UNS S31254, commonly known by its trade name 254 SMO (Sandvik trade designation), is a super austenitic stainless steel specifically engineered for applications requiring exceptional corrosion resistance in chloride-containing environments. This high-performance alloy represents the pinnacle of austenitic stainless steel development, offering corrosion resistance approaching that of nickel-based alloys while maintaining the fabricability and cost advantages of stainless steel. Understanding the precise chemical composition of UNS S31254 is crucial for engineers, materials specialists, and procurement professionals who specify this material for demanding applications in chemical processing, marine environments, desalination plants, and other aggressive service conditions.
This comprehensive guide explores the detailed chemical composition of UNS S31254, explains the role of each alloying element, examines how the composition impacts material properties, and provides insights into specifications, alternative designations, and practical considerations for material selection.
Overview of UNS S31254
UNS S31254 belongs to the family of super austenitic stainless steels, also known as 6-moly stainless steels due to their molybdenum content of approximately 6%. These materials bridge the performance gap between conventional austenitic stainless steels (like 316L) and expensive nickel-based alloys, providing superior pitting and crevice corrosion resistance in chloride environments.
International Designations
UNS S31254 is known by various designations worldwide:
- UNS S31254 - Unified Numbering System (North America)
- 254 SMO - Trade name (Sandvik)
- 1.4547 - European material number (EN/DIN)
- X1CrNiMoCuN20-18-7 - German designation
- ASTM A240 - Specification for plate, sheet, and strip
- ASTM A276 - Specification for bar and shapes
- ASTM A312 - Specification for welded and seamless pipe
The alloy was developed in the 1970s-1980s as an improvement over standard austenitic stainless steels, specifically targeting applications where 316L and even 317L demonstrated inadequate corrosion resistance.
Detailed Chemical Composition
The chemical composition of UNS S31254 is precisely controlled to achieve its exceptional corrosion resistance and mechanical properties. According to ASTM specifications, the composition requirements are:
Primary Alloying Elements
Carbon (C): 0.020% maximum
The extremely low carbon content is a defining characteristic of UNS S31254:
- Corrosion Resistance: Minimizes chromium carbide precipitation at grain boundaries during welding or high-temperature exposure, preventing sensitization and intergranular corrosion
- Weldability: Eliminates the need for post-weld heat treatment in most applications, as the material remains resistant to intergranular corrosion in the as-welded condition
- Designation: The low carbon content is sometimes indicated by the "L" designation in alternative nomenclature
- Practical Impact: Modern production techniques typically achieve carbon levels of 0.015% or lower, well below the maximum specification
Chromium (Cr): 19.5 - 20.5%
Chromium is the fundamental element providing stainless characteristics:
- Passive Film Formation: Forms a protective chromium oxide (Cr₂O₃) layer on the surface that prevents further corrosion
- Oxidation Resistance: Provides resistance to oxidizing environments and high-temperature oxidation
- Pitting Resistance: Contributes significantly to resistance against localized corrosion in chloride environments
- Typical Range: Usually maintained around 20% for optimal balance of corrosion resistance and other properties
- Critical Threshold: The minimum 19.5% ensures adequate passivation across all production heats
Nickel (Ni): 17.5 - 18.5%
Nickel is the second major alloying element with multiple critical functions:
- Austenite Stabilization: Maintains the face-centered cubic (FCC) austenitic structure across wide temperature ranges, preventing transformation to brittle martensitic or ferritic phases
- Corrosion Resistance: Enhances resistance to reducing acids and provides excellent resistance to stress-corrosion cracking in chloride environments
- Ductility and Toughness: Contributes to the material's excellent ductility, formability, and low-temperature toughness
- Typical Content: Usually around 18%, providing robust austenite stability without excessive cost
- Economic Factor: Nickel is expensive, so the content is optimized to provide necessary properties while controlling costs
Molybdenum (Mo): 6.0 - 6.5%
Molybdenum is the signature element distinguishing super austenitic grades:
- Pitting Resistance: Dramatically enhances resistance to pitting corrosion in chloride-containing solutions
- Crevice Corrosion Resistance: Improves resistance to localized corrosion in crevices, gaps, and under deposits
- General Corrosion: Enhances resistance in reducing acid environments, particularly sulfuric and phosphoric acids
