Evolution of Precipitate Phases During Long-Term Isothermal Aging at 1083 K (810 degrees C) in a New Precipitation-Strengthened Heat-Resistant Austenitic Stainless Steel

Abstract

Evolution of precipitate phases in a precipitation-strengthened high nitrogen 15Cr-15Ni-4Mn-3Cu-1.5Mo-0.46Si-Nb-C-N-Al-B austenitic heat-resistant stainless steel during 10,000 hours of isothermal aging at 1083 K (810 A degrees C) was investigated. The major precipitate phases found in microstructure after 330 hours of aging are coarser residual primary Nb(C,N) precipitates, Z-phase precipitates, (Nb,Cr)N precipitates with fcc crystal structure, Cu precipitates, and coarser Cr2N precipitates. No significant changes were noted in the chemical compositions, crystal structure of the precipitate phases, or the phase species after 10,000 hours at 1083 K (810 A degrees C). The coarsening behavior of Z-phase, (Nb,Cr)N, and Cu precipitates was the only observed notable change in the microstructures during the performed aging. Z-phase and (Nb,Cr)N precipitates exhibited superior dimensional stability and morphology with extremely low coarsening rates, while Cu precipitates coarsened at the fastest rate. In addition, Z-phase particles showed a remarkable lower coarsening rate as compared to that of (Nb,Cr)N precipitates. The abnormal slow coarsening kinetics of Z-phase can be explained by the nucleation energy barrier (NEB) limited coarsening mechanism applied for faceted ceramic crystals.