2507 Super Duplex Stainless Steel: Properties, PREN and Uses

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Category: Super Duplex Stainless Steel

1. What Is 2507 Super Duplex Stainless Steel?

2507 is a super duplex stainless steel grade designated as UNS S32750 (EN 1.4410). It belongs to the duplex stainless steel family, characterized by a microstructure consisting of approximately equal proportions of austenite and ferrite phases. This dual-phase structure gives 2507 a unique combination of high strength and exceptional corrosion resistance.

The term "Super Duplex" distinguishes 2507 from standard duplex grades such as 2205 (UNS S32205). Compared with standard duplex stainless steels, 2507 contains:

• Higher chromium (~25%) — enhances general corrosion resistance and contributes to a higher PREN
• Higher molybdenum (~4%) — significantly improves resistance to chloride pitting and crevice corrosion
• Higher nitrogen (~0.3%) — strengthens the austenite phase, improves pitting resistance, and maintains phase balance
• Higher PREN (typically ≥40) — indicating superior resistance to localized corrosion in chloride-rich environments

These compositional enhancements provide 2507 with superior resistance to chloride pitting, crevice corrosion, and stress corrosion cracking (SCC) compared with standard duplex and conventional austenitic stainless steels.

The ferritic phase makes 2507 magnetic. This is a normal characteristic of duplex stainless steel and does not indicate inferior corrosion resistance. Unlike martensitic stainless steels, the magnetism in 2507 results from its ferrite content, not from a transformation that compromises corrosion performance.

2. Chemical Composition of 2507 Super Duplex Stainless Steel

The chemical composition of 2507 is defined by ASTM A240. The following table presents the typical composition ranges for UNS S32750:

Function of Major Alloying Elements

Chromium (Cr): Chromium is the primary element responsible for stainless steel's corrosion resistance. At approximately 25%, it forms a stable passive oxide film on the surface, improving general corrosion resistance and directly increasing the PREN (Pitting Resistance Equivalent Number).

Molybdenum (Mo): At approximately 4%, molybdenum is critical for enhancing resistance to chloride-induced pitting and crevice corrosion. It works synergistically with chromium to stabilize the passive film in aggressive halide environments.

Nitrogen (N): Nitrogen serves multiple functions: it strengthens the austenite phase through solid-solution strengthening, significantly improves pitting resistance (as reflected in the PREN formula), and helps maintain the desired duplex phase balance during welding and heat treatment.

Nickel (Ni): Nickel stabilizes the austenite phase and improves overall toughness. It ensures the microstructure maintains the correct austenite-ferrite ratio, which is essential for the balanced properties of duplex stainless steel.

3. Super Duplex Metallurgy: How 2507 Achieves High Performance

The defining feature of 2507 is its duplex microstructure, consisting of two distinct phases: ferrite (body-centered cubic) and austenite (face-centered cubic) in approximately equal proportions. This balanced structure is achieved through precise alloy design and controlled solution annealing followed by rapid cooling.

Contribution of Each Phase

Ferrite Phase: Provides high resistance to chloride stress corrosion cracking (SCC) — a failure mode to which conventional austenitic stainless steels such as 304 and 316 are susceptible. The ferrite phase also contributes significantly to the material's elevated yield strength, typically more than double that of 316L.

Austenite Phase: Contributes ductility, toughness, and general corrosion resistance. The austenite phase ensures that 2507 retains good formability and weldability despite its high strength.

Manufacturing Requirements

Producing 2507 with optimal properties requires:

• Solution annealing at approximately 1020–1100°C to dissolve precipitates and achieve the correct phase balance
• Rapid cooling (water quenching) to suppress the formation of detrimental intermetallic phases such as sigma and chi
• Controlled phase balance — typically 40–60% ferrite with the balance austenite

Phase Balance Importance

Incorrect phase balance — whether from improper heat treatment or uncontrolled welding — can result in:

• Reduced toughness, particularly at low temperatures
• Reduced corrosion resistance due to chromium and molybdenum depletion in localized areas
• Formation of intermetallic phases (sigma, chi) that cause embrittlement

2507 requires tighter metallurgical control than conventional austenitic stainless steels. This is a key consideration for both mill production and fabrication — not all suppliers possess the expertise to consistently deliver properly heat-treated super duplex products.

4. PREN and Corrosion Resistance of 2507

The Pitting Resistance Equivalent Number (PREN) is a predictive indicator used to compare the relative resistance of stainless steels to chloride-induced pitting corrosion. The most commonly used formula is:

PREN = %Cr + 3.3 × %Mo + 16 × %N

It is important to understand that PREN is a comparative ranking tool — not a guarantee of actual service performance. Real-world corrosion behavior depends on temperature, chloride concentration, pH, and the presence of crevice geometries.