- 6-Moly Designation: The approximately 6% molybdenum content gives rise to the "6-moly" classification
- Performance Impact: This high molybdenum level, combined with chromium and nitrogen, creates exceptional localized corrosion resistance
- Cost Factor: Molybdenum is expensive; the high content significantly impacts material cost but is essential for performance
Nitrogen (N): 0.18 - 0.22%
Nitrogen is a critical alloying element in modern super austenitic stainless steels:
- Pitting Resistance: Significantly enhances resistance to pitting corrosion, contributing to the high PREN value
- Strength Enhancement: Provides solid-solution strengthening without sacrificing ductility or toughness
- Austenite Stabilization: Helps maintain austenitic structure, allowing reduction in nickel content
- Interstitial Strengthening: Occupies interstitial sites in the crystal lattice, increasing strength
- Controlled Addition: Nitrogen is intentionally added during melting under pressure to achieve specified levels
- Typical Range: Usually maintained around 0.20% for optimal property balance
Copper (Cu): 0.50 - 1.00%
Copper addition is a distinguishing feature of UNS S31254:
- Acid Resistance: Significantly improves resistance to sulfuric acid, particularly in reducing conditions
- Corrosion Enhancement: Contributes to overall corrosion resistance in specific environments
- Typical Content: Usually around 0.7%, balanced to provide benefits without compromising hot workability
- Historical Context: Copper addition was a key innovation in developing super austenitic grades
- Limitation: Excessive copper can negatively impact hot ductility; content is carefully controlled
Secondary Alloying Elements
Manganese (Mn): 1.00% maximum
Manganese serves multiple metallurgical purposes:
- Deoxidation: Acts as deoxidizer during steel production
- Sulfur Control: Binds with sulfur to form manganese sulfide inclusions rather than harmful nickel sulfides
- Austenite Stabilization: Contributes to maintaining austenitic structure
- Nitrogen Solubility: Enhances nitrogen solubility in the melt
- Typical Content: Usually 0.5-0.8%, sufficient for metallurgical purposes
Silicon (Si): 0.80% maximum
Silicon functions primarily as a processing aid:
- Deoxidation: Strong deoxidizer during melting and casting
- Oxidation Resistance: Contributes moderately to high-temperature oxidation resistance
- Scale Formation: Affects surface oxide characteristics
- Typical Content: Usually 0.3-0.6%
- Limitation: Excessive silicon can promote sigma phase formation; kept below specification maximum
Phosphorus (P): 0.030% maximum
Phosphorus is considered a residual impurity:
- Detrimental Effects: Can reduce toughness, particularly at low temperatures
- Minimization: Modern steelmaking practices typically achieve levels well below 0.020%
- Specification Limit: Maximum 0.030% ensures adequate toughness and ductility
- Quality Indicator: Lower phosphorus levels often indicate higher quality production practices
Sulfur (S): 0.010% maximum
Sulfur is another impurity that must be strictly controlled:
- Hot Workability: Excessive sulfur severely reduces hot ductility and can cause hot cracking during processing
- Corrosion Impact: Can reduce corrosion resistance, particularly pitting resistance
- Strict Control: The very low maximum (0.010%) reflects modern clean steel production
- Actual Practice: Premium grades often achieve sulfur levels below 0.005%
- Inclusion Control: Sulfur forms sulfide inclusions that can act as initiation sites for corrosion
Iron (Fe): Balance
Iron comprises the remainder of the composition after accounting for all specified elements:
- Matrix Element: Forms the base matrix of the steel
- Typical Content: Approximately 55-57% by weight
- Cost Factor: Iron is the least expensive component, helping control overall alloy cost
- Property Impact: The specific iron content (within the compositional balance) has minimal direct impact on properties
Compositional Balance and Interactions
The exceptional performance of UNS S31254 results not just from individual elements but from their synergistic interactions and carefully balanced proportions.
Pitting Resistance Equivalent Number (PREN)
The PREN is a widely used index for predicting pitting corrosion resistance in chloride environments:
PREN Formula: PREN = %Cr + 3.3(%Mo) + 16(%N)
UNS S31254 PREN Calculation:
- Chromium contribution: 20% × 1 = 20
- Molybdenum contribution: 6.2% × 3.3 = 20.5
- Nitrogen contribution: 0.20% × 16 = 3.2
- Total PREN ≈ 43-44
This PREN value significantly exceeds conventional austenitic stainless steels:
- 316L (UNS S31603): PREN ≈ 24
- 317L (UNS S31703): PREN ≈ 29
- 904L (UNS N08904): PREN ≈ 35
The high PREN of UNS S31254 indicates superior resistance to pitting and crevice corrosion, enabling use in aggressive chloride environments where lower-PREN alloys fail.
