Duplex & Super Duplex Steel Pipes

The main difference lies in the chemical composition and resulting corrosion resistance. Super duplex pipe (like UNS S32750) has higher chromium (24-26% vs 21-23%), molybdenum (3-5% vs 2.5-3.5%), and nitrogen (0.24-0.32% vs 0.14-0.20%) content than standard duplex (UNS S32205). This gives super duplex a higher PREN value (>40 vs 32-38), offering superior corrosion resistance particularly in chloride environments and higher tensile strength (116 ksi vs 95 ksi minimum).

Standard duplex steel pipe is suitable for temperatures up to 300°C and moderate corrosive environments, while super duplex 2507 pipe can handle more aggressive conditions including seawater at elevated temperatures, sour gas service, and high-chloride industrial environments. Super duplex also commands a premium price, typically 30-50% higher than standard duplex grades.

ASTM A790 (equivalent to ASME SA 790) is the standard specification for seamless and straight-seam welded ferritic/austenitic (duplex) stainless steel pipe intended for general corrosive service, with a particular emphasis on resistance to stress corrosion cracking. The specification covers pipes in nominal sizes from NPS 1/2 to NPS 48 in various schedules and wall thicknesses.

The standard defines chemical composition requirements, mechanical property minimums, heat treatment procedures (solution annealing), testing protocols (hydrostatic, NDT, ferrite content), and acceptance criteria. Common grades covered include UNS S31803, S32205, S32304, S32750, S32760, and S32906. Our ASTM A790 UNS S32750 pipes meet all requirements including corrosion testing per ASTM G48.

Yes, duplex stainless steel pipes have good weldability when proper procedures are followed. However, welding must be carefully controlled to maintain the correct phase balance (40-60% austenite/ferrite ratio) and avoid the formation of detrimental intermetallic phases like sigma phase, chi phase, or chromium nitrides that form at elevated temperatures (300-1000°C).

We recommend using matching filler metals (e.g., ER2209 for 2205, ER2594 for 2507) or slightly overalloyed consumables to compensate for nitrogen loss during welding. Welding should be performed with heat input between 0.5-2.5 kJ/mm, interpass temperature below 150°C, and no preheat required (except for very thick sections >25mm). Post-weld heat treatment is generally not required, but solution annealing at 1050-1150°C may be specified for critical applications. Our technical team can provide detailed Welding Procedure Specifications (WPS) for your specific application.

ASTM A789 covers ferritic/austenitic stainless steel tubing for general service, while ASTM A790 covers pipe for general corrosive service. Though both specifications include the same duplex grades, the key differences lie in dimensional tolerances (tubing has tighter tolerances), size ranges (A789 covers smaller diameters), and testing requirements. A789 is typically used for heat exchangers, instrumentation, and hydraulic lines, while A790 is used for structural and process piping systems. Both our ASTM A789 UNS S32750 tubes and SA 790 UNS S32750 pipes are manufactured to the highest quality standards.

Standard duplex grades like 2205 (S32205) can operate continuously up to 300°C (572°F), though corrosion resistance may diminish above 250°C. Super duplex stainless steel pipe grades like 2507 (S32750) have similar temperature limits. The critical temperature range is 300-1000°C where embrittlement from sigma phase formation can occur with prolonged exposure (>2 hours). For applications above 250°C, austenitic stainless steels or nickel alloys may be more appropriate. At cryogenic temperatures, duplex steels can be used down to -50°C while maintaining excellent toughness and ductility.

Grade selection depends on several factors: operating temperature, chloride concentration, pH level, presence of H2S or CO2, pressure requirements, and cost constraints. For seawater applications at ambient temperature, standard duplex 2205 pipe (UNS S32205) is usually sufficient. For hot seawater (>60°C), high-chloride brines, or sour gas service, super duplex 2507 pipe (UNS S32750) or S32760 is recommended. Our corrosion engineering team can perform isocorrosion diagrams and PREN calculations to determine the most cost-effective grade for your specific service conditions. Contact us with your process parameters for a detailed material recommendation.

While duplex steel pipes are more robust than austenitic stainless steels, proper handling prevents surface contamination. Avoid contact with carbon steel (use dedicated lifting equipment and storage racks), prevent exposure to chloride sources during storage, and protect pipe ends with plastic caps. Clean thoroughly before welding to remove any oil, grease, or iron contamination. Unlike carbon steel, duplex pipes do not require protective coatings for most applications, though organic or inorganic coatings may be applied for extreme conditions or extended storage. Store in dry, well-ventilated areas with adequate support to prevent distortion.

Every shipment includes comprehensive documentation: Mill Test Certificates (MTC) per EN 10204 3.1 showing chemical analysis of each heat/lot, mechanical test results (tensile, yield, elongation, hardness, impact energy), ferrite content measurement, heat treatment records, dimensional inspection reports, and PMI (Positive Material Identification) verification. For export orders, we provide commercial invoice, packing list, certificate of origin, export license (if required), and fumigation certificates. Third-party inspection certificates from Lloyd’s, Bureau Veritas, TUV, or other approved agencies can be arranged. All documents are provided in hard copy and digital format for your records and project files.