Understanding Duplex Stainless Steels
Duplex stainless steels are a unique class of materials characterized by a two-phase microstructure, consisting of both austenite and ferrite phases. This dual-phase composition endows them with a blend of excellent corrosion resistance and high strength, making them invaluable across various industries. From oil and gas to chemical processing and marine engineering, duplex stainless steels have become a staple due to their durability and reliability.
The history of duplex stainless steels dates back to the early 20th century, but it was not until the 1970s and 1980s that significant advancements were made. The development of new alloy compositions and improved manufacturing processes led to the creation of widely used alloys like 2205 (UNS S31803 / S32205). This alloy is known for its good corrosion resistance, high strength, and moderate cost, striking a balance that makes it a popular choice over traditional austenitic stainless steels.
The Rise of Super Duplex Stainless Steels
Building on the foundation of duplex stainless steels, super duplex stainless steels emerged, offering even greater performance. These alloys typically contain higher levels of chromium (around 25%), along with increased amounts of molybdenum and nitrogen. This enhanced composition results in superior corrosion resistance, particularly in aggressive environments like offshore oil and gas production. Furthermore, super duplex alloys exhibit improved mechanical properties, including higher tensile and yield strength, as well as better resistance to stress corrosion cracking and pitting corrosion.
Introducing Alloy F255
Among the super duplex stainless steels, Alloy F255 (UNS S32550) stands out as a material designed to meet the rigorous demands of highly corrosive environments. Alloy F255 combines a balanced mix of chromium, molybdenum, and nitrogen, augmented by other alloying elements such as nickel and copper, to deliver exceptional performance.
A unique aspect of Alloy F255 is the addition of copper, which enhances its corrosion resistance by inhibiting the growth of corrosion pits through the redeposition of copper at active pit sites. This feature is particularly beneficial in prolonging the material’s lifespan in harsh conditions.
Another significant advantage of Alloy F255 is its reduced susceptibility to sigma phase formation. The sigma phase is a brittle intermetallic compound that can form when stainless steels are exposed to temperatures between 1050°F and 1800°F. The unique chemistry of Alloy F255 slows down the formation of the sigma phase, enhancing the material’s durability and maintaining its mechanical properties even in high-temperature applications.
An Outstanding Choice Versus 300 Series and Standard Duplex Stainless Steels
F255 is an excellent choice for applications requiring higher corrosion resistance and strength than 300 series stainless steels and standard duplex stainless steels can offer. The dual microstructure offers the higher strength from the ferritic phase and the corrosion resistance and toughness from the austenitic phase. Its modified chemistry with the increased amount of copper makes it unique among other super duplexes with regard to inclusion mitigation and corrosion resistance.
| Property | F255 | 2507 | 2205 | 316 |
| UNS Number | S32550 | S32750 | S32205 | S31600 |
| Chemical Composition (Fe balance) | ||||
| %Cr | 24 - 27 | 24-26 | 22 - 23 | 16 - 18 |
| %Ni | 4.5 - 6.5 | 6.0-8.0 | 4.5- 6.5 | 10 - 14 |
| %Mo | 2.9 - 3.9 | 3.0-5.0 | 3.0 - 3.5 | 2.0 - 3.0 |
| %Cu | 1.5 - 2.5 | 0.50 (max) | - | - |
| %N | 0.10 - 0.25 | 0.24-0.32 | 0.14 - 0.20 | 0.10 (max) |
| %Si (max) | 1 | 0.8 | ≤ 1.0 | 1 |
| %Mn (max) | 1.5 | 1.2 | 2 | 2 |
| %P (max) | 0.04 | 0.035 | 0.03 | 0.045 |
| %S (max) | 0.03 | 0.02 | 0.015 | 0.03 |
| %C (max) | 0.04 | 0.03 | 0.03 | 0.08 |
| Minimum Mechanical Strength | ||||
| Tensile Strength | 110 | 110 | 95 ksi | 75 ksi |
| 0.2 % Yield Strength | 80 | 75 | 65 ksi | 30 ksi |
| Elongation | 15 | 15 | 25 | 30 |
| Hardness (max) | 297 | 310 | 290 HB | 217 HB |
| PREN* | 40 | 41.4 | 35.1 | 24.6 |
*PREN = Cr% + 3.3 * (Mo% +0.5 * W%) + 16 * N% (calculated value is based on nominal composition from manufacturers data sheets. PREN is not a guarantee of corrosion performance and doesn’t take all factors and elements into consideration)
Applications and Advantages of Alloy F255
Alloy F255 excels in environments where chloride-induced corrosion is a major concern. Its excellent resistance to such corrosion makes it particularly suitable for use in marine environments, offshore oil and gas extraction, and chemical processing facilities. The alloy’s outstanding performance in these challenging settings ensures reliability and longevity, reducing maintenance costs and downtime.
The evolution from conventional duplex stainless steels to advanced super duplex alloys like alloy F255 represents a significant leap in material science. Alloy F255 exemplifies this progression with its superior corrosion resistance, enhanced mechanical strength, and overall reliability in demanding conditions. As industries continue to seek materials that can withstand harsh environments and deliver consistent performance, Alloy F255 emerges as a superior choice, pushing the boundaries of what duplex stainless steels can achieve.
EXPLORE ALLOY F255
To learn more about alloy F255, we invite you to download our datasheet. If you have any questions or need further information about how alloy F255 can meet your requirements, please fill out the form below to contact one of our knowledgeable sales representatives. We’re here to help you find the best corrosion-resistant solutions to fit your specific needs.