Austenite Stability
The nickel, manganese, nitrogen, and copper content collectively ensure the material maintains a stable austenitic structure:
- Room Temperature: Fully austenitic FCC structure
- Cryogenic Temperatures: Remains austenitic down to -196°C (liquid nitrogen temperature)
- Elevated Temperatures: Maintains austenitic structure up to approximately 850°C
- No Phase Transformations: Does not undergo ductile-to-brittle transitions or martensitic transformations during normal thermal cycling
Intermetallic Phase Formation
The composition must be carefully balanced to avoid detrimental intermetallic phases:
Sigma Phase (σ): A hard, brittle intermetallic phase that can form during prolonged exposure to 600-900°C
- The high molybdenum and chromium content increases sigma phase susceptibility
- Rapid cooling through the critical temperature range prevents sigma formation
- Solution annealing dissolves any sigma phase that may have formed
Chi Phase (χ): Another potentially embrittling phase in the same temperature range
- Similar formation conditions as sigma phase
- Controlled through proper heat treatment and avoiding critical temperature ranges
Proper Heat Treatment: Solution annealing at 1100-1150°C followed by rapid cooling prevents these phases and ensures optimal properties
Strength-Ductility Balance
The composition achieves an excellent balance of strength and ductility:
- Solid-Solution Strengthening: Molybdenum, nitrogen, and other substitutional elements provide strengthening
- Interstitial Strengthening: Nitrogen occupies interstitial sites, increasing strength without embrittlement
- Ductility Maintenance: Austenitic structure and low interstitial content (carbon) ensure excellent ductility
- Result: Yield strength around 300 MPa with elongation exceeding 35%, providing both structural capability and formability
Compositional Specifications and Standards
UNS S31254 is covered by multiple international standards that specify composition limits:
ASTM Specifications
ASTM A240/A240M: Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications
ASTM A276/A276M: Standard Specification for Stainless Steel Bars and Shapes
ASTM A312/A312M: Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes
ASTM A479/A479M: Standard Specification for Stainless Steel Bars and Shapes for Use in Boilers and Other Pressure Vessels
European Standards
EN 10088-2: Stainless steels - Part 2: Technical delivery conditions for sheet/plate and strip of corrosion resisting steels for general purposes
EN 10088-3: Stainless steels - Part 3: Technical delivery conditions for semi-finished products, bars, rods, wire, sections and bright products of corrosion resisting steels for general purposes
Other International Standards
JIS G4305: Cold-rolled stainless steel plate, sheet and strip (Japanese standard)
GB/T 4237: Hot rolled stainless steel plate, sheet and strip (Chinese standard)
Each specification may have minor variations in allowable ranges or testing requirements, but the fundamental composition remains consistent.
Composition Impact on Material Properties
The specific chemical composition directly determines UNS S31254's performance characteristics:
Corrosion Resistance
Exceptional Pitting Resistance: The combination of 20% Cr, 6% Mo, and 0.20% N creates outstanding resistance to pitting in chloride solutions, with Critical Pitting Temperature (CPT) typically exceeding 50°C in 6% FeCl₃ solution.
Superior Crevice Corrosion Resistance: High molybdenum and nitrogen content provide excellent resistance to crevice corrosion, the most common failure mode in seawater and other chloride environments.
Stress-Corrosion Cracking (SCC) Resistance: High nickel content (18%) ensures excellent resistance to chloride-induced SCC that plagues lower-nickel austenitic grades.
Acid Resistance: Copper addition significantly improves resistance to sulfuric acid, while high molybdenum enhances resistance to phosphoric acid.
Seawater Performance: The composition is optimized for outstanding performance in natural and chlorinated seawater, including resistance to microbiologically influenced corrosion (MIC).
