Creep behaviour and microstructural evolution in AISI 316LN + Nb steels at 650 °C

The paper deals with the effect of niobium in the wrought AISI 316LN steels on the long-term creep characteristics at 650 °C. Casts B and C contained 0.1 and 0.3 wt.%Nb, respectively. As a reference material the niobium free Cast A was used. Small additions of niobium to the AISI 316LN steel resulted in a significant reduction of the minimum creep rate and shortening of the tertiary creep stage. At time to rupture exceeding 104 h the creep rupture strength of the niobium-bearing Casts B and C was slightly inferior to the Cast A. Two nitrides formed in the Casts B and C: Z-phase and M6X. The minimum creep rate in niobium-bearing casts was favourably affected by precipitation of the Z-phase. The dimensional stability of Z-phase particles was very high, but niobium additions also accelerated the formation and coarsening of η-Laves and σ-phase. Coarse σ-phase particles at grain boundaries contributed significantly to the shortening of the tertiary creep stage.

Research highlights
► We studied the effect of Nb additions to AISI 316LN steels on creep and microstructure at 650 °C.
► Nb additions resulted in a reduction of secondary creep rate and shortening of the tertiary stage.
► Two nitrogen rich minor phases were present in the niobium-bearing casts: Z-phase and M6X.
► The dimensional stability of Z-phase during creep at 650 °C was much better than that of M6X.
► Nb accelerated σ-phase and η-Laves formation and this surpassed the positive effect of Z-phase.