Study On Preparation And High-Temperature Oxidation Behavior Of Fe-25Cr High Temperature Alloy By Powder Metallurgy

Fe-25Cr high temperature alloys with a relative high density were prepared by powder metallurgy from pure Fe and Cr powders following by hot-forging at 1220℃, and then isothermal oxidation at 1100℃for different times. The density of the samples was measured by Archimedes principle. The microstructure of as-sintered and as-forged alloys was observed by optical microscope(OM). Phase identification of the surface scale was carried out using X-ray diffraction (XRD). The morphology of the oxidation scale and the elements distribution the oxide in cross-section were characterize by scanning electron microscopy (SEM) coupled with energy dispersive spectrum(EDS). The effect of pressing, sintering and forging on the microstructure and density was investigated, and the formation process and mechanism of oxidation scales was discussed. The main results of this paper are presented as follows:1. The density of the sintered body increases with the increase of press pressure, sintering temperature and sintering time in the research dimension of this paper, but the incremental velocity decreases gradually. When the sintering temperature is 1300℃, the size of the grains enlarges obviously.2. The relative density of with press pressure 800MPa, sintering temperature 1280℃and sintering time 60min is about 95.23%. After forging at 1220℃the relative density is 97.77%, which is larger than that of the as-sintered alloys by 2.67%. The pores have been disappeared and the grains have also been refined.3. Althought the microstructure of Fe-25Cr alloys is consisted of a single-phase solid solution a-Fe(Cr), the content of Cr element is not homogeneity. The content of Cr is higher in the grain boundary, while that of Fe is relative higher in the grains.4. In the initial stage of oxidation, Cr2O3 formed rapidly and the thickness of it increases with the increase of oxidation time, which is controlled by the diffusion of Cr from the matrix in to the oxidation scales. A prolongation of oxidation time, initial Cr2O3 scale reacts with O and evaporates from due to the long time exposure in the high temperature oxidative environment.5. The Fe element diffused from the matrix in to the initial Cr2O3 scale reacts with O to form FeO and finally forms spinel FeCr2O4 because the initial Cr2O3 scale is loose and it can't impede the entad diffusion of O, which leads the oxidation scales transferring from the single Cr2O3 to a complex scales of Cr2O3 and FeCr2O4.6. The oxidation kinetics curve of Fe-25Cr alloys at 1100℃agrees well with the dual parabolas. When the oxidation time is shorter than 25h, the kinetics equation is y2=2.39×10-11 t. While when the oxidation time is 25-100h, the kinetics equation is y2=8.50×10-11 t.