Effects of Al additions on precipitation, recrystallization and mechanical properties of hot-rolled ultra-super ferritic stainless steels

In this study, 1.0 wt%Al element was added into the traditional 26.8Cr-3.7Mo-2Ni super-ferritic stainless steel to tailor the brittle phase precipitation, recrystallization microstructure and mechanical property via solution annealing at temperatures of 950-1150 degrees C. The experimental results demonstrated that hot-rolled microstructure for 1.0Al sample consisted of surface fine grain region and core elongated grains together with TiN, NbC and Laves phase, while none of these fine grains appeared in 0Al samples. After annealing below 1050 degrees C, both sigma-phase and Laves phase were formed in Al-free samples, while only abundant Laves phase precipitated in 1.0Al sample indicating the inhibition effect of Al on the sigma-phase and the promotion effect on Laves phase. When annealing above 1050 degrees C, recrystallization was completed and neither sigma-phase nor Laves phase were observed. Relatively stable mean grain size was produced due to the Laves phase pinning effect during annealing at 1050 degrees C, while the Nb/Al solute drag effect bore responsibility for the situation at 1050-1150 degrees C. Sudden grain coarsening phenomenon appeared after annealing above 1150 degrees C. Al additions gave a solution strengthening and intensive Laves phase precipitation strengthening for the designed alloys, and good comprehensive mechanical properties were obtained demonstrating the probability of developing of the Al-modified ultra-super stainless steels.