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The influence of σ -phase on mechanical properties in type ER309L stainless steel has been investigated by small punch test. The SA508 cl.3 reactor pressure vessel steel plates were overlay-cladded with the type ER309L welding consumable with different heat input rates. The microstructure of the clad was composed of fcc γ -austenite, a few percent of bcc δ-ferrite and bct σ -phase. Area fraction of σ -phase was ranging approximately 2-8% depending on the welding condition. Ductile to brittle transition behavior was observed in the clad, and it was strongly affected by the amount of brittle σ -phase. When the σ -phase content was about 8 percent, the transition behavior almost disappeared due to severe cracking. Fracture appearance gradually changed from circumferential to radial cracking not only as test temperature became low but as the volume fraction of σ -phase increased. The results are manifested in terms of fracture stress of σ -phase and δ-ferrite combined with the roles of radial and circumferential stress by means of the stress analysis of deformed specimen using finite element method.

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