High-hop Fe - Based Sintering Densification And Organizational Performance

In this research, two kinds of sintered steels with high content of alloying elements were studied, the chemical composition of which are Fe-11Mo-4Cr-2V-1.45C and Fe-4Cr-4Mo-6Co-1.5Ni-1.2C respectively. The first one is a kind of sintered steel with high Mo and C content, while the second one contains Co. Differential thermal analysis(DTA), optical microstructure, X-ray diffraction(XRD), scanning electron microscopic(SEM), energy disperse X-ray Analysis(EDXA) were used to analysis the liquid forming temperature, phase composition and the microstructure.The densification behaviour of the sintered steel with high Mo and C content was investigated. The two-layer structure was sintered in vacuum atmosphere at 1260℃ and certain pressure was applied on the compacts in order to keep shape. The C, V content is the key factors for densification, while Mo has minor effect on densification. The results show that C can impede densification, but V can hinder the densification. It is proved that the mechanical properties of the two layer structure are improved at a great extent. The TRS reached 1980MPa and the impact energy was 18J/cm². The distribution maps of the alloying elements show that the diffusion depth of carbon is remarkably smaller than which can be expected because of the high potential of Mo, V to form carbide, Mo and V mainly distributed in the needle shaped carbides, while Cr was more homogeneous than Mo, V.The evolution of needle shaped carbides was investigated in detail. The metallurgical changes were investigated through annealing at high temperature, adding alloying elements (l%Si, 0.1%B) and sintered in nitrogen atmosphere. The results show that needle shaped carbide present in the microstructure is verified to be M₂C carbide by X-ray, metallurgical microstructure and EDS techniques. Since the liquid phase has high content of alloying elements and the solidification condition can not homogenize during sintering, the two factors promote the formation of M₂C eutectic.M2C is a metastable phase which can decompose during annealing, Annealing temperature has great effect on the spheroidizing and decomposition of M₂C The needle shaped carbide can also be changed by the addition of silicon because silicon can stabilize M₆C carbide and prevent the formation of M2C. Boron strongly enhanced sintering by the formation of a liquid phase when sintering at temperature higher than 1200℃. Boron acts as a sintering additive to achieve higherdensity. The sintering behavior and final microstructure is affected when sintered under nitrogen atmosphere. N has some influence on the densification and the morphology of the carbides. The factors that affecting nitrogen absorption include nitrogen pressure, V content of the alloy, C activity and holding time.Direct sintering and infiltration process were applied to enhance the densification behavior of the sintered steel containing Co. The results show that raising sintering temperature and carbon content can enhance the densification of iron-based material with high content of carbide forming elements. Carbide network present in the microstructure. High density (7.6g/cm3) can be achieved through traditional powder metallurgy technology. Carbide network present in the microstructure, which is consist of MgC and M23C6 type carbides. The hardness of the material is high(61HRC) due to the formation of martensite and high content of alloying elements, but this kind of carbide is detrimental to the mechanical properties. The TRS, a02and or are 547MPa, 277MPa and 388MPa respectively. The skeleton carbides are formed through eutectic reaction (L-^y+MeC), white area is N^C carbides and the black area is the matrix (y-Fe). After the steel was infiltrated, the density reached 7.85g/cm3 and the mechanical properties were raised a lot. The hardness, TRS, tensile strength, impact energy was 51HRC, 1282MPa, 1140MPa and 8.1 Jem"2 respectively.