Duplex Stainless Steel: An Overview
Duplex stainless steel has had a rich history since 1927. It's gone through three main stages. Each stage has unique features. Let's explore its journey and benefits.
History of Duplex Stainless Steel
First Generation
The start was with AISI 329 steel. It has lots of chromium and molybdenum. This means it's good against some types of rust. However, it has a high carbon content (0.1%). This can make welding tricky. Welded parts can be less strong. Their rust resistance can also go down. So, it's often used without welding.
Second Generation
Things changed with a new process called argon-oxygen decarburization. This brought in nitrogen-rich duplex steels. They have very little carbon. They also have elements like molybdenum, copper, or silicon. These help them resist rust. A well-known type is SAF2205.
Third Generation
Then came the super duplex steels. Names like SAF2507, UR52N+, and Zeron100 are in this group. They have even less carbon and more of other elements. Around 40-45% is a type of iron called ferrite. This mix is great against certain rust.
Organization and Properties
Duplex stainless steel is special. It has two main parts: ferrite and austenite. They're almost equal. But one should be more than 30%. This mix has many benefits.
Chemical Composition of Duplex Stainless Steels
| Comparison of typical chemical compositions of selected austenitic and duplex stainless steels (approximate values) |
| Categories | Steel grades | Chemical composition % | PREN |
| C | Cr | Ni | Mo | N |
|
| Austenitic | 304L | 0.03 | 19 | 12 | ---- | 0.1 | 21 |
| 316L | 0.03 | 18 | 12 | 2 | ---- | 25 |
| Duplex Stainless Steel | 2205 | 0.03 | 22 | 5 | 3 | 0.15 | 34 |
| 255 | 0.03 | 25 | 6 | 3 | 0.2 | 38 |
| 2507 | 0.03 | 25 | 7 | 4 | 0.25 | 42 |
Studies show the best mix is 40-60% ferrite and 60-40% austenite. Other things can change steel's strength and stretchiness.
Mechanical Properties
| Room temperature mechanical properties of typical duplex stainless steel grades |
| Steel grades | Standard
grades | σ0.2/MPa | σb/Mpa | δ5/% | AK/J |
| AISI304 | UNS S30400 | 210 | 515～690 | 45 | ＞300 |
| AISI430 | UNS S43000 | 205 | 450 | 20 |
|
| SAF2304 | UNS S32304 | 400 | 600～820 | 25 | 300 |
| SAF2205 | UNS S31803 | 450 | 680～880 | 25 | 250 |
| SAF2507 | UNS S32750 | 550 | 800～1000 | 25 | 230 |
Corrosion Resistance
Duplex steel is tough against rust. It stands up to different rust types. Even the joints are strong against rust. It's also good against wear. Many examples show it's good against two rust types: intergranular and uniform.
Limitations
It's not perfect, though. It has some downsides compared to another type called austenitic. It works best in a certain temperature range. It's also a bit less bendy. And there's a certain temperature where it can become brittle.
Conclusion
Duplex stainless steel has grown a lot in a century. It's used in many industries now. It's great for strength and rust resistance. But, knowing its limits is key. The right choices can make it work best.