Both austenitic and ferritic stainless steel are used in various industrial applications, from automotive to aerospace and food processing. However, did you know there’s another material that combines the properties of both stainless steels? It’s called duplex stainless steel, which offers better strength and high corrosion resistance to easily withstand harsh conditions.
Keep reading to learn more about duplex stainless steel grades, their benefits, history, and more!
What is Duplex Stainless Steel?
Duplex stainless steel is distinguished by its microstructure, which usually contains 50% austenite (face-centered cubic) and 50% ferrite (body-centered cubic). However, this ratio may vary between different duplex stainless steel grades.
The chromium content in this material is quite high compared to traditional stainless steel, standing at 20% to 28%. That’s why it provides better corrosion resistance, particularly when it comes to chloride pitting.
Brief History of Duplex Stainless Steel
If we look at the history of duplex stainless steel, the first combination of austenitic and ferritic phases was developed back in 1939 by Avesta Ironworks in Sweden. This material offered good corrosion resistance and strength but had limited applications.
So, the innovations continued, and in the 1960s, the concept of decarburization emerged, which made it possible to create steel with low levels of carbon. Then, in the 1980s, Super and Hyper duplex stainless steel came into existence, which were way stronger than the traditional duplex 2205 grade.
Duplex Stainless Steel Grades
Here’s an overview of the duplex stainless steel grades, along with their unique applications and characteristics:
Duplex 2205
Duplex 2205 is one of the duplex stainless steel grades that comprise austenitic, ferritic, 22% chromium, 3% molybdenum, and 5% to 6% nickel.
It’s one of the most commonly used grades and has high yield strength, which is almost double that of austenitic stainless steel. Some common applications are in chemical processing, marine conditions, and the oil and gas industry.
Duplex 2507
Duplex 2507 has around 24% to 26% chromium, 3% to 5% molybdenum, and 6% to 8% nickel. It has high compressive strength and offers incredible resistance to pitting and corrosion.
On top of this, the 2507 stainless steel grade is known to have a low thermal expansion coefficient and high thermal conductivity. It’s typically used in the pulp and paper industry, seawater desalination equipment, and the oil and gas industry.
Duplex 2101
Another duplex stainless steel grade is duplex 2101, which is low-nickel and nitrogen-enhanced. It has around 21% to 22% chromium and 0.1% to 0.8% molybdenum.
This grade also has equal amounts of ferrite and austenite and was developed for general-purpose use. Common uses of duplex 2101 are bridges, flood gates, storage tanks, and water heaters.
Duplex 2304
Duplex 2304 has 23% chromium, 4% nickel, and is molybdenum-free. It has good corrosion resistance properties similar to those of austenitic stainless steel grade 316L.
Not only this but its mechanical characteristics, such as the yield strength, are also double that of the austenitic grades 304 and 316. It’s suitable for the pulp and paper industry, coal handling, potash, and food and beverage processing.
Duplex 2760
One of the common duplex stainless steel grades is duplex 2760, which is known to have 24% to 26% chromium, 6% to 8% nickel, 3% to 4% molybdenum, and 0.2% to 0.3% nitrogen.
It provides advanced pitting and crevice corrosion resistance, which makes it suitable for use in different saline and other harsh environments.
Duplex 2760, which resembles S32760, is commonly used in chemical, marine equipment, oil and gas, and petrochemical production.
Chemical Composition of Duplex Stainless Steel Grades
Here’s an overview of the chemical composition of two different grades or alloys of duplex stainless steel:
| Duplex Stainless Steel Grades | C | Si | Mn | Cr | Ni | S | P | Mo | N |
| Duplex 2205 | 0.030 | 1.00 | 2.00 | 22.0-23.0 | 4.5-6.5 | 0.020 | 0.030 | 3.0-3.5 | 014-0.20 |
| Duplex 2507 | 0.030 | 0.80 | 1.20 | 24.0-26.0 | 6.0-8.0 | 0.020 | 0.035 | 3.0-5.0 | 0.24-0.32 |
Benefits of Duplex Stainless Steel
Well, duplex stainless steel grades constitute a small portion of the stainless steel market; it has a range of benefits, such as:
Improved Strength
Firstly, duplex stainless steel grades have a dual microstructure comprising both austenitic and ferritic phases. Therefore, the strength it offers is also way higher than either phase alone. It is more formable under pressure than ferritic stainless steel but not as much as austenitic material. However, you can still use this material in a variety of applications without worrying about cracking or damage.
Affordability
Another benefit of duplex stainless steel is its affordability. But what makes it cheaper than other options like austenitic stainless steel? The answer is that it requires much lower levels of nickel and molybdenum compared to other types of stainless steel.
Since both these metals are expensive, their smaller quantities automatically result in lower costs. That’s not all; the high strength of this steel enables you to use thin sheets in different applications, which further makes the material cheap.
More Corrosion Resistance
One of the most impressive aspects of duplex stainless steel is that it offers exceptional corrosion resistance. It’s primarily due to the presence of high levels of chromium, molybdenum, and nitrogen that form a protective oxide layer upon the steel’s surface. This reduces the risk of corrosion and scaling, ensuring the material performs well in all conditions.
Although duplex stainless steel has many advantages, it’s not without its share of downsides. For instance, they have very poor impact toughness at low temperatures and often develop seam phases during welding.
Mechanical Properties of Duplex Stainless Steel Grades
Here’s an overview of the mechanical properties of two different duplex stainless steel grades:
| Duplex Stainless Steel Grades | Tensile Strength Rm(MPa) | Yield Strength RPO.2N/MPa | Elongation 85(%) |
| 2205 | ≥620 | ≥450 | ≥25 |
| 2507 | ≥800 | ≥550 | ≥25 |
Applications of Duplex Stainless Steel
The applications of duplex stainless steel aren’t limited to one industry or process but are widespread. Let’s look at a few:
- Suitable for oil drilling, desalination, and water treatments
- In chemical labs or processing, it’s used to handle corrosive substances
- Production of pollution control equipment
- Part of demanding offshore applications such as pipelines and storage tanks
- Used in pulp and paper manufacturing
Duplex Vs Super Duplex Stainless Steel
Many people often confuse duplex and super duplex stainless steel grades. Well, super duplex stainless steel actually has the properties of typical duplex steel but features increased chromium, molybdenum, and nitrogen content.
For instance, it has an alloying addition of 25% chromium, which is quite high compared to the addition of 22% chromium in standard duplex grades.
Simple duplex steel is easier to machine, while super duplex stainless steel requires high precision, which may slow down the overall machining speed. However, both types need preheating and post-weld treatments to eliminate chromium carbide precipitation.
Conclusion
Duplex stainless steel is useful if you want a reliable material for your industrial applications without the worry of intergranular corrosion, which is common in ferritic stainless steel. Wondering where you can get excellent-quality duplex stainless steel grades? Jianglin is the ultimate one-stop shop. Contact us today to get a variety of stainless steel that is affordable and offers superior strength!
