Abstract: In this work, 16Cr-25Ni austenitic stainless steel weld metals (WM) were prepared with Mo contents of 6.3 wt.% and 2.4 wt.%. The evolution of precipitates and their influence on the impact fracture behavior of two WMs under as-weld, post-weld heat treatment (PWHT), and aging after PWHT conditions were systematically studied. The results showed that the reduction of Mo content changed the precipitation behavior of precipitated phases in the WMs. The 6.3 wt.% Mo WM contained M₆C carbides, M₂₃C₆ carbides, and M₂(C, N) carbonitrides. However, only the M₂₃C₆ carbides were present in the 2.4 wt.% Mo WM. The M₆C carbides remained stable in the interdendritic regions (IDRs) of the 6.3 wt.% Mo WM for aging 5000 h after PWHT. The M₂₃C₆ carbides and M₂(C, N) carbonitrides aggregated in IDRs. Additionally, approximately 71 nm of M₂₃C₆ carbides precipitated semi-continuously along the grain boundaries (GBs). In the 2.4 wt.% Mo WM for aging 5000 h after PWHT, M₂₃C₆ carbides were distributed in the IDRs and dendrite core regions (DCRs). Along the GBs, continuous M₂₃C₆ precipitates about 252 nm in size were observed. As the Mo content of the WM decreased, the tensile strength decreased slightly, but its impact toughness increased significantly under different conditions. During the impact of the WMs, deformation twins, deformation bands, dislocation slip, and secondary phases interacted to influence impact toughness. Deformation twins and deformation bands contributed to the impact toughness of the WMs. However, the number of deformation twins in the 2.4 wt.% Mo WM was always higher than that in the 6.3 wt.% Mo WM. For aging 5000 h after PWHT, micron-size M₆C carbides caused crack initiation, and nano-size M₂₃C₆ carbides and M₂(C, N) carbonitrides clusters in the IDRs of the 6.3 wt.% Mo WM strongly impeded dislocations, making IDRs preferred paths for crack propagation. In the 2.4 wt.% Mo WM, M₂₃C₆ along GBs provided significant dislocation pinning. This caused GBs to be the primary crack initiation and propagation paths.

Key words: Mo content, Austenitic stainless steel weld metals, Aging, Microstructural evolution, Impact toughness