Failure Investigation of a Duplex Stainless Steel Flange: Role of Improper Heat Treatment and Sigma Phase Embrittlement

Duplex stainless steels (DSSs) are widely used in pressure-retaining components due to their high strength, toughness, and corrosion resistance arising from a balanced ferrite–austenite microstructure. However, exposure to inappropriate thermal conditions during fabrication or heat treatment can promote precipitation of intermetallic phases, particularly sigma (σ) phase, which severely degrades fracture toughness and corrosion performance. This study presents a detailed failure investigation of a DSS flange that fractured during hydrostatic pressure testing.

The failure investigation comprised visual examination, fractography, optical microscopy, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), hardness mapping, ferrite content measurement, Charpy V-notch impact testing, and ASTM G48 corrosion testing. Material traceability records were reviewed to correlate metallurgical observations and property variations across different heat numbers supplied for identical service conditions.

Microstructural analysis revealed extensive sigma phase precipitation along ferrite–austenite interfaces in the failed flange, accompanied by chromium and molybdenum segregation consistent with intermetallic phase formation. Ferrite measurements indicated deviation from the recommended phase balance for DSS. Charpy impact testing showed a pronounced reduction in absorbed energy, confirming severe embrittlement, while ASTM G48 testing demonstrated localized corrosion attack associated with chromium-depleted regions. Fractographic examination revealed chevron patterns characteristic of rapid brittle fracture, with crack initiation associated with sigma-rich embrittled regions and unstable propagation during hydrostatic loading. Comparative assessment across multiple heat numbers demonstrated that these degradations were confined to a single material batch, whereas components from other heats exhibited acceptable microstructure, toughness, and corrosion resistance.

The combined evidence of intermetallic precipitation, microchemical segregation, mechanical embrittlement, corrosion susceptibility, and batch-specific occurrence indicates improper thermal exposure during heat treatment of the affected heat number as the root cause of failure. Sigma phase–induced embrittlement significantly reduced fracture toughness, causing brittle failure during hydrotesting. The findings highlight the critical importance of strict thermal process control, heat number traceability, and phase balance verification.