| Alumina-forming austenitic heat-resistant steels (AFAs) which showed a large oxidation resistance at elevated temperatures and have great potential to be utilized as next-generation structural materials have attracted extensive attention in recent years. Up to date, research in this field have been mainly placed on high temperature oxidation-resistance, however, no systematic and in-depth stutides on the high-termpeature mechanical properties of these steels were conducted. Our group has successfully developed a new AFA steel with excellent high-temperature oxidation resistance, nevertheless, the mechanical properties of this steel at elevatated temperatures need to be improved which limited its practical engineering applications. In order to fully understand deformation mechansism and improve high-temperature mechanical properties of AFA steels, effects of precipitates evolution, pre-cold work treatment and recrytallization on deformation behavior and the high-temperature mechanical properties of our AFA steels were systematically investigated.Firstly, it was found that the steady-state flow behavior could be well described by a stress power-law with consideration of the threshold stress, and temperature dependence of the shear modulus and self-diffusion coefficient, suggesting that the deformation of the austenite matrix was controlled by the recovery process and dictated by lattice diffustion. The threshold stress was temperature-dependent and appeared to be resulted from the Orowan bowing stress. The interaction energy between the matrix and precipitates also decreased from163to34kJ/mol with increasing temperature, which was probably associated with formation of the two different precipitates.Secondly, dynamic evolution of precipitates and its influences on the strengthening mechanism of AFA steels were also systematically studied. At1023K or above, it was observed that B2-NiAl, Laves-Fe2Nb and δ/σ-FeCr mainly formed in the base steel. The major strengthening phase is the Laves-Fe2Nb, however, this intermetallic compound coarsened quickly, leading to the undesirable creep properties. In contrast, nanosized secondary NbC phase is more effective in strengthening and has low tendency to be coarsened, but is only stable at low temperatures. Phase competition between the most effective strengthening NbC and the Laves-Fe2Nb phase was then analyzed, and it was revealed that adjusting the Nb/C ratio in the steels could enable precipitation of highly stable, fine NbC particles. In addition, formation of detrimental σ-FeCr phase could be suppressed by lowering the Mo and Si content in the alloy.In addition, effects of processing conditions and recrystallization on high-temperature creep properties of AFA steels were also investigated. Appling10%cold-work could lead to formation of a high density of nanosized NbC particles dispersed homogeneously in the matrix, doubling the creep rupture lifetime at1023K under100MPa. Our studies also indicate that the grain growth exponent, n, reaches about3and the apparent activation energy for grain growth is234.7kJ/mol, which is consistent with that of the Nb diffusion along the grain boundary in the austenite. In other words, precipitation of nanoszied NbC phase also occurred during the recrystallization process.On the base of the above stuieds, a novel AFA steel (Fe-18Cr-25Ni-3Al-0.8Mo-0.5Nb-0.08Si-0.08C-0.01B-0.04P-0.1Y-0.1Ti,wt.%) consisting of a high density of nanosized NbC particles homogeneously dispersed in the austenitic matrix was successfully developed via adjusting the Nb/C ratio and lowering the Mo and Si concentration. With optimized processing conditions, significant enhancement in the creep resistance which is superior to that of the HTUPS AFA steels was achieved. |
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