The precipitation behavior of σ phase and its effect on mechanical properties of ER347 deposited metal during long-term aging at 750 ℃ are studied. The results show that the ferrite content

hardness and impact toughness of ER347 deposited metal are closely related to the aging time. When ER347 deposited metal is aged at 750 ℃

with the extension of aging time

σ phase precipitation increases

hardness increases

while the ferrite content and toughness decrease. ER347 deposited metal has good as-welded impact toughness

but the toughness decreases after aging treatment. The root cause of the embrittlement is that δ-ferrite decomposes to form a hard and brittle σ phase

which significantly reduces the toughness of ER347 deposited metal. There are a large number of high-energy σ/γ2 and δ/σ/γ2 incoherent interface inside（σ +γ2） structure

and the σphase itself is hard and brittle

which is easy to act as a potential crack source

under high strain rate

brittle cracking occurs and spreads rapidly

resulting in the deposited metal toughness decreased.