Mechanical-Microstructural degradation mapping of thermally aged duplex stainless Steel: Linking Sigma (σ) phase evolution with small punch test response and embrittlement risk

Duplex stainless steels (DSS) are widely used in energy, marine, and chemical industries for their balanced combination of strength, toughness, and corrosion resistance. However, exposure to intermediate temperatures promote sigma (σ) phase precipitation, which can lead to severe embrittlement and loss of structural integrity. This study investigates the thermal embrittlement of 2205 DSS aged at 850 °C using the Small Punch Test (SPT) combined with quantitative microstructural characterization. Maximum load (Fm), displacement at maximum load (Um), and absorbed energy (ESP) were correlated with σ phase evolution. Three degradation regimes were identified: stable response (≤ 6 min), rapid embrittlement (30 min-1 h), and saturation beyond 3 h with fully brittle behaviour. Absorbed energy decreased by 32 % after 30 min and by 96 % after 17 h, while ductility and strength decreased by 85 % and 78 %, respectively. SEM-based image analysis showed a strong correlation between increasing σ phase fraction mechanical property degradation. A mechanical-microstructural degradation map was developed to quantitatively link σ phase fraction with reduction in toughness, ductility, and strength, providing a predictive framework for embrittlement assessment. The integrated SPT-microstructure approach demonstrates high sensitivity to early-stage degradation and offers a practical approach relevant to service-induced embrittlement and failure risk assessment.