Creep resistance of austenitic steels can be improved by small additions of strong carbide and nitride forming elements. Additions of such elements to steels have to be followed by detailed investigations on microstructural stability and surveys of creep failure mechanisms.This paper deals with creep behaviour and microstructure evolution in the AISI 316LN + 0.1 wt. % Nb steel during long-term creep exposure at 600 and 625 °C. Creep tests with a constant tensile load were carried out in air up to times to rupture of 200 000 hours. Microstructure evolution during long-term creep exposure was studied using light microscopy, scanning and transmission electron microscopy. A small addition of niobium resulted in a reduction of the minimum creep rate and shortening of the tertiary creep stage. Investigations on microstructure evolution in creep ruptured specimens revealed the following minor phases: Z-phase (NbCrN), M23C6, M6X (Cr3Ni2SiX type), -Laves and -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 -phase, -Laves and M6X. Coarse particles, especially of -phase, facilitated the development of creep damage, which resulted in poor long-term creep ductility.Keywords: Heat resistance, AISI 36LN + 0.1 % Nb, precipitation reactions, stability of minor phases.
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