Stainless Steel: The Basics

Comparing Duplex Stainless Steels: Grades, Properties, Selection Guidelines

When comparing duplex stainless steels, understanding the different grades, their properties, and selection guidelines is crucial.

This stainless steel class offers a unique combination of strength and corrosion resistance that surpasses austenitic and ferritic grades.

In this post, we delve into an overview of duplex stainless steels, exploring their composition and how they differ from other types.

We'll also highlight factors influencing grade selection, such as application requirements, cost considerations, and availability.
We'll further discuss the mechanical properties of various duplex grades.

A key focus will be on the corrosion resistance offered by these materials in different environments, including resistance to stress corrosion cracking.

Lastly, we provide comprehensive selection guidelines for duplex stainless steels aimed at assisting engineers and corporate buyers make informed decisions when choosing these materials for specific applications in different industries.

Overview of Duplex Stainless Steels

Duplex stainless steels are a type of stainless steel that combine the beneficial properties of austenitic and ferritic stainless steels.

The name "duplex" originates from the dual-phase microstructure of these materials, comprised of approximately equal parts austenite and ferrite.

Duplex stainless steels were developed to overcome some limitations of traditional stainless steel grades, such as the popular 304 and 316 types.
Compared to these conventional stainless steels, duplex stainless steels exhibit superior mechanical properties—especially regarding strength and toughness.

They also have improved resistance to stress corrosion cracking, a common issue with standard austenitic stainless steels when used in certain aggressive environments.

The enhanced corrosion resistance of duplex steels is primarily due to their higher chromium, molybdenum, and nitrogen content.

Furthermore, duplex stainless steels offer better price stability as they have lower nickel content than austenitic grades, making them less sensitive to market fluctuations in nickel prices.

Types Of Duplex Stainless Steels

  • Lean Duplex (PREN 22-27): Lean duplex stainless steels are a subset of duplex stainless steels that are formulated with lower amounts of expensive alloying elements, like nickel and molybdenum, while still maintaining good corrosion resistance and higher strength than conventional austenitic stainless steels. This makes them a more cost-effective option for specific applications. Popular lean duplex stainless steel grades include LDX 2101 (UNS S32101), 2304 (UNS S32304), and 2003 (UNS S32003.) 
  • Standard Duplex (PREN 28-38): Standard duplex stainless steels are a group of stainless steels with a two-phase microstructure of roughly equal proportions of ferrite and austenite. Grade 2205 (UNS S32205) is a popular example of standard duplex stainless steel grades.
  • Super Duplex (PREN 38-45): Super duplex stainless steels are a subgroup that exhibits even higher corrosion resistance and mechanical strength. They have a higher alloy content, particularly in chromium, molybdenum, and nitrogen. Popular super duplex stainless steel grades include F53 (UNS S32750) and F55 (UNS S32760.)
  • Hyper Duplex (PREN >45): Hyper duplex stainless steels offer maximum resistance and strength with 2707 (UNS S32707) showing PRENs for 49-50. Other specialized alloys can reach numbers as high as 53

The Role Of Nitrogen In Duplex Stainless Steel

Nitrogen plays a vital role in the creation of duplex stainless steel alloys. Four key areas where nitrogen provides benefits include:

  1. Austenite Stabilization: Nitrogen is an alloying element that helps stabilize the austenite phase in duplex stainless steels. Compared to ferrite alone, this phase improves strength, toughness, and corrosion resistance, as noted at Outokumpu.
  2. Corrosion Resistance: Nitrogen enhances the resistance of duplex stainless steels to various forms of corrosion, such as pitting and crevice corrosion. Nitrogen promotes the formation of stainless steel's renowned passive protective film on the surface, which helps resist attack from corrosive environments.
  3. Strength and Toughness: Nitrogen strengthens the alloy matrix by solid solution strengthening, which improves the mechanical properties of duplex stainless steels. It enhances the material's yield strength, tensile strength, and impact toughness, making it suitable for demanding structural applications.
  4. Cost-Effective Additions: Nitrogen alloying offers a cost-effective alternative to increasing the alloy's performance compared to other alloying elements like nickel or molybdenum. It allows foundries to create alloys with desirable properties without significantly increasing the material cost.
Adding nitrogen in duplex stainless steel alloys is crucial for achieving an optimal balance of corrosion resistance, strength, and toughness.

