Selection and Heat Treatment of Duplex Stainless Steel
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December 22, 2025
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Technical Guide
Duplex stainless steel has gained widespread attention and application due to its superior performance characteristics. Selecting the appropriate grade and heat treatment process based on specific environmental conditions and performance requirements is essential for optimal results.
Table of Contents
- Overview of Duplex Stainless Steel
- Classification of Duplex Stainless Steel
- Phase Characteristics and Heat Treatment Transformation
- Corrosion Resistance in Various Media
- Selection Guidelines Based on Environmental Conditions
- Practical Applications and Case Studies
- Frequently Asked Questions
- Conclusion
Overview of Duplex Stainless Steel
Duplex stainless steel represents a significant branch of the stainless steel family, first discovered and applied in the 1930s. One of its defining characteristics is excellent resistance to localized corrosion. As corrosion failure incidents with conventional stainless steels increased during practical applications, developing new duplex stainless steel grades received considerable attention worldwide.
Since the 1980s, the development and application of duplex stainless steel has accelerated dramatically. It has become a major category of stainless steel, standing alongside austenitic, ferritic, and martensitic types.
Definition: Duplex stainless steel refers to stainless steel grades whose solution-treated microstructure primarily contains both ferrite and austenite phases, with each phase comprising a minimum of 30% of the total metallurgical structure.
Through proper control of chemical composition and heat treatment processes, duplex stainless steel combines the toughness and weldability of austenitic steel with the strength and stress corrosion resistance of ferritic steel.
Key Advantages of Duplex Stainless Steel
- Excellent pitting corrosion resistance – Superior protection against localized attack in chloride environments
- Outstanding chloride stress corrosion cracking resistance – Significantly better than austenitic grades
- Good corrosion fatigue and erosion-corrosion resistance – Ideal for dynamic loading conditions
- Excellent overall mechanical properties – Higher strength than austenitic stainless steels
Classification of Duplex Stainless Steel
Based on their development history and alloying content, duplex stainless steels are primarily divided into four categories:
| Category | Alloy Content | Typical Applications |
|---|---|---|
| Low Alloy Type | Lower Cr, Mo content | Mild corrosive environments, general structural applications |
| Medium Alloy Type | Moderate Cr, Mo, N content | Chemical processing, offshore platforms |
| High Alloy Type | Higher Cr, Mo, N content | Aggressive chloride environments, seawater systems |
| Super Duplex | Highest Cr, Mo, N content (PREN > 40) | Most demanding applications, hot seawater, aggressive chemicals |
These duplex stainless steel grades provide different levels of corrosion resistance in various environmental media. Generally, as alloy content increases and smelting technology improves—particularly with reduced carbon content and lower inclusion levels—the corrosion resistance of duplex stainless steel becomes progressively stronger.
Phase Characteristics and Heat Treatment Transformation
Dual-Phase Structure
In the solution-treated microstructure of duplex stainless steel, both ferrite (α) and austenite (γ) phases are present, with the proportions of both phases being relatively close. However, the same duplex stainless steel grade will exhibit different phase proportions at different temperatures.
Element Distribution: Under isothermal aging conditions, alloying elements distribute differently between the α and γ phases. The α phase concentrates ferrite-forming elements (Cr, Mo), while the γ phase concentrates austenite-forming elements (Ni, N). During heat treatment or hot forging, as temperature increases, element distribution between the two phases becomes more uniform.
Secondary Phase Precipitation
The ferrite phase at high temperatures is inherently unstable. During isothermal aging below certain temperatures or during improper heat treatment, secondary precipitate phases may form, including:
- Secondary austenite (γ₂)
- Carbides and nitrides (M₂₃C₆, Cr₂N)
- Intermetallic phases (σ phase, χ phase)
⚠️ Important: These secondary precipitate phases significantly impact mechanical properties and corrosion resistance. Carbide precipitation causes chromium depletion at grain boundaries, leading to intergranular corrosion. Intermetallic phases are brittle and affect both mechanical properties and corrosion resistance. Therefore, these phase precipitations should be minimized or avoided.
