δ-ferrite transformation mechanism and its effect on mechanical properties of 316H weld metal
来源期刊：JOURNAL OF MATERIALS SCIENCE TECHNOLOG2020年第22期
论文作者：Langlang Zhao Shitong Wei Dong Wu Dianbao Gao Shanping Lu
摘 要：The microstructure evolution and mechanical properties of the 316 H 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 σ-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 σ 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 of σ 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 ene rgy.
Langlang Zhao1,2,3，Shitong Wei1，Dong Wu3，Dianbao Gao4，Shanping Lu3
1. Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences2. School of Materials Science and Engineering, University of Science and Technology of China3. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences4. Nuclear Power and Petro-Chemical Business Group,China First Heavy Industries
摘 要：The microstructure evolution and mechanical properties of the 316 H 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 σ-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 σ 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 of σ 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 ene rgy.
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