It makes them well-suited for various industries such as oil and gas, chemical processing, and marine applications.

Corrosion Resistance of Duplex Stainless Steels

Corrosion resistance is a fundamental property of stainless steel.

Stainless steel's corrosion resistance refers to its ability to withstand the detrimental effects of chemical reactions and degradation when exposed to various corrosive environments.

Understanding Corrosion Resistance

The corrosion resistance of stainless steel is primarily due to the presence of chromium in its composition. 
Chromium forms a passive oxide layer on the surface of the steel, known as the chromium oxide (Cr2O3) layer or the passive film.

This acts as a protective barrier that prevents direct contact between the underlying steel and the corrosive environment.

For more information on this crucial topic, consult our guide on understanding stainless steel's corrosion-resistant properties.

We also offer deeper dives into common corrosion types, including crevice corrosion, stress corrosion cracking, microbial corrosion, and intergranular corrosion.

Factors Affecting Corrosion Resistance

In the case of duplex stainless steel, other alloying elements like molybdenum, nickel, and nitrogen further enhance the corrosion resistance of stainless steel.

These elements help form a more robust and stable passive film, especially in aggressive or specialized corrosive environments.

But duplex stainless steel's secret weapon against pitting and crevice corrosion is its balanced microstructure.  

Also known as a biphasic structure, duplex steels combine ferrite and austenite phases to offer the best of both with fewer downsides.

Ferrite is a body-centered cubic (BCC) crystal structure that provides excellent strength and resistance to stress corrosion cracking.

It is magnetic and has a lower nickel content compared to austenite. Ferrite is more corrosion-resistant in chloride-rich environments. The lower nickel content also makes it more ideal for use in medical sectors, where nickel allergies can often cause issues with medical implants and equipment.

Austenite is a face-centered cubic (FCC) crystal structure that offers superior toughness, ductility, and high-temperature strength.

It is non-magnetic and contains a higher nickel content, contributing to its excellent corrosion resistance in various environments.

The relative amounts of ferrite and austenite in duplex stainless steel can be controlled through alloy composition and heat treatment, allowing for customization of the material's properties to suit specific applications.

Common Duplex Stainless Steel Grades Explored

The ideal ratio of ferrite to austenite depends on desired mechanical properties, corrosion resistance requirements, and service conditions.

Different duplex stainless steel grades will offer different ratios to provide an option for nearly any use case or environment.
Let's look at some of the most popular grades and see how they handle various corrosive situations:

2205 (UNS S32205/S31803)

This grade is known for good general corrosion resistance, offering superior performance to 316L in most environments.

Its high chromium, molybdenum, and nitrogen content resist pitting, crevice, and general corrosion.

However, as noted in AZO Materials, it's unsuitable for applications requiring extended exposure to temperatures above 300°C due to precipitation of brittle microstructures.

But, as pointed out by Matmatch, the increased availability of duplex alloys with PREN above 40 have created a wealth of opportunities for use in the industries that previously relied on more expensive options—or more in-depth maintenance and replacement routines.
 
Typical applications include:

  • Chemical processing, transport, and storage: Grade 2205 is used in pressure vessels, tanks, piping, and heat exchangers in the chemical industry. 
  • Oil and gas exploration and processing: It is suitable for offshore platforms, pipelines, and equipment exposed to corrosive environments in the oil and gas industry.
  • Desalination plants: Grade 2205 is used in seawater desalination plants due to its resistance to chloride corrosion.
  • Marine applications: It is utilized in shipbuilding, offshore structures, and marine equipment.
  • Pulp and paper industry: Grade 2205 is used in pulp and paper production equipment, where corrosion resistance is crucial.

