Influence of Solution Annealing Time on the Hydrostatic Integrity of Super Duplex Steel 3D Hot Induction Bends

Abstract

UNS stainless steel S32760 is commonly used in the petrochemical and gas industries for its superior mechanical properties and corrosion resistance. Its primary application in offshore systems are predominantly in topside process piping. It consists of extensive network of pipes located on the deck of offshore oil and gas platforms. These high-temperature pipes transport hydrocarbons and chemicals for processing, drilling, and utility services, exposing them to highly corrosive processed fluids and saltwater. Despite its superior performance, both austenitic (γ) and ferritic (α) phases could be altered by shifts in the microstructure, by excessive heat input during welding, potentially affecting their mechanical properties and corrosion resistance. The research investigates the impact of hot induction bending on stainless steel S32760 by examining its post-bending effects at recommended temperatures of 850 to 1150 °C. Metallurgical analysis was performed to evaluate the austenite (γ) and ferrite (α) contents in the base metal (BM), heat-affected zone (HAZ) and weld metal (WM) followed by positive material identification (PMI). The results observed an increase in chromium content by 25% reflects microstructural phase distribution and elemental partitioning rather than direct evidence of intermetallic phase formation. Mechanical properties and corrosion resistance were tested after the bending process through hardness and pitting resistance equivalent number (PREN). Higher hardness measurements and corrosion resistance demonstrated robust structural integrity of samples tested. Further hydrostatic pressure test and radiography examination were performed to evaluate the integrity of the welded materials in multiple solution annealing treatments at 1120 °C and 1110 °C. The data suggest multiple heat treatments during solution annealing revealed well-distributed mixture of ferrite and austenite indicated by 381 MPa. Yet, lower ferrite content demonstrated failure at 266 MPa due to the brittleness. This research suggested that multiple heat treatments and solution annealing above 1000 °C are effective to prevent formation of intermetallic phase and enhance materials’ structural integrity.