| Main Grade Classifications (Pyramid Structure) |
| Category |
Representative Grade |
Typical Composition |
Corrosion Resistance (PREN Value) |
Main Applications |
| Lean Duplex |
2304 |
23Cr-4Ni |
24-26 |
Replaces 304/316 for applications requiring higher strength but less severe corrosion (e.g., food equipment, water storage tanks). |
| Standard Duplex |
2205 |
22Cr-5Ni-3Mo |
32-35 |
The most widely used workhorse grade. The top choice in chemical processing, shipbuilding, desalination, environmental FGD systems, etc. |
| Super Duplex |
2507 |
25Cr-7Ni-4Mo |
>40 |
Extremely high corrosion resistance for the most demanding environments (e.g., deep-sea oil & gas, offshore platforms, FGD systems). |
| Note: PREN stands for Pitting Resistance Equivalent Number; the higher the value, the stronger the resistance to chloride pitting corrosion. |
Core Advantages (Why Choose It?)
High Strength: This is its most prominent advantage. Its yield strength is typically twice that of common 304/316 stainless steel. This allows for reduced wall thickness in the design and manufacture of pressure vessels, storage tanks, or pipelines, thereby reducing weight and material costs.
Excellent Resistance to Stress Corrosion Cracking: Ordinary austenitic stainless steels (e.g., 304, 316) are prone to cracking when stressed in hot, humid environments containing chloride ions. Due to its ferrite content, duplex stainless steel has strong immunity to chloride-induced stress corrosion cracking.
Good Economy: Due to its high strength, duplex stainless steel (e.g., 2205) can often replace 316L stainless steel. Although the unit price might be slightly higher, the overall cost can be lower due to the reduced material required. At the same time, it is significantly cheaper than high-nickel alloys (e.g., Hastelloy).
Usage Limitations to Note
Although its performance is excellent, there are some points to consider during use:
Temperature Range: It is generally recommended for use between -50°C and 300°C. Prolonged operation at excessively high temperatures (>300°C) can lead to the precipitation of the harmful sigma (σ) phase, causing embrittlement. At very low temperatures, toughness decreases (especially for grades with higher ferrite content).
Processing Difficulty: Cold working (e.g., bending) results in greater springback and requires higher forming forces than in austenitic stainless steels.
Welding: Although modern duplex steels (especially nitrogen-containing 2205) have good weldability, strict control of heat input and interpass temperature during welding is necessary to ensure a balanced phase ratio in the weld metal and heat-affected zone, preventing an excess of one phase that could degrade properties.
Typical Application Scenarios
Chemical Industry: Heat exchangers, reactors (handling media containing chloride ions).
Marine Engineering: Seawater cooling piping, seawater pumps and valves, ship propellers.
Bridges & Construction: Reinforcement bars for cross-sea bridges, load-bearing structures in coastal buildings (leveraging high strength and chloride corrosion resistance).
Pulp & Paper: Digesters, bleaching equipment.
Selection Advice
If you are considering using duplex stainless steel, it’s usually best to start evaluating with 2205, as it offers the best combination of comprehensive performance, cost-effectiveness, and availability. If higher corrosion resistance or strength is required, consider upgrading to 2507; if the application is cost-sensitive and the environment is mild, 2304 could be a good choice.