2507 (UNS S32750)

This super duplex grade has very high pitting and crevice corrosion resistance thanks to its high molybdenum and chromium content.

It offers high strength but can also undergo embrittlement if exposed to high temperatures for an extended time.

Grade 2507 duplex stainless steel is frequently used in extreme environments like desalination plants, seawater systems, and petrochemical industries.

Typical applications include:

  • Chemical processing equipment: Grade 2507 is suitable for equipment used in producing and handling chemicals, acids, and solvents. 
  • Oil and gas industry: It is used in offshore platforms, subsea equipment, and pipelines due to its resistance to corrosion in seawater and harsh environments.
  • Desalination plants: The alloy's corrosion resistance makes it suitable for use in desalination plants where it comes into contact with chloride-rich seawater.
  • Pulp and paper industry: Grade 2507 is used in equipment such as digesters, bleach washers, and evaporators due to its resistance to corrosive chemicals used in the papermaking process.

LDX 2101 (UNS S32101)

As a lean duplex stainless steel, LDX 2101 offers good general corrosion resistance, superior to 304 but not as good as 316.

It's characterized by high strength and good resistance to stress corrosion cracking.

It's less expensive because it uses less nickel and molybdenum but can be harder to source depending on market conditions.  

Common applications include:

  • Chemical processing: LDX 2101's corrosion resistance makes it suitable for equipment and components used in chemical processing plants.
  • Oil and gas industry: LDX 2101 can be used in offshore platforms, pipelines, and other equipment exposed to corrosive environments in the oil and gas industry.
  • Desalination plants: LDX 2101's resistance to corrosion and chloride stress-corrosion cracking makes it suitable for desalination plants where seawater is processed.
  • Food processing: LDX 2101's corrosion resistance and good machinability make it suitable for food processing and storage equipment.
  • Construction: LDX 2101 can be used in structural applications requiring corrosion resistance, such as bridges and buildings.

2304 (UNS S32304)

Another lean duplex, 2304 offers corrosion resistance similar to 316L but with higher strength.

However, it's susceptible to pitting corrosion in chloride-rich environments.
 
It's often used in construction, architecture, and in the pulp and paper industry.

Some ideal specific applications include:

  • Chemical Processing: The construction of chemical tankers, cargo tanks, and storage tanks due to grade 2304's corrosion resistance properties.
  • Oil and gas industry: Offshore platforms, pipelines, and subsea equipment in the oil and gas industry.
  • Pulp and paper industry: Heat exchangers, digesters and bleach towers, recovery boilers, piping systems, and other equipment.
  • Construction: Architectural structures and components, including facades, beams, columns, trusses, handrails, decorative elements, roofing, and cladding.

Zeron 100 (UNS S32760)

Like 2507 (UNS S32750), this super duplex grade offers high pitting and crevice corrosion resistance. It also provides good resistance to sulphide stress corrosion cracking.

However, like other super duplex grades, it's susceptible to embrittlement at high temperatures.

Typical applications include:

  • Oil and gas industry: Zeron 100 is commonly used in offshore oil and gas production, providing reliable corrosion resistance in seawater and sour gas environments.
  • Chemical processing: It is used in chemical processing plants throughout piping systems, heat exchangers, valves and fittings, pressure vessels, and chemical storage tanks.
  • Desalination plants: It can withstand the corrosive effects of seawater, making it ideal for use in reverse osmosis (RO) equipment, filler vessels, sulphate removal units, and flue gas desulphurization (FGD) equipment.
  • Petrochemical, paper and pulp, and marine industries: This grade is often found in heat exchangers, pressure vessels, and piping systems. 

Factors Influencing Grade Selection

Selecting the correct grade of duplex stainless steel is crucial for optimal performance.