For 2507, the typical PREN is ≥40, which places it significantly above standard duplex grades (2205: PREN ~35) and far above conventional austenitic grades (316L: PREN ~25).

What This Means in Practice

2507 provides excellent resistance to:

• Chloride pitting — the localized breakdown of the passive film in chloride-containing environments
• Crevice corrosion — corrosion occurring in stagnant, confined geometries such as gasket surfaces and under deposits
• Seawater exposure — making 2507 suitable for submerged and splash-zone service in marine environments
• Offshore environments — where combined mechanical loading and aggressive chloride conditions are present

Typical Application Environments

These properties make 2507 the material of choice for seawater systems, desalination equipment (RO high-pressure piping), offshore oil and gas production systems, and chemical process equipment handling chloride-bearing streams.

Limitations

Despite its excellent corrosion resistance, 2507 is not immune to all environments. Known limitations include:

• Strong reducing acids (e.g., hot hydrochloric acid, sulfuric acid at certain concentrations)
• Very high temperature chloride environments — above approximately 250–300°C, where the passive film may become unstable
• Highly specialized chemical conditions involving oxidizing halides or complex process streams

When 2507 is not sufficient, higher alloy nickel-based materials (e.g., Alloy 625, Alloy C-276) or 6Mo super austenitic grades (e.g., UNS N08367, UNS S31254) may be required depending on the specific environment.

5. Mechanical Properties of 2507 Super Duplex Stainless Steel

The following table presents the minimum mechanical properties for 2507 in the solution-annealed condition per ASTM A240:

2507 has significantly higher strength than conventional austenitic stainless steels. For comparison, its yield strength is approximately 2–3 times that of 316L (Rp0.2 ≥ 170 MPa). This strength advantage enables several practical benefits:

• Reduced wall thickness — thinner material can withstand the same pressure rating, saving material cost and weight
• Lower equipment weight — beneficial for offshore topside structures where weight reduction directly impacts structural support costs
• Improved pressure capability — higher allowable working pressures for piping and pressure vessel applications

It is important to note that the strength of 2507 comes from its duplex microstructure — not from heat treatment. 2507 cannot be hardened by thermal treatment, and attempting to do so may degrade both mechanical properties and corrosion resistance.

6. Welding of 2507 Super Duplex Stainless Steel

2507 is weldable, but successful welding requires strict procedure control. The goal is to preserve the duplex phase balance and corrosion resistance in the weld metal and heat-affected zone (HAZ).

Welding Requirements

• Qualified welding procedures — welding procedure specifications (WPS) should be qualified per ASME Section IX or equivalent with corrosion testing per ASTM A923
• Controlled heat input — typically 0.5–2.5 kJ/mm, depending on thickness and process. Excessive heat input promotes intermetallic phase precipitation; insufficient heat input may result in excessive ferrite
• Interpass temperature control — maximum interpass temperature typically 150°C to prevent deleterious phase formation

Applicable Welding Processes

Common welding processes for 2507 include:

• TIG (GTAW) — preferred for root passes and thin sections; provides the best control over heat input
• MIG (GMAW) — suitable for fill passes in thicker sections
• SMAW (MMA) — used for field welding and repair work
• SAW — for high-deposition shop welding of thick sections

Filler Metal

The common filler metal for 2507 is ER2594 (AWS A5.9). This over-alloyed filler ensures:

• Corrosion resistance in the weld metal matches or exceeds that of the base metal
• Proper phase balance is restored in the as-welded condition

Welding Risks

Excessive Ferrite: If the weld cools too quickly (low heat input), the ferrite content in the weld metal and HAZ may exceed acceptable limits, resulting in reduced toughness and corrosion resistance.

Excessive Heat Input: If heat input is too high, intermetallic phases (sigma, chi) may precipitate, causing embrittlement and reduced corrosion performance. These phases cannot be removed without re-solution annealing.

When welding procedures are properly controlled, post-weld heat treatment is generally not required. However, for critical applications or when corrosion testing indicates it is necessary, full solution annealing may be specified.

7. Temperature Limitations of 2507 Super Duplex Stainless Steel

2507 is designed for corrosion resistance in aggressive environments — not for elevated-temperature service. Understanding its thermal limitations is essential for correct material selection.

Above approximately 300°C, several degradation mechanisms become active:

• Phase stability decreases — the carefully controlled duplex structure begins to decompose
• Sigma phase formation risk increases — chromium-rich sigma phase precipitates at grain boundaries, causing embrittlement and reducing corrosion resistance
• 475°C embrittlement — ferrite phase decomposition in the range of 280–500°C can cause severe loss of toughness

For applications requiring elevated-temperature strength and oxidation resistance, consider these alternatives:

2507 should not be selected as a replacement for heat-resistant stainless steels. Doing so may result in premature failure due to embrittlement, even at temperatures well below typical furnace service conditions.