Effect of Heating Temperature and Cooling Rate
Research has demonstrated that after solution treatment at 1000-1100°C, the maximum σ phase precipitation varies little with heating temperature. However, changing the cooling method produces significantly different results.
| Cooling Method | σ Phase Precipitation | Effect on Properties |
|---|---|---|
| Water Quenching | Minimal to none | Optimal corrosion resistance |
| Air Cooling | Moderate precipitation possible | Reduced corrosion resistance |
| Furnace Cooling | Significant precipitation | Severely degraded properties |
This demonstrates that both heating temperature and cooling method have significant effects on σ phase precipitation quantity, which in turn has a major impact on the performance of duplex stainless steel.
Corrosion Resistance in Various Media
Duplex stainless steel is primarily used in environments requiring resistance to localized corrosion. Its uniform corrosion resistance depends on the stability of the passive film in the corrosive environment. The corrosion potential of duplex stainless steel lies within the passivation region of both phases, and the passivation of both phases provides excellent corrosion resistance.
Performance in Acidic Solutions
Duplex stainless steel demonstrates strong corrosion resistance in mild reducing acids, such as dilute sulfuric acid and dilute hydrochloric acid. Through experimental research, experts have developed iso-corrosion diagrams for several typical duplex stainless steels in sulfuric acid environments.
✓ Excellent Performance:
- Dilute sulfuric acid and dilute hydrochloric acid
- Organic acids (formic acid, acetic acid)
- Petrochemical acidic environments
✗ Poor Performance:
- Concentrated sulfuric acid
- Concentrated nitric acid and other strong oxidizing acids
Performance in Alkaline Solutions
Through testing in alkaline solutions, duplex stainless steel demonstrates corrosion resistance only in low-concentration alkaline solutions. As the concentration of alkaline solutions increases, the corrosion rate of duplex stainless steel accelerates. In low-concentration alkaline solutions, duplex stainless steel shows superior corrosion resistance.
Comparative Corrosion Data
Research institutions have conducted comparative corrosion data studies between duplex stainless steel and austenitic stainless steel in acid-alkaline solutions. The results clearly demonstrate the superior corrosion resistance of duplex stainless steel compared to conventional stainless steels.
| Test Condition | Austenitic SS | Duplex SS |
|---|---|---|
| 5% H₂SO₄, Boiling | Higher corrosion rate | Lower corrosion rate |
| 10% HCl, 60°C | Higher corrosion rate | Lower corrosion rate |
| 20% NaOH, Boiling | Moderate resistance | Better resistance |
| Chloride SCC Test | Susceptible | Highly resistant |
Selection Guidelines Based on Environmental Conditions
Due to the special properties that duplex stainless steel possesses—which conventional stainless steels do not have—it has gained widespread application. However, as an iron-based material, duplex stainless steel cannot meet performance requirements in strongly acidic or strongly alkaline environments.
Key Insight: Duplex stainless steel truly demonstrates its advantages in various mild acids and bases, low-corrosion salt ion environments, and various corrosive gas atmospheres, showing significant differences from conventional stainless steels. It is particularly suitable for components used in environments with relatively complex media.
Recommended Application Areas
Neutral Chloride Environments
Seawater systems, brackish water handling, cooling water applications
Petroleum Refining
Distillation columns, heat exchangers, piping systems
Chemical Industry
Reactors, storage tanks, process equipment
Oil & Gas Industry
Offshore platforms, pipelines, downhole equipment
Pulp & Paper Industry
Digesters, bleach plant equipment, washers
Fertilizer Industry
Ammonia and urea production equipment
Heat Treatment Considerations
Different duplex stainless steels have varying corrosion resistance properties, and different heat treatment regimes also significantly affect their performance. The ratio of the two-phase structure after heat treatment particularly impacts corrosion resistance.
For some components requiring specific mechanical properties, different isothermal aging temperatures may be needed to achieve the required service conditions. However, in the post-heat treatment service state, it is essential to maintain an appropriate dual-phase structure ratio; otherwise, corrosion resistance will be significantly reduced.
