Strength, Structure & Superiority
Duplex & super duplex stainless steels are high grade stainless steels superior to the more common stainless steels such as 304, 316, and 409. But what are they, and why are they superior? In this blog, we will briefly discuss the various stainless steel structures and what makes duplex and super duplex stronger, and more resistant materials.
The most common and economical type of stainless steel is austenitic stainless steel. It gets its name from its austenite crystal structure. At its most fundamental level, this structure has a face-centered cubic (FCC) arrangement. That is, its atomic bonds are formed between atoms located at the corners and the centers of each of the six faces of a cube, leaving the internal volume void. This shows that its unit structure is primarily external.
This external bonding structure, combined with its chemical composition, gives austenitic stainless steel excellent surface corrosion resistance, but it is relatively weaker in terms of mechanical strength.
Another common type of stainless steel is ferritic stainless steel, which is also named after its crystal structure. Ferritic steel has a ferrite (or body-centered cubic, BCC) structure. These structures contain a higher proportion of iron and are connected more internally.
In contrast to the face-centered structure of austenitic steel, ferritic steel has a cube-centered structure. Imagine redrawing the previous cube, this time replacing the six face-centered spheres with a single sphere in the center of the cube. If you visualize pulling one of the corner spheres with an imaginary tether (symbolized by a dashed line), the cube is less likely to deform because of the central sphere’s position and the tension in its internal bonds.
As a result, ferritic stainless steel has higher strength than austenitic steel. However, due to the lack of surface-bonded atoms, it is more susceptible to surface corrosion and thus less resistant to external environmental attacks.
Note: This discussion of strength primarily refers to yield strength and tensile strength. While ferritic stainless steel does offer corrosion resistance due to its chemical properties, we are not addressing other material properties such as toughness and ductility here.
Duplex stainless steel refers to its dual-phase structure, which is a combination of both austenitic and ferritic phases. You can visualize its structure as a blend of both external and internal bonding. By adding both face-centered and cube-centered atoms to our cube diagram, we illustrate how duplex and super duplex steels benefit from both types of structures.
This dual-phase configuration makes duplex stainless steels both stronger and more corrosion-resistant than either austenitic or ferritic stainless steels on their own. In the case of super duplex, the difference lies in its higher levels of alloying elements such as nickel and chromium, which further enhance both strength and corrosion resistance.
Below is a comparison of various stainless steel types, including specific examples of their strength and corrosion resistance. Duplex and super duplex stainless steels consistently offer significant advantages in both categories.
Comparison between Austenitic, Ferritic, & Duplex Stainless Steel | |||
Material | Corrosion Resistence: PREN | Yield Strength | Tensile Strength |
Austenitic 304 | 20 | 31 KSI | 73 KSI |
Austenitic 316 | 27 | 30 KSI | 75 KSI |
Ferritic 430 | 18 | 40 KSI | 65 KSI |
Duplex 2205 | 36 | ~ 73 KSI | ~ 103 KSI |
Super Duplex 2507 | 43 | 80 KSI | 116 KSI |
After examining these values, it’s clear that 2205 duplex and 2507 super duplex are superior stainless steel options compared to austenitic (304, 316) and ferritic alloys particularly in terms of corrosion and mechanical strength.
We also offer other advanced duplex stainless steels such as Ferralium® 255 or Zeron® 100. Want to learn more? Explore details about PREN (Pitting Resistance Equivalent Number) and how it can guide your material selection.
Reach out to us and get a quotation on our Stainless Steel Duplex materials today.
