316H austenitic stainless steel is widely used in the manufacture of nuclear reactor components owing to its excellent comprehensive properties, such as main vessel, support assembly, etc. During the fusion welding of austenitic stainless steel, the tendency of hot cracking tends to occur when the structural restraint is too large. For preventing the cracking, it is usually desirable to form a certain amount of δ-ferrite in the weld. However, the δ-ferrite is harmful to the mechanical and corrosion properties of the weld during high temperature service process, so the δ-ferrite content in weld metal must be strictly controlled. The microstructure evolution and mechanical properties of the 316H stainless steel weld metals with different C contents were studied at the aging temperature of 600 ℃ for different times. The results indicated that for the as-welded weld metal, with the increasing C content, the yield and tensile strengths increased, while the elongation decreased owing to the increasing C solid solution strengthening effect. 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. Furthermore, after a long enough aging time, a transformation from M23C6 to σ occurred. The C content has a significant influence on the δ-ferrite transformation behavior, δ-ferrite in the low and medium C weld metals transforms into M23C6 and σ phase successively, while δ-ferrite in high C weld metal only transforms into M23C6 carbides. The variations of mechanical properties with aging conditions depended mainly on the microstructures at different aging conditions. For the low C weld metal, as the aging time increased, fine M23C6 first precipitated, after that σ phase formed, the increasing σ phase content improved the strength obviously. For the medium and high C weld metals, as the aging time increased, first the depletion of the solid solution C as a result the M23C6 precipitation deteriorated the strength, then the formation of σ phase improved the strength. Furthermore, with the increasing aging time, the precipitation and coarsening of M23C6 and σ phase deteriorated the elongation and impact energy. This research provides theoretical and practical guidance for the control of the chemical composition and δ-ferrite content in the 316H weld metal for high temperature service.
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|Affiliation/Organization||Institute of Metal Research, Chinese Academy of Sciences|