2507 Duplex Stainless Steel: Equivalents, Compositions, Properties and Applications

2507 duplex stainless steel is a material used in many different industries. Selecting the right material for an application is a key part of engineering. The choice of material can influence how long something lasts and how it performs. Duplex stainless steels are a class of materials that combine the properties of both ferritic stainless steel and austenitic stainless steels. This combination gives them higher strength and better resistance to corrosion than many other stainless steels.

Duplex stainless steels are called “duplex” because they have a two-phase microstructure of ferrite and austenite. This structure is what gives them their useful properties. The 2507 grade is known as a super duplex stainless steel because of its high content of chromium, molybdenum, and nitrogen. These elements give it superior strength and resistance to various types of corrosion, especially in environments with chlorides. In this article, we will look at the main characteristics of 2507 duplex stainless steel. We will explain its properties, how it is made using investment casting, and where it is used.

2507 Duplex Stainless Steel International Equivalents

One of the challenges in working with engineering materials is the wide variety of naming conventions used across different international standards. To ensure clarity and consistency, it’s important to understand the equivalent designations for a specific material like 2507 super duplex stainless steel. This grade has a variety of designations depending on the country or organization that produced the standard.

The most common designation in the United States is the UNS S32750, which is overseen by the Unified Numbering System for metals and alloys. This is often paired with specific product form standards from organizations like the American Society for Testing and Materials (ASTM) and the American Society of Mechanical Engineers (ASME), where it is designated as ASTM A182 Grade F53 and ASME SA-182 Grade F53 for forged products.

In Europe, the material is designated by the EN 1.4410 standard. A more technical name within this system is X2CrNiMoN25-7-4, which describes the chemical composition (e.g., approximately 25% chromium, 7% nickel, 4% molybdenum). Other European standards that cover this material include EN 10088-3.

Other international standards also provide equivalent designations. For example, the French standard organization, AFNOR, uses Z2CND25-07N. The Japanese Industrial Standards (JIS) may refer to it as JIS G4304 SUS329J4L. In China, the equivalent standard is often referred to by its GB/T 3280 designation for certain product forms. Furthermore, the material is also used in oil and gas applications under the NACE MR0175/ISO 15156 standard. These different designations all refer to the same basic material with its specific chemical composition and properties.

Standard Body / Region	Common Name / Grade	Designation / Standard No.	Notes
Unified Numbering System (UNS)	Super Duplex 2507	UNS S32750	The primary global designation.
Europe (EN)	Super Duplex	EN 1.4410	European material number.
Europe (EN – Chemical)	Super Duplex	X2CrNiMoN25-7-4	Name based on chemical composition.
United States (ASTM)	Super Duplex 2507	F53 (ASTM A182, A479)	Forged products (e.g., flanges, bars, fittings).
United States (ASME)	Super Duplex 2507	SA-182, SA-240, SA-479	Equivalent ASME standards for pressure vessels.
China (GB/T)	2507 Duplex Steel	GB/T 3280	Chinese National Standard.
Japan (JIS)	SUS329J4L	JIS G4304	Japanese Industrial Standard.
Casting Standard (ASTM)	CE3MN or Grade 5A	ASTM A890	Specific designation for investment castings.
Oil & Gas (NACE / ISO)	Super Duplex 2507	NACE MR0175 / ISO 15156	For use in H₂S-containing environments.
Trade Name	SAF 2507	–	Original trademarked name by Alleima (formerly Sandvik).

2507 Duplex Stainless Steel Chemical Composition

The exceptional properties of 2507 super duplex stainless steel are a direct result of its carefully controlled chemical composition. Each element plays a specific and crucial role, working together to achieve a unique balance of high strength and outstanding corrosion resistance. The material’s “duplex” nature comes from a microstructure that is approximately 50% ferrite and 50% austenite, a balance that is maintained by the ratio of these alloying elements.

Primary Alloying Elements

The three most important elements that define 2507 are Chromium, Molybdenum, and Nitrogen. Their combined effect is often measured by the Pitting Resistance Equivalent Number (PREN), a formula used to predict an alloy’s resistance to pitting corrosion in chloride environments. A higher PREN indicates better resistance. For super duplex 2507, the PREN is typically above 40, which is why it is considered a “super” duplex.

