Densification Of Powder Metallurgy Iron-based Superalloy

Fe based high temperature alloy which has high creep strength and fatigue resistance at high temperatures is one kind of advanced iron alloy with superior combination properties. This alloy has been used in many technology fields, such as aerospace vehicles, nucleus engineering, energy and power, traffic engineering and petrochemical engineering; et al. At present, this alloy is mainly produced by mechanical alloying(MA), hot extrusion and heat treatment, however, this procedure costs much and introduces extraneous component easily during MA which contaminates the alloy.In this investigation, the pre-alloyed powder Fe-12Cr-2.5W-0.4Ti-0.25(Y₂O₃) made by inert-gas atomization was used as raw material, non impurity Fe-based high temperature alloy was prepared using pre-alloyed traditional powder metallurgy technology, including press molding,sintering,hot-forming and heat treatment. Mechanical properties and microstructure of this high temperature alloy were investigated by means of metallographic analysis(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM),differential scanning calorimetry(DSC) and mechanical testing. The major results can be summarized as follow:(1)The particle size distribution,microstructures and micro-hardness of Fe-Cr-W-Ti-Y-O pre-alloyed powder were tested, the powder were annealed at different temperatures and the influence of powder performance on pressing was analyzed.(2)The influences of die wall lubrication procedure on mechanical property were compared to admixed powder lubrication method and the influencing factors of iron-based high temperature alloy powder by pressing were investigated. The results show that the lubrication procedure has a great influence on the consolidation and microstructural features. Die wall lubrication avoids the appearance of binder and offers the possibility of making pore-free material. After vacuum sintering, the density of sintered body is 0.3～0.4g/cm³ higher than the one prepared by admixed powder lubrication. It is also found that the particles bind together more tightly by die wall lubrication under higher pressures, which leads to better mechanical properties.(3)Sintered materials were heavily deformed by hot forging and hot rolling, as a result, the density of the sampples increased significantly. By observing the microstructures, lots of aggregative dislocations and grain boundaries were found to be pinned by second phase, which is the main reason for hardening.(4)Second phase was observed after annealing, they pinned grain boundaries and strongly restrained recrystallization. Annealing temperature plays an important role in second phase precipitation, it was found that the second phase particles were the finest and most homogeneously and intensively distributed when annealed at 1300℃, consequently most strengthened. The microstructural evolution and hardness change were examined and contribution of annealing temperature to the precipitation and material properties was analyzed.