σ-phase precipitation in weld metal of austenitic stainless steels and its effect on properties

Low-temperature conditions necessitate high toughness in the welded joints of austenitic stainless steel structures. However, the impact toughness of the weld metal decreases significantly when using electrodes developed for SMAW under such conditions. To address this issue, a comparative study was conducted on the structure and properties of the weld metal by selecting welding electrodes with similar structures. The results indicate that the high Mo content in the weld metals post-welding triggers the eutectic decomposition of some δ-ferrite between the interdendrites during the cooling process: δ → γ′ + σ. This leads to the formation of a mixed structure comprising δ-ferrite, σ-phase, and γ′ between the interdendrites. Consequently, this results in a complete brittle fracture of the impact section when subjected to impact loading at low temperatures. The precipitation of a large number of σ-phase between the interdendritic regions during the deformation process makes it the preferential site for cracking. This shortens the crack propagation path, diminishes the plastic deformation ability of the base material, and ultimately results in an average absorbed energy of only 6.33 J at −196 ℃. This is more than an 85% reduction compared to the low Mo weld metal impact value at −196 ℃. A small amount of δ-ferrite did not reduce the low-temperature toughness of the weld metal; however, precipitation of the σ-phase can seriously deteriorate the weld toughness.