Microstructure and mechanical properties evolution of 16Cr-25Ni-4Mo superaustenitic stainless steel weld metal during 550°C aging

The long-term aging behavior at 550°C was investigated for a 16Cr-25Ni-4Mo weld metal (WM) and to elucidate how precipitation, deformation twins, microbands, and deformation bands jointly affect its plasticity and toughness. Specifically, aging promoted the precipitation of M₂₃C₆ carbides and M₂(C, N) carbonitrides. M₂₃C₆ carbides were distributed along grain boundaries (GBs) and interdendritic regions (IDRs), while M₂(C, N) carbonitrides were found only in IDRs. At GBs, M₂₃C₆ carbides coarsened, following the Lifshitz-Slyozov-Wagner (LSW) model. At aging 5000 h, their average size reached 402.1 nm. Moreover, their number density peaked at 1.6 μm⁻² at aging 1000 h. In the IDRs, M₂₃C₆ carbides showed multipoint nucleation with limited growth, whereas M₂(C, N) carbonitrides exhibited fewer nucleation with slow growth. At aging 5000 h, the number density and average size of M₂₃C₆ carbides were 15 μm⁻² and 47.0 nm, respectively, while those of M₂(C, N) carbonitrides were 6.0 μm⁻² and 162.5 nm, respectively. Both the plasticity and the toughness of the WM deteriorated with increasing aging time, but a more rapid decline occurred when cracks were initiated simultaneously at GBs and in IDRs. During tensile loading, cracks primarily initiated at GBs and gradually intensified with aging. At aging 3000 h, additional IDR cracks formed due to precipitate accumulation in IDRs, together with the marked reduction in the number of deformation twins and microbands, jointly contributed to a faster drop in elongation from 33.0% to 27.2% between 1000 and 3000 h. In contrast, impact cracks mainly originated in the IDRs, where crack severity also increased over time. At aging 5000 h, further GBs cracking induced by coarsened and continuously distributed M₂₃C₆ carbides at GBs, together with the disappearance of deformation twins near the impact fracture surface, jointly contributed to a faster drop in impact toughness from 164.0 J to 141.5 J between 3000 and 5000 h.