Factors like application requirements, environmental conditions, cost considerations, fabrication process compatibility, and maintenance needs play a role in choosing the perfect grade.

Application Requirements

Understand your application's needs. Consider strength, durability, heat resistance, and formability.  

Application requirements may dictate the necessary mechanical properties, such as strength, toughness, and ductility.

Factors like load-bearing capacity, impact resistance, and dimensional stability should be considered to ensure the chosen grade can withstand the expected operational conditions.

Duplex stainless steels can also exhibit changes in mechanical properties at elevated temperatures, as reported by the British Stainless Steel Association (BSSA).

It is essential to understand the maximum operating temperature and potential thermal cycling to select a grade that can maintain its structural integrity and performance under those conditions.

Consider how the application aligns with applicable industry standards, codes, and regulations relevant to your project.

Choosing a duplex stainless steel grade that meets the required standards ensures compliance and ensures that the material is suitable for the intended application.

Environmental Conditions

Different duplex stainless steel grades offer varying levels of corrosion resistance.  

The grade choice should align with the specific corrosive environment the material will be exposed to, considering factors such as temperature, acidity, chloride concentration, and other corrosive agents.

For example, an excellent grade for an offshore piping process might differ from the perfect grade for a pulp and paper processing component.

As with all grades of stainless steel, aligning the grade's strength to the environment will yield the best service life and help improve long-term cost efficiency.

Cost Considerations

The budget and cost limitations of the project will always play a role in grade selection.
Different grades have varying costs due to alloy composition, availability, and market demand differences.

Moreso, these costs can fluctuate based on global markets, events, and trends.

Balancing performance requirements with cost considerations is vital to optimize value and meet budgetary constraints.

Fabrication Process Compatibility

Application requirements may involve specific fabrication processes like welding, forming, or machining.

When choosing the ideal grade for your project, it's crucial to consider the weldability, formability, and ease of fabrication of the selected duplex stainless steel grade to ensure it can be effectively and efficiently processed to meet the project's needs.

Outokumpu breaks down many of these considerations—and the science behind them—in their helpful overview of duplex stainless steel weldability. SciElo Brazil also highlights studies showing that through proper techniques, you can weld many duplex alloys without any effective loss of corrosion resistance—but this is only possible with a thorough understanding of the composition of the alloy(s) you choose.

Remember this when choosing valves, flanges, and other components to use alongside your duplex stainless steels to avoid unwanted complexity during installation.

Maintenance Needs

Grades requiring less upkeep save time and resources in the long run. 

Duplex stainless steels are known for their excellent corrosion resistance.

However, different grades may exhibit varying resistance to specific corrosive environments.

By understanding the maintenance requirements of the application, such as exposure to chemicals, seawater, or high temperatures, one can choose a grade with the appropriate corrosion resistance, ensuring long-term durability and minimizing the need for frequent maintenance.
This same consideration extends beyond the grade of duplex you choose, including surface finish and cleanability, maintenance frequency, the need for specialized cleaners or maintenance materials, and the difficulty of repairing or restoring the various parts and components you plan to use.

Considering maintenance needs upfront helps ensure the material remains in optimal condition over its intended service life while minimizing the need for extensive and costly maintenance interventions.

Frequently Asked Questions About Duplex Stainless Steel Grades, Properties, and Selection Guidelines

What is the difference between duplex and super duplex stainless steel?

Duplex stainless steelsare characterized by a balanced microstructure of approximately equal amounts of ferrite and austenite, imparting good mechanical properties and corrosion resistance

Super duplex stainless steels, on the other hand, contain higher levels of chromium, molybdenum, and nitrogen which significantly enhance their pitting and crevice corrosion resistance compared to regular duplex steels.

However, super duplexes can be more challenging to work with due to their high alloying element content, requiring careful attention to heat treatment and welding processes.

What is the significance of the PREN value in duplex stainless steels? 

