Microstructure and Creep Properties of AISI 316LN Steels with Niobium Additions

摘要

The long-term creep rupture tests have been carried out on three casts of a type AISI 316LN steel at 600 and 650 deg C. Two of the casts investigated contained additions of 0.1 and 0.3 wt. percent of niobium. The growing niobium content strongly reduced the minimum creep rate and prolonged the time to the onset of the tertiary stage of creep and also shortened this stage. The enhanced creep resistance of niobium containing steels is not accompanied by the longer creep life that might have been expected. At both temperatures of creep exposure the niobium-bearing casts displayed an inferior creep ductility. Mierostnictural investigations revealed that niobium provoked significant grain size refinement and the formation of Z-phase. Particles of this phase were considerably dimensionally stable. Furthermore, niobium accelerated the formation and coarsening of sigma-phase, eta-Laves and Mg(C,N). The coarse intergranular particles facilitated the formation of cavities which resulted in intergranular failure mode.