Creep Behavior and Microstructure Evolution in AISI 316LN + 0.1 Wt.% Nb Steel at 600 and 625°C

The paper deals with microstructural evolution in the AISI 316LN + 0.1 wt.% Nb steel during long-term creep exposure at 600 and 625°C. The following minor phases formed: Z-phase (NbCrN), M₂₃C₆, M6X (Cr3Ni2SiX type), η-Laves (Fe2Mo type) and σ-phase. M6X gradually replaced M₂₃C₆ carbides. Primary Z-phase particles were present in the matrix after solution annealing, while secondary Z-phase particles formed during creep. Precipitation of Z-phase was more intensive at 625°C. The dimensional stability of Z-phase particles was excellent and these particles had a positive effect on the minimum creep rate. However, niobium also accelerated the formation and coarsening of σ-phase, η-Laves and M6X. Coarse particles, especially of σ-phase, facilitated the development of creep damage, which resulted in poor long-term creep ductility.