Characterization of σ-phase precipitation and effect on performance in duplex stainless steel S31803

•Duplex stainless steel flange has undergone corrosion fatigue failure.•Quantitative analysis of σ-phase was used by electron backscatter diffraction.•Impact toughness of duplex stainless steel was deteriorated by σ-phase.•Formation of σ-phase were affected by heat treatment and welding processes. A flange made of duplex stainless steel S31803 prematurely failed due to corrosion fatigue cracking in an offshore oilfield. This paper was carried out to determine the correlation among manufacturing processes, microstructures and properties of the failed flange by using various analysis methods. The results indicate that the σ-phase was present in heat affected zone and base metal, and the content of σ-phase was 17.0 vol% and 4.7 vol%, respectively. The hardness of heat affected zone was extremely high, up to 394HV0.3. The tensile and impact fractures of base metal were brittle cleavage fractures. The impact toughness of duplex stainless steel was remarkablely deteriorated due to the precipitation of σ-phase. The precipitation of σ-phase in base metal was owing to improper aging process. A large amount of σ-phase precipitation was caused by excessive welding heat input in heat affected zone.