Fabrication Of Pure Iron Strips By Direct Powder Rolling Technique And Magnetic Properties

In this thesis, the fabrication technique of pure strips by direct powder rolling technique was studied. Effect of rolling coefficients and powder properties on the density , thickness and homogeneity of green strips was analyzed. Function of sintering and re-rolling on the density was assessed, as well as magnetic properties of the strips.Raw material of atomized iron powder was classified into four types of grain with various initial density, and then the powder was rolled to strips at different roll gap. Density, thickness and microstructure of strips were measured. Effect of rolling conditions on density and thickness was researched and unhomogeneity was charactered by density and microstructure. After being sintered, strips were re-rolled. Strength, resistance and density were measured to study the effect of re-rolling on properties of strips, especially on the density. Various green strips of different density were sintered on different conditions so as to research the effect of density on H_c, and the relationship between H_c and microstructure.The results show that roll gap and powder properties have influence on properties of green strips. Coarsening iron powder of higher initial density can be rolled into strips with higher density and thickness. With the roll gap increasing, thickness of strips rolled by the four sorts of powder get down, and density rise. None of the four sorts of powder can be rolled to strips when roll gap is 0.2mm. Unhomogeneity exists in length, width and thickness. When roller rotates for one fourth of circumference, thickness level off. In the dimension of width and thickness, density in the middle of strips is higher than that in the edge. After being sintered for one hour at 900℃ ~ 1300°C, density changes little ,and the density of sintered strips depends on that of green strips mainly. The highest density of sintered strips is 7.58g/cm~3, and that of re-rolled strips is 7.73 g/cm~3. After re-rolling, effect of roll gap on the density, resistance and strength get down. For the coercive force of sintered iron parts, the sintering conditions(temperature and time) are more important than the density. Even with highly porous samples, practically the same low H_c values can be obtained as with 'samples of high density, if the former are sintered at high temperatures and for long times. By re-pressing and annealing at relatively low temperatures , H_c is hardly lowered;in the case of originally highly dense samples ,itmay even be increased. Only re-sintering results in coercive forces comparable to those of well-sintered samples of equal density. After re-pressing, annealing and re-sintering, difference between He values of sintered strips with various density diminishes. Therefore the coercive force is determined more by parameters depending on the sintering condition, e.g. pore growth and pore rounding, than by the density. At the same time, the share of the pores is predominate, being 2/3 or more of the total He.