Effect of Ni–Mn–N alloying on ferrite suppression and weld solidification mode in 316L-series austenitic stainless steels

Abstract 316L-series austenitic stainless steels are important candidate materials for structural and blanket-related components in nuclear fusion reactors, while small variations in Ni, Mn, and N contents can markedly affect phase constitution, magnetic and electrochemical behavior, and weld-metal solidification. In this study, the microstructures of 316L and 316LN stainless steels were characterized by EBSD and XRD and correlated with their magnetic and electrochemical responses. Equilibrium phase diagrams and Scheil solidification paths were further calculated for 316L(N)/CLF-1 dissimilar weld metals to clarify the coupled effect of Ni-Mn-N on solidification behavior. 316LN exhibited an almost single-phase austenitic microstructure, whereas 316L retained a small amount of ferrite, giving relative permeabilities of 1.004 and 1.165 at 300 K, respectively. The more homogeneous austenitic microstructure of 316LN improved passive-film stability, with a polarization resistance approximately 2.3 times that of 316L. Low Ni-Mn-N weld metal solidified predominantly through the L → L + δ path, with limited L + δ → γ peritectic reaction and incomplete subsequent δ → γ transformation, resulting in a ferrite + martensite microstructure. Higher Ni-Mn-N contents expanded the γ stability field and promoted more extensive peritectic and δ → γ transformations. Meanwhile, the calculated Ms decreased by approximately 30 °C, enhancing austenite stability and producing columnar austenite with interdendritic lath martensite. These results demonstrate the sensitivity of phase stability and weld solidification to coupled Ni-Mn-N variations and provide a thermodynamic basis for composition and solidification-mode control in 316L(N)/CLF-1 dissimilar joints.