Duplex Stainless Steels 101
Duplex Stainless Steels are commonly used in offshore marine applications because of their superior chloride and pitting resistance.
There's something special about a good combination. Think mac and cheese, peanut butter and jelly, or even the sometimes controversial French fries and Wendy's Frosties. By combining the strengths of multiple things, you often elevate the entire experience. Today, we're going to cover a unique combo material in our industry - Duplex Stainless Steel. Let's get into it!
Types of Stainless Steel
One of the most popular materials in our investment casting foundry is stainless steel (SS). SS offers a broad range of benefits, including corrosion resistance, oxidation resistance, wear resistance, sharp aesthetics, and more. Within the larger umbrella of stainless steels, you may have heard of different categories such as 300 series stainless (which includes the super popular 304 Stainless). Today's blog focuses on Duplex Stainless Steel, which is a combination of the Austenitic and Ferritic families. Let's cover both of these families before getting deeper into Duplex.
Austenitic Stainless Steel (300 Series):
Is the most common type of stainless steel, known for its excellent corrosion resistance, formability, and non-magnetic properties. It contains chromium (Cr) and nickel (Ni), with a primary microstructure of austenite (a face-centered cubic crystal structure). Common grades include 304 and 316 (with added molybdenum for better corrosion resistance).
Ferritic Stainless Steel:
A magnetic, chromium-based stainless steel with low carbon and little to no nickel, offering moderate corrosion resistance, high resistance to stress corrosion cracking, and good formability. Its body-centered cubic (BCC) structure makes it less ductile than austenitic steels but more affordable. Common grades include 430 and 439.
What is Duplex Stainless Steel?
Duplex stainless steel combines Austenitic and Ferritic phases in its metallurgical structure in roughly a 50/50 mix. In simpler terms, the unique makeup that gives the 300 Series Stainless its characteristics and ferritic stainless its characteristics are combined to create a hybrid material featuring the strengths of both!
You may recognize the term "Duplex" from Duplex housing or apartments. In this case, two domiciles are combined into one building. Duplex stainless steel is called Duplex because it combines these two microstructures of stainless into a new material with unique characteristics. Let's talk more about what makes it unique.
Duplex stainless steel combines the strengths of austenitic and ferritic steels, offering double the strength, superior corrosion resistance, and better-stress-cracking resistance than these standard grades. Its 50/50 austenite-ferrite microstructure provides high durability in harsh environments (like seawater, oil & gas), while also using less nickel than austenitic steels, making it more cost-effective for demanding applications.
Key Advantages Over Other Stainless Steels:
✔ Higher strength (2x yield strength of 304)
✔ Excellent corrosion resistance (especially to chlorides & pitting)
✔ Good weldability & toughness (unlike ferritic steels)
✔ Cost savings (lower nickel content than austenitic)
Common Grades of Duplex Stainless
Here are some of our most commonly cast Duplex steels and a few usage scenarios for each:
Duplex 1A:
is lean duplex stainless steel (meaning it has a lower "leaner" amount of alloying materials like nickel or molybdenum). It is used for its high strength, cost efficiency, and good corrosion resistance.
Commonly used in:
Water Treatment: Piping, tanks, and filtration
Construction: Bridges, roofing, and structural components
Marine: Boat shafts and propellers, seawater systems, Splash zone structural components
Food Processing
Paper Processing
and more
Duplex 1B
Another lean stainless steel with a different makeup than 1A (it includes more nickel in its composition) for better corrosion resistance in moderate chemical/water environments.
Usage scenarios
Freshwater treatment plants
Storage tanks and piping systems for moderate corrosive environments
Bridge components in coastal areas
and more
Duplex 4A:
Duplex 4A is a super duplex stainless steel (which means it is a high-performance SS including a higher amount of alloying materials like chromium, molybdenum, nickel, etc.). Duplex 4A features ultra-high corrosion resistance and strength (2x stronger than 316 stainless), designed for extreme environments like seawater, offshore oil/gas, and chemical processing.
Usage Scenarios:
Offshore/oil & gas: Subsea pipelines, risers, valves.
Desalination plants: Reverse osmosis systems.
Chemical processing: parts with aggressive acid/chloride exposure.
Power plants: Flue gas scrubbers
Ferralium 255:
Another Super Duplex stainless steel that includes ~2% copper in its composition, which provides extreme corrosion resistance (sulfuric acid resistance) with superior mechanical strength and wear resistance. This gives it a long service life in very challenging environments.
Usage Scenarios:
Oil & gas: Subsea manifolds, pump shafts, downhole tools.
Marine: Propeller shafts, seawater piping.
Chemical: Reactors and instruments in sulfuric acid service.
Power Generation: FGD (flue gas desulfurization) systems.
Duplex 5A:
A duplex alloy surpassing standard and super duplex grades, with one of the highest PREN ratings of Duplex steels. PREN stands for Pitting Resistance Equivalent Number, which showcases its ability to resist pitting in corrosive environments (e.g. hydrochloric acid)
Duplex 5A has exceptional chloride resistance, engineered for ultra-demanding applications like deep-sea oil/gas and chemical processing. It can even serve as a replacement for superalloys in some instances.
Usage Scenarios:
Subsea choke valves (10,000+ psi pressure).
Geothermal brine handling (high-Cl⁻, H₂S, CO₂).
Nuclear waste containment.
Chemical reactors with mixed acids (HCl + H₂SO₄).
When to use Duplex Stainless Steel:
Duplex Steel is awesome! It's incredible the performance that can be obtained by mixing ferritic and austenitic segments. But, when should you use it? Like any precision casting project, it is crucial to consider your budget and the requirements of its environment.
Here are a few things to consider about Duplex:
When to Use Duplex SS?
Your part will be in a Chloride-rich Environment
Why? Resists pitting/crevice corrosion better than 316 or 304.
Your part finds itself in a high-stress / high-corrosion application
Why? 2x stronger than austenitics (e.g., 316 Stainless), reducing material thickness/cost.
Cost-Sensitive Projects Needing Nickel Reduction
Why? Lean duplex (e.g. Duplex 1A) cuts nickel use vs. 316 Stainless, saving 20–30% cost.
Need for abrasion & corrosion resistance combination
Example: Slurry pumps, mining equipment.
Why? The ferritic phase adds wear resistance (grades like Ferralium 255 include copper).
When to Avoid Duplex?
❌ Highly acidic (pH <2) or oxidizing environments (e.g., concentrated sulfuric acid – use a superalloy).
❌ Cryogenic temperatures (<–50°C) (ferritic phase becomes brittle).
❌ Simple, low-stress applications (don't go overboard when a simpler, more cost-effective alloy will get the job done).
Conclusion:
In this blog, we covered the basics of Duplex Stainless Steel.
The Duplex family combines the Ferritic and Austenitic microstructure families to create a unique type of stainless steel with a hybridization of their strengths. We also covered a few of the most common alloys of Duplex steel and some common usage scenarios.
If you're interested in casting with Duplex stainless steel or any of our materials, contact our team today! With more than 45 years of investment casting experience, IPC Foundry Group can help you pick the perfect alloy for your project and save you time and money throughout your next production cycle.
