Phase Transformation During The Sintering Of The ODS Ferritic Heat-Resistant Steel

ODS ferritic steel has excellent high temperature performance and low activity as a nuclear reactor cladding material representative. elements of ODS steel play an important role during sintering, So it has become more and more important to study the effect of these elements on the austenitizing temperature during sintering. two systems had been studied in this paper: Fe-0.3Y₂O₃ (wt. %) and Fe-9Cr-0.2Ti-0.3Y₂O₃ (wt.%). First, the heating rate which would be used in later experiment was selected. Then, the phase transformation was analyzed by means of differential thermal analysis(DTA), XRD, SEM, powder sintering theory and the second phase pinning theory. The main content and conclusions are as follows:The Fe-0.3Y₂O₃ (wt. %) powders were milled at different time(0h,12h,20h,30h)respectively. The compacts were sintered in the DTA at the temperature of 1000℃with the heating rate of 10K/min. The effect of different milling time and the addition of Y₂O₃ on the austenite transformation temperature was studied. The results show that the austenitizing temperature will decrease because of the increasing of the dislocation and decreasing of the grain size which was caused during the milling process. On the other hand, Y₂O₃ can pin austenite grain boundaries and impede the transformation of austenite, so that the region of austenite will be expanded too.9%Cr (wt. %) and 0.2%Ti (wt. %) were added in the Fe-0.3Y₂O₃ (wt.%) system. Powders were milled at different time (0h,10h,20h and 40h) respectively. The effect of Cr and Ti on the austenitizing temperature during sintering process under different milling time was studied. The mass fraction of Ti and Y₂O₃ were expanded to10 times in order that Ti-Y-O can be easily detected by XRD to prove if there was the existence of Ti-Y-O at the sintering temperature of 1000℃. The results show that Cr can dissolve in the matrix only by diffusion during sintering when Fe-Cr-Y system powders without being milled, so the concentration of Cr was low and below 7% which would cause the transition point of ferrite to austenite decrease with the increasing of Cr concentration. When the powders were milled for 10h, the concentration of Cr was up to 7% in the matrix through the further diffusion during sintering that will cause the transition point of ferrite to austenite increase and will be extended with the increasing milling time. When the milling time increased to 20h, Cr completely dissolved in the matrix and the concentration distribution was more uniform with the milling time, When the milling time reached 40h,Cr no longer played a significant role on the austenitizing temperature. In Fe-Cr-Ti-Y system, Experiments also showed that Ti could promote the decomposition of Y₂O₃ during the milling; Y₂O₃ precipitated in the matrix when Fe-Cr-Ti-Y was sintered at 1000℃, though without the existence of Ti-Y-O complex oxides. So we can infer that the precipitation of the Ti-Y-O may require higher temperature.