PREN (Pitting Resistance Equivalent Number)measures a stainless steel grade's relative pitting corrosion resistance in chloride-containing environments.

It's calculated based on the percentages of chromium, molybdenum, and nitrogen in the steel.

Higher PREN values typically indicate better pitting and crevice corrosion resistance, significant for duplex stainless steels in aggressive environments like marine and chemical processing industries.

To learn more about this critical value, consult our pitting resistance equivalent numbers (PRENs) guide.

Why is welding of duplex stainless steels considered challenging?

Duplex stainless steels are sensitive to heat input and cooling rates during welding, affecting their balanced ferrite and austenite microstructure.

Excessive heat can promote the formation of detrimental intermetallic phases like the sigma phase, which reduces corrosion resistance and toughness.

Carefully controlled welding procedures, including preheating, using low heat input, and post-weld heat treatment, are often needed.

For more about welding duplex stainless and other grades of stainless steel, consult our stainless steel welding considerations guide.

Are duplex stainless steels magnetic?

Yes, duplex stainless steels are magnetic due to the presence of ferrite in their microstructure. This property can be used for material identification or sorting during production or recycling.

Can you use duplex stainless steels at high temperatures?

While duplex stainless steels exhibit good mechanical properties at room temperature, their performance at high temperatures is limited.

Prolonged exposure to temperatures above 300°C can lead to the precipitation of harmful phases, reducing the corrosion resistance and toughness of the material.

Other stainless steel grades or high-temperature alloys are usually more suitable for high-temperature applications.

Why are duplex stainless steels often preferred in marine environments?

Duplex stainless steels offer superior corrosion resistance to chlorides, common in marine environments, compared to conventional austenitic grades like 304 and 316. Their high chromium, molybdenum, and nitrogen content provide excellent resistance to pitting and crevice corrosion, which are common issues in these environments, providing far greater reliability and longer service life for a marginal cost increase.

How does the cost of duplex stainless steels compare to austenitic grades?

Duplex stainless steels can be more cost-effective than equivalent austenitic grades.

They offer higher strength, allowing less material to be used, and they have lower nickel content, making their cost less sensitive to nickel price fluctuations.

However, the initial material cost can be higher due to the alloying elements needed for their superior properties.

How does the formability of duplex stainless steels compare to austenitic grades?

Duplex stainless steels are generally less formable than austenitic grades due to their higher yield strength and lower elongation.

Therefore, more powerful equipment may be required for forming operations.

 Also, due to the presence of the ferritic phase, they are more prone to spring back.

What is the machinability of duplex stainless steels like?

Duplex stainless steels are typically more challenging to machine than austenitic grades due to their high strength and work hardening rate. Specific machining techniques and tool materials are often needed to ensure satisfactory results.

Are there any environmental concerns with using duplex stainless steels?

Stainless steels, including duplex grades, are highly recyclable, which is a significant advantage from an environmental perspective.

However, the production process can be energy-intensive and involve mining for raw materials, which can have environmental impacts.

Also, care must be taken in the disposal or recycling of waste material, especially from manufacturing processes like pickling or machining, which can generate harmful waste.

If you would like more information, you can consult our stainless steel recycling guide.

Conclusion

Duplex stainless steels offer exceptional corrosion resistance and high tensile strength, making them an ideal choice in applications that traditional stainless steel grades cannot handle.

From the extremes of offshore oil and gas processing to the heat and chemical bombardment of pulp and paper processing, duplex stainless steel benefits service life, maintenance requirements, and lifetime cost.

As with any materials selection process, it is vital to consider any application requirements and the unique strengths and weaknesses of the duplex grade to ensure the optimal balance of cost and performance. 


If you're considering duplex steel for your next project, the experts at Unified Alloys are ready to help! Call us today to speak with one of our knowledgeable sales analysts. As a leading provider of stainless steel and stainless steel products throughout North America for more than 40 years, we can help you source the perfect parts, components, or materials for projects of any size.

References

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