• Chromium (Cr): Present in the highest concentration, chromium is the primary element responsible for the alloy’s corrosion resistance. It forms a thin, stable, and self-healing passive layer on the surface of the steel. With a high content of 24-26%, it provides excellent resistance to oxidation, corrosion, and erosion.
• Molybdenum (Mo): Molybdenum is a powerful corrosion fighter, working in synergy with chromium to enhance resistance to localized corrosion, particularly pitting and crevice corrosion in harsh chloride-containing environments. Its presence at 3-5% significantly improves the stability of the passive layer and helps prevent it from breaking down.
• Nitrogen (N): Nitrogen is a key element that distinguishes super duplex steels. It acts as a powerful austenite former, helping to maintain the balanced duplex microstructure. It also significantly improves strength and hardness without sacrificing ductility. Most importantly, it dramatically increases resistance to localized corrosion, particularly pitting and crevice corrosion, by concentrating in the austenite phase and enhancing its stability.

Minor Elements and Impurities

• Nickel (Ni): With a content of 6-8%, nickel also helps to stabilize the austenitic phase of the microstructure and contributes to the material’s overall toughness. It’s a critical component for achieving the desired duplex phase balance.
• Carbon (C): The carbon content is kept very low, typically below 0.03%. This is essential to prevent the formation of chromium carbides at grain boundaries during welding or high-temperature exposure. Such carbides can lead to a phenomenon called “sensitization,” which makes the steel susceptible to intergranular corrosion.
• Manganese (Mn) and Silicon (Si): These elements are present in small amounts and contribute to deoxidation during the steel-making process.

Element	UNS S32750 Composition (Weight %)	Role
Chromium (Cr)	24.0 – 26.0	Forms passive layer; provides corrosion and pitting resistance.
Nickel (Ni)	6.0 – 8.0	Stabilizes austenite; enhances toughness.
Molybdenum (Mo)	3.0 – 5.0	Improves resistance to pitting and crevice corrosion.
Nitrogen (N)	0.24 – 0.32	Enhances strength and pitting resistance; stabilizes austenite.
Carbon (C)	≤0.03	Kept low to prevent sensitization and intergranular corrosion.
Manganese (Mn)	≤1.20	Deoxidizer; improves weldability.
Phosphorus (P)	≤0.035	Impurity; kept low to maintain ductility.
Sulfur (S)	≤0.020	Impurity; kept low for machinability and ductility.
Silicon (Si)	≤0.80	Deoxidizer; strengthens ferrite phase.

Mechanical Properties of 2507 Duplex Stainless Steel

The mechanical properties of 2507 super duplex stainless steel are what make it suitable for highly demanding applications. The dual-phase (ferrite-austenite) microstructure provides a unique combination of high strength from the ferrite phase and excellent toughness from the austenite phase. This material is designed to offer performance superior to that of standard austenitic or duplex stainless steels.

Strength and Hardness

The high nitrogen content, along with chromium and molybdenum, significantly contributes to the alloy’s high strength. This makes it an ideal choice for applications where a strong material is required to handle high stress or heavy loads.

• Tensile Strength: This property measures the material’s ability to withstand a pulling force before it breaks. For super duplex 2507, the tensile strength is typically over 750 MPa (109 ksi), which is significantly higher than most austenitic grades like 316L.
• Yield Strength: This is a crucial property for structural applications, as it measures the point at which the material begins to deform permanently. The yield strength of SS 2507 is typically above 550 MPa (80 ksi), more than double that of 316L. This high yield strength allows for the design of lighter and thinner-walled components.
• Hardness: The hardness of 2507 stainless steel, often measured on the Brinell or Rockwell C scales, indicates its resistance to indentation. A typical hardness for this alloy is approximately 290 HB or 30 HRC, reflecting its high strength.

Toughness and Ductility

While its high strength is a primary advantage, 2507 duplex stainless steel also maintains good toughness and ductility, which are crucial for preventing brittle fracture.

• Elongation: Elongation is a measure of ductility, representing the percentage a material can be stretched before it fractures. Super duplex 2507 typically has an elongation of more than 25%, demonstrating its ability to deform plastically without breaking.
• Impact Toughness: This property measures the material’s ability to absorb energy and resist fracture at a given temperature. 2507 stainless steel has a high impact toughness, which is particularly important for cold-weather applications or those with sudden loading.

