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The microstructure evolution and mechanical properties of the 316H stainless steel weld metals with different C contents were studied at the aging temperature of 550 ℃ for different aging holding time.The transformation behavior of δ-ferrite and precipitation mechanisms of M23C6 and σ phase in the as-aged weld metal were investigated.The results indicated that for the as-welded weld metal,with increasing C content,the yield and tensile strengths increased,while the elongation decreased owing to the increase of C solid solution strengthening effect.Moreover,both the high δ-ferrite content in low C weld metal and the precipitated M23C6 carbide in high C weld metal deteriorated the impact energy obviously.During the aging process,the rapid precipitation of M23C6 carbide occurred in δ-ferrite firstly owing to the high diffusion rate of C.Once the carbon is depleted by precipitation of M23C6,the slow formation of σ phase occurred through eutectoid transformation (δ → σ + γ) depending on the diffusion of Cr and Mo.Moreover,increasing C content promoted the formation of M23C6 carbides and inhibited the formation of σ phase.Therefore,increasing C content accelerated the transformation of δ-ferrite in weld metal during aging process.Furthermore,after a long enough aging time,a transformation from M23C6 to σ occurred.The variations of mechanical properties with aging conditions depended to a large extent on the microstructures at different aging conditions.For the low C weld metal aged at 550 ℃,with the increase of the aging time,fine M23C6 first precipitated,then coarsened,after that σ phase formed,which caused that the yield and tensile strengths first increased,then decreased,and finally increased slightly again.For the medium C weld metal,as the aging time increased,first the depletion of the solid solution C as a result the M23C6 precipitation deteriorated the strength,and then the formation ofσ phase improved the strength.For the high C weld metal,with the increase of the aging time,the depletion of the solid solution C and the coarsening of the M23C6 precipitates deteriorated the strength.Furthermore,with increasing aging time,both the precipitation and coarsening of M23C6 and increasing σ phase content deteriorated the elongation and impact energy.