Mechanical Properties
Tensile Strength: 650-800 MPa, higher than conventional austenitic stainless steels due to nitrogen strengthening
Yield Strength: Approximately 300-350 MPa (0.2% offset)
Elongation: Minimum 35%, typically 40-50% in annealed condition
Hardness: Approximately 200-250 HB (Brinell), reflecting the solid-solution strengthened condition
Toughness: Excellent impact toughness across wide temperature range, maintaining ductility at cryogenic temperatures
Physical Properties
Density: 8.0 g/cm³ (0.289 lb/in³)
Thermal Expansion: Approximately 16 × 10⁻⁶ /°C (20-100°C)
Thermal Conductivity: Approximately 14 W/m·K at 100°C (lower than lower-alloy stainless steels due to high alloy content)
Electrical Resistivity: Approximately 85 μΩ·cm at 20°C
Magnetic Properties: Non-magnetic in annealed condition (magnetic permeability approximately 1.02)
Quality Control and Composition Verification
Ensuring actual composition meets specification requirements is critical for material performance:
Chemical Analysis Methods
Optical Emission Spectroscopy (OES): Most common method for rapid analysis of all major elements
X-Ray Fluorescence (XRF): Non-destructive method for composition verification, particularly useful for product identification
Combustion Analysis: Specific methods for accurate determination of carbon, sulfur, and nitrogen
Wet Chemical Analysis: Traditional methods still used for referee analysis and nitrogen determination
Inductively Coupled Plasma (ICP): Highly accurate for trace element analysis
Material Certification
Mill Test Reports (MTR): Provide certified chemical composition from heat analysis, mechanical properties, and heat treatment details
Heat Number Traceability: Each production lot is assigned a unique heat number, allowing complete traceability
Third-Party Testing: Critical applications may require independent laboratory verification of composition
Positive Material Identification (PMI): Handheld XRF analyzers enable field verification of material grade
Composition Variations and Special Grades
While standard UNS S31254 composition is tightly controlled, variations exist:
Standard Production
Regular production material meeting ASTM specifications suitable for most applications
Controlled Chemistry Grades
Some suppliers offer tighter compositional control for critical applications:
- Narrower ranges for key elements (Cr, Ni, Mo)
- Lower maximum limits for impurities (P, S)
- Enhanced cleanliness (reduced inclusion content)
Alternative Compositions
Higher Nitrogen Variants: Some producers offer grades with nitrogen toward the upper specification limit (0.22%) for enhanced strength and pitting resistance
Lower Carbon Variants: Ultra-low carbon versions with <0.015% C for maximum intergranular corrosion resistance
Comparison with Related Alloys
Understanding compositional differences helps in material selection:
UNS S31254 vs. 316L (UNS S31603)
Major Compositional Differences:
- Molybdenum: 6% vs. 2-3%
- Nitrogen: 0.20% vs. <0.10%
- Copper: 0.7% vs. none
- Nickel: 18% vs. 10-14%
Performance Impact: S31254 provides dramatically superior pitting resistance (PREN 43 vs. 24) and is suitable for aggressive chloride environments where 316L fails
UNS S31254 vs. 904L (UNS N08904)
Compositional Comparison:
- Chromium: 20% vs. 19-23%
- Molybdenum: 6% vs. 4-5%
- Nitrogen: 0.20% vs. minimal
- Copper: 0.7% vs. 1-2%
Performance: S31254 offers higher pitting resistance (PREN 43 vs. 35) and better seawater performance; 904L provides excellent sulfuric acid resistance
UNS S31254 vs. Other Super Austenitics
AL-6XN (UNS N08367): Similar 6-moly grade with comparable performance; composition differences are minor
1925hMo (UNS N08926): Another 6-moly grade with slightly different Cr/Ni/Mo balance
Selection: Differences between 6-moly grades are often less significant than availability, cost, and specific application requirements
Practical Considerations for Specification
When specifying UNS S31254, consider:
Composition Verification
- Request certified mill test reports for all material
- Verify heat numbers match between certification and actual material
- Consider PMI testing for critical applications
- Ensure composition meets intended service requirements
Special Requirements
- Specify tighter composition ranges if needed for critical applications
- Request lower impurity limits (P, S) for maximum corrosion resistance
- Specify nitrogen content range if strength is critical
- Consider enhanced cleanliness for fatigue-critical applications
Form and Condition
- Verify solution annealed condition for optimal corrosion resistance
- Confirm appropriate product form (plate, sheet, bar, pipe, etc.)
- Check that thickness and dimensions are within specification capabilities
- Ensure surface finish meets application requirements
Conclusion
The chemical composition of UNS S31254 represents decades of metallurgical development aimed at creating an austenitic stainless steel with exceptional corrosion resistance in chloride environments. The carefully balanced combination of 20% chromium, 18% nickel, 6% molybdenum, 0.20% nitrogen, and copper addition creates a super austenitic stainless steel with a PREN value of approximately 43-44, providing performance that approaches nickel-based alloys while maintaining the fabricability and relative cost-effectiveness of stainless steel.
Each element in the composition serves specific purposes: chromium provides passivity, nickel ensures austenitic stability and SCC resistance, molybdenum dramatically enhances pitting resistance, nitrogen strengthens and improves localized corrosion resistance, and copper boosts acid resistance. The extremely low carbon content ensures excellent weldability and freedom from sensitization.
For engineers specifying materials for aggressive environments—whether seawater systems, desalination plants, chemical processing equipment, or offshore platforms—understanding UNS S31254's composition and how it translates to performance is essential for optimal material selection. The alloy's composition positions it as a premium solution for applications where conventional austenitic stainless steels prove inadequate but the full cost of nickel-based alloys is not justified.
Proper specification, composition verification through certified mill test reports, and understanding of how composition impacts properties ensure successful application of this high-performance material in demanding service conditions.