Property	Typical Value	Standard Unit
Tensile Strength	750	MPa
	109	ksi
Yield Strength	550	MPa
	80	ksi
Elongation	25	%
Hardness	290	HB
	30	HRC

Physical Properties of 2507 Duplex Stainless Steel

The physical properties of 2507 super duplex stainless steel are important for understanding its thermal and electromagnetic behavior, which dictates its suitability for various industrial applications.

Density and Thermal Conductivity

• Density: The density of SS 2507 is similar to that of other stainless steels, approximately 7.8 g/cm³ (or 0.282 lb/in³). This is a useful property for calculating the weight of components for design and engineering purposes.
• Thermal Conductivity: The thermal conductivity of a material describes its ability to conduct heat. 2507’s thermal conductivity is relatively low compared to standard carbon steels but slightly higher than standard austenitic stainless steels like 304. It is typically around 14.2textW/mK at room temperature. This low thermal conductivity can be a beneficial property in applications where heat transfer needs to be minimized.

Magnetic Properties

• Magnetic Permeability: Unlike austenitic stainless steels (such as 304 and 316), which are non-magnetic, duplex stainless steels like 2507 are ferromagnetic and therefore attracted to a magnet. This is due to the presence of a significant amount of the ferrite phase in its microstructure. The magnetic permeability of 2507 is typically between 1.02 and 1.10.

Property	Typical Value	Standard Unit
Density	7.8	g/cm³
	0.282	lb/in³
Thermal Conductivity	14.2	W/mK
Magnetic Permeability	1.02 – 1.10	–

Super Duplex 2507 Corrosion Resistance

The outstanding corrosion resistance of 2507 super duplex stainless steel is its most defining characteristic, making it the material of choice for the most aggressive environments. This resistance stems from its high content of chromium, molybdenum, and nitrogen, which form a strong and stable passive layer on the surface.

Mechanisms of Corrosion

Stainless steels are susceptible to several types of localized corrosion, particularly in environments containing chlorides.

• Pitting Corrosion: This is a localized form of corrosion that results in small pits or holes on the surface of the metal. It occurs when the passive layer is locally breached, often by chloride ions. This creates an anode (the pit) and a large cathode (the remaining passive surface), leading to a rapid and concentrated attack.
• Crevice Corrosion: This is similar to pitting but occurs within tight, stagnant crevices (e.g., under bolt heads, gaskets, or deposits). The restricted oxygen flow inside the crevice leads to the breakdown of the passive layer and the concentration of aggressive ions like chlorides, accelerating corrosion.
• Stress Corrosion Cracking (SCC): This is a particularly dangerous form of corrosion that occurs when a material is subjected to a combination of tensile stress and a specific corrosive environment (most notably hot chlorides). It results in brittle fracture along grain boundaries without significant plastic deformation.
• General Corrosion: This is a more uniform, widespread form of attack over the entire surface of the metal, typically in strong acid or alkali solutions. While SS 2507 excels in resisting localized attack, its performance in general corrosion is also excellent compared to lower-alloyed stainless steels.

Superior Resistance of 2507 Stainless Steel

The high PREN (Pitting Resistance Equivalent Number) of 2507, which is typically above 40, is a strong indicator of its superior resistance to localized corrosion. This high value is achieved by the synergistic effect of its key alloying elements:

• Chromium (24-26%): Provides a robust passive layer that is the first line of defense.
• Molybdenum (3-5%): Significantly improves the stability of the passive layer, particularly against pitting and crevice corrosion.
• Nitrogen (0.24-0.32%): Strengthens the passive layer and enhances the resistance to localized corrosion by concentrating in the austenite phase. It also improves the material’s resistance to stress corrosion cracking.

Due to its balanced ferrite-austenite microstructure, super duplex 2507 stainless steel also exhibits exceptional resistance to chloride-induced stress corrosion cracking, a common failure mechanism for many austenitic stainless steels.

Comparison with Other Stainless Steel Grades

The following table provides a comparison of 2507 duplex stainless steel’s corrosion resistance with other commonly used stainless steel grades.

Property	304/304L (Austenitic)	316/316L (Austenitic)	2205 (Standard Duplex)	2507 (Super Duplex)
PREN	18-20	24-27	34-36	>40
Pitting & Crevice Corrosion	Poor to Fair	Fair to Good	Good	Excellent
Stress Corrosion Cracking (SCC)	Poor (very susceptible)	Poor (susceptible)	Good	Excellent
General Corrosion	Fair	Good	Good	Excellent
Key Use Environment	Low chlorides	Moderate chlorides	Moderate to high chlorides	High chlorides & acids

Manufacturing and Fabrication Properties

The fabrication of 2507 super duplex stainless steel requires careful consideration of its unique properties, which differ significantly from those of conventional austenitic stainless steels. Its high strength and dual-phase microstructure present both advantages and challenges during manufacturing processes like casting, machining, forming, and welding.

