General Properties

Alloy 309 (UNS S30900) is an austenitic stainless steel specifically developed for high-temperature corrosion resistance applications. Here are some key points about Alloy 309:Alloy 309 exhibits good resistance to oxidation at high temperatures. It can withstand oxidation up to 1900°F (1038°C) under non-cyclic conditions. However, frequent thermal cycling can reduce its oxidation resistance to approximately 1850°F (1010°C).Sulfur-containing atmospheres: Due to its high chromium and low nickel content, Alloy 309 can be used in environments with sulfur content up to 1832°F (1000°C). This makes it suitable for applications where sulfur-containing atmospheres are present.Carburizing atmospheres: Alloy 309 is not recommended for use in highly carburizing atmospheres as it exhibits only moderate resistance to carbon absorption. It may not provide sufficient protection against carbon diffusion and subsequent carburization.Applications: Alloy 309 can be utilized in slightly oxidizing, nitriding, cementing, and thermal cycling applications. However, the maximum service temperature must be reduced in these applications compared to non-cycling conditions.When heated between 1202 – 1742°F (650 – 950°C), Alloy 309 is susceptible to sigma phase precipitation. This can result in reduced toughness and mechanical properties. To restore toughness, a solution annealing treatment at 2012 – 2102°F (1100 – 1150°C) is recommended.

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 specifically designed for service in wet corrosive environments. The high carbon content in Alloy 309, which is added to enhance creep properties, can have a detrimental effect on its resistance to aqueous corrosion. Here are some additional points regarding its corrosion resistance: The high carbon content in Alloy 309 can make it more susceptible to corrosion in aqueous environments. Prolonged exposure to high temperatures can lead to intergranular corrosion in this alloy.Corrosion resistance compared to heat-resistant alloys: Despite its limitations in wet corrosive environments, Alloy 309 offers better corrosion resistance than many other heat-resistant alloys. This is primarily due to its high chromium content, which is 23% in Alloy 309.

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

Chemical Analysis

Weight % (all values are maximum unless a range is otherwise indicated)

Mechanical Properties

Typical Values at 68°F (20°C)