Study On Preparation, Microstrucutre And High Temperature Properties Of Powder Metallurgy Iron-based Superalloy

Fe-based superalloy which has high creep strength and neutron irradiation-resistance at high temperatures is one important kind of advanced materials. The major goal is to research the preparation process and the properties of the oxide diffusion strengthen (ODS) ferrite alloy. In this study, oxidation kinetics and the form of oxygen in the atomized powder, mechanical alloyed powder and the iron-based alloy Fe-14Cr-3W-0.5Ti-0.3Y-0.3O(wt,%) were researched by direct oxidation or oxygen addition by atomization; the factors that affect the compaction and the precipitation phase formation of the sintered alloy were represented through enhancing the content of the precipitated phase; the iron-based alloy with the excellent room and high temperature properties was manufactured by the hot working and heat treatment process, and the microstructure evolution and the strengthen mechanism were studied, then the conclusions are drawn as below:(1) The oxidation behavior of iron-based metal powder Fe-14Cr-3W-0.5 Ti-0.3Y-0.03O(wt,%) at 500℃has been studied. The oxidation process obeys the oxidation kinetics (ΔW/S) 2=kpt+c. Oxidation mechanism is controlled by surface reaction within 16 hours. The formation of the Fe2O3 oxide surface can be controlled by increasing the oxidation temperature and shortening time.(2) The existential manner of oxygen in the atomized powders is solid solution. The O dissolves uniformly in the oxidized powder and a very thin layer of oxides forms on the powder surface after sintering. The solid solubility of oxygen in the iron-based powder expands during the mechanical alloying process. The oxygen as solid solute is evenly distribution in the mechanical alloyed powders.(3) During sintering at temperatures from 800 to 1000℃, large number of equiaxed grains fine as hundreds of microns are found embedding in the matrix of Fe-14Cr-3W-0.5Ti-0.3Y-0.22O alloy (wt,%). The recrystallized grains are ultra-stable and show minor dependence on sintering temperature and time. When heated to high temperature, the recrystals turn to lamellar grained structure; while abnormal grain growth occurs after prolonged annealing and results in bimodally grained structure. The inhomogeneous distribution of Y-Ti-O nano-oxides throughout the matrix, which is the main reason of bimodally grained structure. The MA-ed Fe-14Cr-3W-0.5Ti-0.3Y-0.220 alloy (wt,%) shows the better friction properties at 600 and 700℃, and the wearing mechanism is changed from adhesive wear to oxide wear and abrasive wear with the temperature and sliding speed increasing.(4) The precipitated phase Ti2Y2O7 in the Fe-14Cr-3W-5Ti-3Y-2.2O (wt,%) alloy is the dispersion strengthen phase when the densification of the sintered alloy ended. The size of precipitated particle grows and the enhancing effect decreases with the increasing of the sintering temperature. The high temperature compressibility strength is associated with the precipitated particle size and the weight percentage of the particles in the forged alloy. The hardness of the forged alloy by using atomized powder is HRC54.8 at 1300℃, because the grains occur full recrystallization, and the grain size becomes fine. The hardness decreases with annealing temperature increasing, because the grain size of recrystallization unchanged but softening occurred. After tempering at 700℃, crystal size changed unobvious, the hardness value decreases which is because of the decrease of solid solubility of Cr.(5) The results show that the addition of Al facilitates its diffusion in the iron-based alloy and the grain size of the alloy is largest among the iron-based alloy with additions. The alloy adding Fe2O3 also has some residual porosity left by the solution of the oxide which hinders the grain growth. Grain growth obeys the Beck equation. Adding Al increases the grain growth exponent while adding Fe2O3 decreases the grain growth exponent.(6) The high density iron-based alloy has been prepared by swaging process. The grain size of the alloy growths with the heat treatment temperature increasing. The Y-Ti-O nano-oxides play a vital role in the stable of the high temperature strength. The precipitated particle size show minor dependence on annealing temperature and time. The compressibility strength at 850℃remains constant.