8. Applications of 2507 Super Duplex Stainless Steel

The combination of high strength, excellent chloride corrosion resistance, and good fabricability makes 2507 the preferred material for demanding industrial environments. The following table summarizes key application sectors:

9. Limitations and Common Selection Mistakes

Despite its excellent properties, 2507 is not a universal solution. The following are common mistakes encountered in material selection:

Mistake 1 — Selecting 2507 Solely for Its High Strength: While 2507 offers significantly higher yield strength than 316L, the primary driver for selecting super duplex should be the corrosion environment. If chloride resistance is not required, other high-strength materials may provide more cost-effective solutions.

Mistake 2 — Using 2507 for High-Temperature Service: As discussed in Section 7, super duplex stainless steel is not designed for elevated-temperature applications. Specifying 2507 for furnace components, exhaust systems, or process heaters will result in rapid material degradation.

Mistake 3 — Ignoring Welding Requirements: Improper fabrication can irreversibly damage the duplex phase balance. Fabricators without super duplex experience may apply procedures developed for austenitic stainless steels, leading to excessive ferrite or sigma phase formation.

Mistake 4 — Ordering Without Complete Specification: Using only the generic term "super duplex" in a purchase order is insufficient. Suppliers require the full UNS designation, applicable ASTM standard, product form, and documentation requirements to provide material that meets your requirements.

10. How to Select 2507 Super Duplex Stainless Steel

Proper selection of 2507 requires evaluation across multiple dimensions. The following framework provides a structured approach:

Step 1: Corrosion Environment Assessment

Evaluate the specific service conditions: chloride concentration, seawater exposure type (submerged, splash zone, atmospheric), operating temperature, pH, and the presence of any additional aggressive species such as H₂S. Confirm that 2507's corrosion resistance profile aligns with these conditions.

Step 2: Mechanical Requirements Verification

Determine the required pressure rating, structural loading, and whether weight reduction is a design objective. Verify that 2507's strength allows for the desired wall thickness reduction while maintaining sufficient stiffness and buckling resistance.

Step 3: Fabrication Planning

Assess whether your fabricator has documented experience with super duplex welding. Confirm that qualified WPS documentation exists and that ASTM A923 corrosion testing will be performed on procedure qualification coupons. Plan for forming requirements, noting that 2507's higher strength requires greater forming forces than austenitic grades.

Step 4: Documentation Specifications

Define required documentation upfront: chemical composition per ASTM A240, tensile properties, hardness testing, phase balance verification if specified, and corrosion testing per ASTM A923 Method C. Align documentation requirements with project quality plans and end-user specifications.

11. Procurement Guidance for 2507 Super Duplex Stainless Steel

An effective purchase specification eliminates ambiguity and ensures that suppliers can provide compliant material. The following table outlines key specification elements:

Example Purchase Specification:

Avoid vague specifications such as "Need super duplex plate." Without the complete specification, a supplier cannot confirm exact chemistry, the applicable standard, testing requirements, or product form — all of which directly affect whether the delivered material meets your project requirements.

12. How Shangyou Supports 2507 Super Duplex Stainless Steel Procurement

Shaanxi Shangyou Stainless Steel Co., Ltd. provides comprehensive support for 2507 super duplex stainless steel procurement with the following quality assurance measures:

• ASTM compliance verification — material supplied against ASTM A240, A789, A790, and other relevant standards
• UNS S32750 confirmation — grade verification against the correct UNS designation
• Chemistry verification — Cr, Ni, Mo, N composition confirmed against ASTM requirements
• PREN calculation review — calculated PREN verified to meet minimum requirements
• EN 10204 3.1 MTC documentation — mill-certified test certificates provided as standard
• PMI testing coordination — positive material identification to confirm grade before shipment
• Ferrite testing coordination — phase balance verification per ASTM A923 or customer requirements
• Dimensional inspection — thickness, width, length, and straightness verified before dispatch
• Surface inspection — visual inspection for surface defects per applicable standards
• Third-party inspection support — coordination with SGS, Bureau Veritas, TÜV, and other agencies

Our team provides technical support throughout the procurement process — from initial grade selection through to delivery and documentation review.

13. Frequently Asked Questions

Q1: What is 2507 super duplex stainless steel?
2507 (UNS S32750, EN 1.4410) is a super duplex stainless steel with approximately 25% chromium, 7% nickel, 4% molybdenum, and 0.3% nitrogen. Its balanced austenite-ferrite microstructure provides a combination of high strength — yield strength exceeds 550 MPa — and excellent resistance to chloride pitting, crevice corrosion, and stress corrosion cracking. It is widely specified for offshore, marine, desalination, and chemical processing equipment exposed to aggressive chloride environments.

