1832°F (1000°C).
The alloy is not recommended for use in highly carburizing
atmospheres since it exhibits only moderate resistance to carbon
absorption. Alloy 309 can be utilized in slightly oxidizing,
nitriding, cementing and thermal cycling applications, albeit, the
maximum service temperature must be reduced.
When heated between 1202 – 1742°F (650 – 950°C) the alloy is
subject to sigma phase precipitation. A solution annealing
treatment at 2012 – 2102°F (1100 – 1150°C) will restore a degree of
toughness.
309S (UNS S30908) is the low carbon version of the alloy. It is
utilized for ease of fabrication. 309H (UNS S30909) is a high
carbon modification developed for enhanced creep resistance. It
most instances the grain size and carbon content of the plate can
meet both the 309S and 309H requirements.
Alloy 309 can be easily welded and processed by standard shop
fabrication practices.
Corrosion Resistance
Wet Corrosion
Alloy 309 is not designed for service in wet corrosive
environments. The high carbon content, which is present to enhance
creep properties, has a detrimental effect on aqueous corrosion
resistance. The alloy is prone to intergranular corrosion after
long term exposure at high temperatures. However, due to its high
chromium content (23%), Alloy 309 is more corrosion resistant than
most heat resistant alloys.
High Temperature Corrosion
Alloy 309 resists high temperature corrosion in most in-service
conditions. Operating temperatures are as follows:
Oxidizing conditions (max. sulfur content – 2 g/m3)
1922°F (1050°C) continuous service
2012°F (1100°C) peak temperature
Oxidizing conditions (max. sulfur greater than 2 g/m3)
1742°F (950°C) maximum temperature
Low oxygen atmosphere (max. sulfur content – 2 g/m3)
1832°F (1000°C) maximum temperature
Nitriding or carburizing atmospheres
1562 –1742°F (850 – 950°C) maximum
The alloy does not perform as well as Alloy 600 (UNS N06600) or
Alloy 800 (UNS N08800) in reducing, nitriding or carburizing
atmospheres, but it does outperform most heat resistant stainless
steels in these conditions.
Creep Properties
Typical Creep Properties
| Temperature | Creep Strain (MPa) | Creep Rapture (MPa) |
|---|
| °C | °F | 1000 H | 10000 H | 100000 H | 1000 H | 10000 H | 100000 H |
|---|
| 600 | 1112 | 120 | 80 | 40 | 190 | 120 | 65 |
| 700 | 1292 | 50 | 25 | 20 | 75 | 36 | 16 |
| 800 | 1472 | 20 | 10 | 8 | 35 | 18 | 7.5 |
| 900 | 1652 | 8 | 4 | 3 | 15 | 8.5 | 3 |
| 1000 | 1832 | 4 | 2.5 | 1.5 | 8 | 4 | 1.5 |
Chemical Analysis
Weight % (all values are maximum unless a range is otherwise
indicated)
| Element | 309 | 309S | 309H |
|---|
| Chromium | 22.0 min.-24.0 max. | 22.0 min.-24.0 max. | 22.0 min.-24.0 max. |
| Nickel | 12.0 min.-15.0 max. | 12.0 min.-15.0 max. | 12.0 min.-15.0 max. |
| Carbon | 0.20 | 0.08 | 0.04 min.-0.10 max. |
| Manganese | 2.00 | 2.00 | 2.00 |
| Phosphorus | 0.045 | 0.045 | 0.045 |
| Sulfer | 0.030 | 0.030 | 0.030 |
| Silicon | 0.75 | 0.75 | 0.75 |
| Iron | Balance | Balance | Balance |
Physical Properties
Density
0.285 lbs/in3
7.89 g/cm3Specific Heat
0.12 BTU/lb-°F (32 – 212°F)
502 J/kg-°K (0 – 100°C)Modulus of Elasticity
28.5 x 106 psi
193 GPaThermal Conductivity 212°F (100°C)
9.0 BTU/hr/ft2/ft/°F
15.6 W/m-°KMelting Range
2500 – 2590°F
1480 – 1530°CElectrical Resistivity
30.7 Microhm-in at 68°C
78 Microhm-cm at 20°CMechanical Properties
Typical Values at 68°F (20°C)
Yield Strength
0.2% Offset | Ultimate Tensile
Strength | Elongation
in 2 in. | Hardness |
|---|
| psi (min.) | (MPa) | psi (min.) | (MPa) | % (min.) | (max.) |
|---|
| 45,000 | 310 | 85,000 | 586 | 50 | 202 (HBN) |
Fabrication Data
Alloy 309 can be easily welded and processed by standard shop
fabrication practices.
Hot Forming
Heat uniformly at 1742 – 2192°F (950 – 1200°C). After hot forming a
final anneal at 1832 – 2101°F (1000 – 1150°C) followed by rapid
quenching is recommended.
Cold Forming
The alloy is quite ductile and forms in a manner very similar to
316. Cold forming of pieces with long-term exposure to high
temperatures is not recommended since the alloy is subject to
carbide precipitation and sigma phase precipitants.
Welding
Alloy 309 can be readily welded by most standard processes
including TIG, PLASMA, MIG, SMAW, SAW and FCAW.
