The effect of sigma phases formation depending on Cr/Ni equivalent ratio in AISI 316L weldments

The crack properties and toughness characteristics in AISI 316L weld metals were investigated in different chemical composition ranges of Cr and Ni contents. Three new filler wires were designed for the flux-cored arc welding (FCAW) process, and the Cr/Ni equivalent ratio and solidification mode of each weld metal was controlled. The microstructure revealed ductility-dip cracking (DDC) generated in the specimens containing <3 wt% of delta-ferrite (δ-ferrite). Sigma (σ) phase precipitation occurred in the weld metals heat-treated at temperatures ranging from 650 °C to 950 °C. The amount of σ in the δ-ferrite region increased significantly with increasing heat treatment time and temperature. At the highest Cr/Ni equivalent ratio, more of the σ phase of the weld metal transformed faster after heat treatment, which deteriorated the toughness. Meanwhile, less σ phase precipitation and DDC occurrence in the specimen containing <3 wt% of δ-ferrite did not affect the toughness reduction.