Castability

2507 super duplex is readily castable, which allows for the production of complex, intricate components that would be difficult or impossible to create through machining or fabrication. However, the casting process must be tightly controlled to ensure that the final product has the desired microstructure. The correct balance of ferrite and austenite is critical for achieving the material’s optimal mechanical properties and corrosion resistance.

Machinability

Due to its high strength, hardness, and work-hardening characteristics, super duplex 2507 stainless steel is classified as being difficult to machine. It requires more power and a lower cutting speed compared to a standard austenitic stainless steel like 316L. To achieve good results, it’s essential to use robust machinery, sharp and hard tooling (such as carbide inserts), and a rigid setup to minimize vibration. Proper heat management is also critical to prevent overheating the material and affecting its properties.

Formability

The high yield strength of grade 2507 means that it requires significantly more force to form than austenitic stainless steels. Cold working processes like bending, deep drawing, and rolling are possible, but they require powerful equipment. Another important consideration is a phenomenon known as “springback,” which is the material’s tendency to partially return to its original shape after forming. Due to its high strength, 2507 has a greater degree of springback, so allowances must be made in tooling design. For cold deformation of more than 10%, a solution annealing heat treatment is recommended to restore the duplex microstructure and its properties.

Weldability

2507 duplex stainless steel is weldable, but the process requires specific care to maintain the material’s properties. The key challenge is to prevent the formation of unwanted secondary phases and to maintain the correct ferrite-austenite balance in the heat-affected zone (HAZ). To achieve this, a low heat input is required, and the interpass temperature should be carefully controlled (typically below 100circC). It is also crucial to use the correct filler metal, often one that is slightly “over-alloyed” with nickel to help promote the formation of austenite during the rapid cooling of the weld. Post-weld heat treatment is generally not necessary, but it may be recommended for certain critical applications.

Investment Casting of 2507 Duplex Stainless Steel

Investment casting, also known as lost-wax casting, is a highly precise manufacturing process used to produce complex, near-net-shape components. While the general process is similar for many metals, casting 2507 super duplex stainless steel is a valuable method for manufacturing parts with unique advantages.

The Investment Casting Process For 2507 Duplex Stainless Steel

1. Wax Pattern Creation: A wax pattern, which is an exact replica of the final part, is created. This can be done by injecting wax into a reusable die. For complex parts, multiple wax patterns may be assembled together onto a central wax “sprue” to form a single casting cluster.
2. Shell Building: The wax cluster is repeatedly dipped into a ceramic slurry and a sand coating. Each layer is allowed to dry, building a strong, multi-layered ceramic shell around the wax pattern. This is a critical step as the shell will form the mold for the molten metal.
3. Dewaxing: The ceramic shell is placed in an autoclave or furnace, where the wax is melted and drained out, leaving a hollow ceramic mold. This process is where the “lost-wax” name originates.
4. Melting and Pouring: The ceramic mold is preheated to a high temperature to remove any remaining wax residue and to ensure the mold is ready to receive the molten metal. The 2507 alloy is then melted in a furnace, and the molten metal is poured into the hot ceramic mold.
5. Cooling and Solidification: The molten metal cools and solidifies within the mold, taking the shape of the original wax pattern.
6. Knockout and Finishing: Once cool, the ceramic shell is broken away from the metal casting. The individual parts are then cut from the sprue and undergo final finishing processes, such as sandblasting, heat treatment, and surface grinding, to meet the specified dimensions and properties.

Benefits of Investment Casting 2507 Stainless Steel

Using the investment casting process for 2507 super duplex stainless steel offers several key advantages, particularly for producing complex and high-performance components.

