Effect of boron and carbon addition on microstructure and mechanical properties of the aged gamma-prime strengthened alumina-forming austenitic alloys

The goal of this work was to understand the effects of aging at 800 °C on the microstructures and mechanical properties of two recently-developed AFA stainless steels based on Fe-14Cr-32Ni-3Nb-3Al-2Ti (wt.%), one of which contained small additions of boron and carbon. To that end both the size distributions and growth kinetics of the B2, Laves phase, L12 precipitates present were quantified. While the lattice parameter, morphology, size and coarsening behavior of the L12 precipitates was the same in both AFA alloys, the B and C enhanced the grain boundary coverage by both Laves phase and B2-NiAl precipitates, but suppressed their coarsening. These interstitial additions also suppressed the formation of twins and discontinuous precipitation, which were observed in the B and C-free material. It is shown that the yield strength at 700 °C is largely controlled by the size of the L12 precipitates, with the largest strengthening effect obtained after aging for 2.4 h for both AFA alloys. Longer aging time led to a loss of strength mainly due to the coarsening of the L12 precipitates.