Q2: What is UNS S32750?
UNS S32750 is the Unified Numbering System designation for 2507 super duplex stainless steel. The UNS number uniquely identifies the alloy's chemical composition within the metals and alloys designation system. Using UNS S32750 in purchase specifications ensures that suppliers and fabricators reference the exact alloy chemistry required, eliminating ambiguity that can arise from trade names alone.

Q3: What is the PREN value of 2507 stainless steel?
The typical PREN of 2507 is ≥40, calculated using the formula PREN = %Cr + 3.3 × %Mo + 16 × %N. This value significantly exceeds standard duplex 2205 (PREN ~35) and conventional 316L (PREN ~25). A higher PREN correlates with greater resistance to chloride-induced pitting corrosion, which is the primary reason 2507 is selected for seawater and aggressive chloride service.

Q4: Is 2507 stainless steel corrosion resistant?
Yes. 2507 offers excellent corrosion resistance, particularly against chloride pitting, crevice corrosion, and stress corrosion cracking. Its high chromium, molybdenum, and nitrogen content creates a stable passive film resistant to breakdown in chloride-rich environments, including seawater. However, 2507 is not immune to all corrosive conditions — strong reducing acids and very high temperature chlorides may exceed its capability.

Q5: Is 2507 stainless steel magnetic?
Yes, 2507 is magnetic due to its ferrite phase content (approximately 40–60%). This is normal for duplex and super duplex stainless steels and does not indicate poor corrosion resistance. The magnetism originates from the ferrite phase, which is intentional and beneficial for resisting chloride stress corrosion cracking. It is not comparable to the magnetism of martensitic grades.

Q6: Can 2507 super duplex stainless steel be welded?
Yes, 2507 can be welded using TIG, MIG, SMAW, and SAW processes. However, welding requires strict procedure control: qualified WPS, controlled heat input (typically 0.5–2.5 kJ/mm), maximum interpass temperature of 150°C, and over-alloyed filler metal (ER2594). Improper welding can result in excessive ferrite or intermetallic phase precipitation, both of which degrade corrosion resistance and toughness.

Q7: Is 2507 suitable for seawater applications?
Yes. 2507 is widely used in seawater systems including subsea equipment, offshore piping, pumps, valves, and heat exchangers. Its high PREN (≥40) provides excellent resistance to chloride pitting and crevice corrosion in submerged and splash-zone seawater service. It is one of the standard materials specified for critical seawater handling equipment in offshore oil and gas projects.

Q8: What is the difference between duplex and super duplex stainless steel?
The primary difference lies in alloy content and resulting corrosion resistance. Super duplex grades like 2507 contain higher chromium (~25% vs ~22%), higher molybdenum (~4% vs ~3%), and higher nitrogen than standard duplex grades like 2205. This results in a higher PREN (≥40 vs ~35) and superior resistance to chloride pitting and crevice corrosion. Both have the duplex austenite-ferrite microstructure and offer high strength.

Q9: What are the limitations of 2507 stainless steel?
Key limitations include: unsuitability for continuous service above approximately 300°C due to phase instability and embrittlement risk; vulnerability to strong reducing acids; higher cost than standard duplex and austenitic grades; and the requirement for experienced fabricators with qualified super duplex welding procedures. 2507 is a corrosion-resistant alloy — not a heat-resistant or universal material.

Q10: What information should I provide when ordering 2507 super duplex stainless steel?
At minimum, specify: grade (Super Duplex 2507), UNS number (S32750), applicable ASTM standard (e.g., ASTM A240 for plate), product form, dimensions, surface finish, and documentation requirements (EN 10204 3.1 MTC). For critical applications, also specify PMI testing, ferrite content verification per ASTM A923, and any third-party inspection requirements. Avoid generic requests such as "super duplex plate."

Technical References

• ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip
• ASTM A789 — Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Tubing
• ASTM A790 — Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe
• ASTM A923 — Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Stainless Steels
• ASTM A1084 — Standard Test Method for Detecting Detrimental Phases in Lean Duplex Stainless Steels
• Outokumpu Stainless Steel Handbook
• ASM Handbook Volume 1 — Properties and Selection: Irons, Steels, and High-Performance Alloys
• Nickel Institute — Duplex Stainless Steels: Guidelines for Fabrication and Application

Related Reading

• [2205 Duplex Stainless Steel: Properties, PREN, Welding and Uses]
• [Stainless Steel Grades Chart: AISI, UNS, EN and JIS Cross-Reference Guide]
• [304 vs 316 Stainless Steel: Differences and Applications]
• [Duplex Stainless Steel: A Complete Guide to 2205 and Beyond]
• [How to Read a Mill Test Certificate: A Buyer's Guide]