• Design Freedom: Investment casting allows for the creation of intricate and complex geometries that would be difficult, expensive, or impossible to achieve through machining or fabrication from wrought forms. This includes features like internal passages, complex curves, and integrated components, which can be cast as a single piece.
• Near-Net-Shape Manufacturing: The process yields parts that are very close to their final shape, requiring minimal post-casting machining. This significantly reduces manufacturing costs and material waste, especially for an expensive and difficult-to-machine alloy like 2507.
• Consistent Microstructure: With proper foundry control, investment casting can produce a consistent and homogeneous microstructure throughout the part. Unlike welded components, which can have varying properties in the heat-affected zones, a cast part has a uniform grain structure, ensuring consistent mechanical properties and corrosion resistance across the entire component.
• Improved Integrity: Casting can produce parts with excellent structural integrity. For certain applications, the continuous grain flow of a cast part can offer superior performance compared to a fabricated component with welds, which can sometimes be stress points.

Applications of 2507 Stainless Steel Castings

The exceptional combination of high strength and superior corrosion resistance makes SS 2507 super duplex stainless steel an ideal material for a wide range of demanding applications. While it is available in many forms, investment castings are particularly useful for producing complex components for these industries.

Oil and Gas Industry

This is a primary market for super duplex 2507 stainless steel due to the harsh conditions found in offshore and subsea environments. The material’s resistance to chloride-induced stress corrosion cracking and pitting is crucial for the safety and longevity of equipment. Specific examples include subsea manifolds, pump bodies, valves, pipe fittings, and other equipment used for oil and gas extraction and processing.

Chemical Processing

In the chemical industry, equipment must withstand exposure to a wide array of corrosive chemicals, including strong acids and chlorides. SS 2507 provides the necessary durability to prevent premature failure. Examples of its use are components for heat exchangers, chemical reactors, pressure vessels, and pumps that handle corrosive media.

Marine and Shipbuilding

The marine environment, particularly saltwater, is highly corrosive to most metals. The excellent pitting and crevice corrosion resistance of 2507 duplex stainless steel makes it a top choice for marine applications, therefore super duplex 2507 can be used to cast various marine cast parts by the investment casting method. For instance, it is used in propeller shafts, pump housings, valves, and other components for saltwater cooling systems.

Pulp and Paper Industry

The pulp and paper industry uses corrosive chemicals, especially in the bleaching process. 2507 is used to resist both general and localized corrosion in these harsh environments. Examples include washers, filters, digesters, and other process equipment that come into contact with aggressive bleach solutions.

Water Treatment and Desalination

With increasing demand for fresh water, desalination plants are becoming more common. These facilities rely on components that can withstand the highly corrosive conditions of high-chloride seawater. Super duplex 2507 stainless steel is an excellent choice for these applications, such as pumps, valves, piping, and pressure vessels for reverse osmosis and other desalination processes.

Power Generation

In power plants, especially those that use flue-gas desulfurization (FGD) systems to reduce emissions, components are exposed to a highly corrosive mixture of sulfur compounds and chlorides. 2507 stainless steel castings’ resistance to this acidic environment makes it ideal for these applications, including absorber towers, scrubbers, and other components in FGD systems.

Industrial Machinery

The exceptional strength, hardness, and wear resistance of 2507 stainless steel castings make them highly suitable for various industrial machinery components that are subject to high stress and abrasive conditions. Its superior corrosion resistance is also critical for parts that handle corrosive fluids or operate in aggressive environments. Applications include precision gears, mixer shafts, pump and agitator components, and other heavy-duty parts that require both durability and resistance to wear and corrosion.

Conclusion

2507 super duplex stainless steel stands out in the world of engineering materials due to its unique combination of high strength and exceptional corrosion resistance. Its carefully controlled chemical composition, particularly the high content of chromium, molybdenum, and nitrogen, gives it a balanced dual-phase microstructure. This results in superior mechanical properties, including high yield and tensile strength, and outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking. Investment casting is a powerful manufacturing method that unlocks the full potential of this alloy by enabling the creation of complex, near-net-shape components with a consistent and reliable microstructure.

Ultimately, super duplex 2507 is a high-performance material chosen for applications where the cost of component failure is unacceptably high. Its unique properties and the flexibility of investment casting make it an indispensable solution for critical components in some of the world’s most aggressive industrial environments.

Taiyuan SIMIS Investment Casting Co., Ltd. is a specialized stainless steel investment casting company. We are equipped with cutting-edge casting technology and facilities, providing you with high-quality custom investment casting parts and components. Our in-house capabilities also include tooling design and manufacturing, CNC machining, heat treatment, and surface treatments. Please contact us if you would like to get a quote or have any questions regarding 2507 duplex stainless steel; we’re here to help.