Selection Recommendations:
- For ordinary acid rain, seawater corrosion, and low-concentration acid-alkaline gas corrosion, use medium to low alloy type duplex stainless steel to meet requirements while reducing costs
- When higher mechanical properties are required, use different isothermal aging treatment temperatures to meet service requirements
Case Studies
Case 1: Pusher Centrifuge Basket for Salt Production
| Environment: | High-concentration brine, continuous operation at elevated temperature |
| Material: | Super duplex stainless steel (PREN > 40) |
| Heat Treatment: | Solution annealing + controlled cooling |
| Result: | 3× service life compared to 316L predecessor; zero unplanned downtime over 18-month evaluation |
Case 2: Decanter Centrifuge Screen for Mining Slurry
| Environment: | Ite particles, acidic slurry (pH 3–4) |
| Material: | High alloy duplex stainless steel |
| Heat Treatment: | Full solution anneal with rapid quench |
| Result: | Significant reduction in pitting corrosion; sustained dimensional accuracy over extended cycles |
Case 3: Centrifuge Screen for Chloride Resistance
| Environment: | Chloride ions, mildly alkaline environment |
| Material: | Medium alloy duplex stainless steel |
| Heat Treatment: | Solution treatment |
| Result: | Longer service life in equivalent media compared to austenitic grades; multiple reorders |
Case 4: Filtration Basket for Pharmaceutical Use
| Environment: | Frequent CIP/SIP cycles, chlorinated cleaning agents |
| Material: | Lean duplex stainless steel with Ra ≤ 0.8 µm finish |
| Heat Treatment: | Low-temperature stress relief to preserve surface finish |
| Result: | Passed FDA surface compliance; validated for repeated sanitation without degradation |
Frequently Asked Questions
What is duplex stainless steel?
Duplex stainless steel is a type of stainless steel with a microstructure containing both ferrite and austenite phases, with each phase comprising at least 30% of the total structure. This dual-phase composition provides excellent corrosion resistance combined with high mechanical strength, offering advantages over both purely austenitic and ferritic grades.
What are the main advantages of duplex stainless steel?
The main advantages include excellent pitting corrosion resistance, superior chloride stress corrosion cracking resistance, good corrosion fatigue and erosion-corrosion resistance, and excellent overall mechanical properties. Duplex stainless steels typically offer about twice the yield strength of austenitic grades while maintaining good toughness and weldability.
What is the recommended heat treatment for duplex stainless steel?
Solution treatment is the primary heat treatment for duplex stainless steel, typically performed at temperatures between 1000°C and 1100°C, followed by rapid cooling (water quenching). This treatment dissolves any precipitated phases and ensures proper ferrite-austenite phase balance, resulting in optimal corrosion resistance and mechanical properties.
What industries commonly use duplex stainless steel?
Duplex stainless steel is widely used in offshore oil and gas platforms, chemical processing, petroleum refining, pulp and paper manufacturing, fertilizer production, desalination plants, and marine applications. It is particularly valuable in environments where resistance to chloride-induced stress corrosion cracking and pitting is critical.
Why is cooling rate important in duplex stainless steel heat treatment?
Cooling rate is crucial because slow cooling allows precipitation of harmful secondary phases such as sigma (σ) phase, chi (χ) phase, carbides, and nitrides. These precipitates significantly degrade both mechanical properties (causing embrittlement) and corrosion resistance (by depleting chromium from the matrix). Rapid cooling, typically by water quenching, prevents these precipitations and maintains the desired dual-phase microstructure.
Conclusion
- The superior performance of duplex stainless steel is gaining increasingly widespread attention and application across multiple industries worldwide.
- Duplex stainless steel is suitable for many industries, particularly defense, petroleum, chemical processing, pharmaceuticals, food processing, and marine applications.
- Selecting the appropriate duplex stainless steel grade based on environmental conditions is essential. The phase transformation characteristics of duplex stainless steel make proper heat treatment critically important. Only through proper material selection and heat treatment can the superior comprehensive performance of duplex stainless steel be fully realized.
FUSHUN METAL
Your trusted partner for specialty steel and alloy solutions. With decades of experience in metallurgical research and application development, we provide high-quality duplex stainless steel products tailored to your specific requirements.
