====================================================================== Weld Procedure Specification (WPS) Addendum for Super Duplex Stainless Steels ====================================================================== DEFINITION ---------------------------------------- A Weld Procedure Specification (WPS) Addendum for Super Duplex Stainless Steels is a formally documented supplement to a base WPS that addresses the unique metallurgical, thermal, and corrosion-related requirements of welding super duplex stainless steels (e.g., UNS S32750, S32760). It specifies critical variables—including heat input limits, interpass temperature control, post-weld cooling rates, and filler metal selection—to preserve the balanced austenite-ferrite microstructure and ensure optimal pitting resistance equivalent number (PREN) and mechanical properties. Unlike generic WPS documents, this addendum mandates strict adherence to time-temperature-transformation (TTT) constraints to avoid deleterious phase formation (e.g., sigma, chi, nitrides). OVERVIEW ---------------------------------------- Super duplex stainless steels (SDSS) exhibit exceptional strength and corrosion resistance due to their approximately 50:50 austenite–ferrite microstructure and high chromium, molybdenum, and nitrogen content. However, this microstructure is highly sensitive to thermal cycles during welding; excessive heat input or slow cooling can precipitate brittle intermetallic phases (e.g., sigma phase above 600–1000°C) or Cr₂N nitrides in the ferrite-rich regions, severely degrading toughness and localized corrosion resistance. The WPS Addendum therefore serves as a domain-specific technical safeguard—building upon ASME Section IX, ISO 15614-1, and ASTM A923 test requirements—to codify empirically validated welding parameters that maintain phase balance (typically 35–65% ferrite), minimize time-at-temperature in critical ranges, and ensure adequate nitrogen retention through controlled shielding gas composition (e.g., N₂ additions to Ar/He mixtures). It also integrates mandatory verification protocols such as ferritoscope measurements, intergranular corrosion testing (ASTM A923 Method C), and microstructural examination (ISO 17638) to confirm conformance. In practice, the addendum functions as both a qualification boundary document and a field execution guide—used by welding engineers, inspectors, and fabricators in offshore oil & gas, desalination plants, and chemical processing systems where SDSS integrity is mission-critical. KEY COMPONENTS ---------------------------------------- 1. Heat Input Limitation (kJ/mm) 2. Interpass Temperature Control (≤150°C) 3. Filler Metal Chemistry Matching (e.g., EN 1600 ER2594) APPLICATIONS ---------------------------------------- - Offshore subsea piping systems - Seawater injection and desalination plant manifolds - Chemical process equipment handling aggressive chloride environments KEY FORMULAS ---------------------------------------- Heat Input: HI = (V × I × 60) / (S × 1000) -> Calculates heat input in kJ/mm, where V = arc voltage (V), I = welding current (A), S = travel speed (mm/min) Pitting Resistance Equivalent Number (PREN): PREN = %Cr + 3.3×%Mo + 16×%N -> Quantifies theoretical pitting corrosion resistance; target PREN ≥ 40 required for super duplex grades Ferrite Number (FN) Target Range: FN = 35–65 (measured per ASTM E562 or ISO 8249) -> Specifies acceptable ferrite content range to ensure microstructural balance and corrosion performance RELATED CONCEPTS ---------------------------------------- - Phase Balance Control - Time-Temperature-Transformation (TTT) Diagrams - Intermetallic Phase Precipitation Kinetics REFERENCES ---------------------------------------- ISO 15614-1:2017 Metallic materials — Qualification testing of welders — Fusion welding — Part 1: Steel (https://www.iso.org/standard/65661.html) ASME BPVC Section IX: Welding, Brazing, and Fusing Qualifications (https://www.asme.org/codes-standards/bpvc-section-ix-welding-brazing-fusing-qualifications) ASTM A923-22 Standard Test Methods for Detecting Detrimental Intermetallic Phases in Duplex Austenitic/Ferritic Stainless Steels (https://www.astm.org/a0923-22.html) TAGS ---------------------------------------- welding, superduplex, corrosionresistance, wps, metallurgy