Magnetic Properties of Iron Compacts in Relation to Sintering Temperature

Abstract

Previous experiments have indicated that the magnetic permeability of sintered iron compacts is to a large extent determined by the final density of the compact. The permeability of five different iron powders has proved to be independent of the origin of the powder, if the compacts were sintered at the same temperature. The results were compared with the theory of Polder and Van Santen and agreed very well with their predicted curve, if the residual pores were assumed to be flat disks. It could be expected that at higher sintering temperatures, the disk-shaped pores would change into a more equiaxed or spherical shape. Experiments to check this theory have been made, and the results are in good agreement. Compared for identical densities, the permeability of different iron powders is appreciably higher if sintering is done at 1250–1350°C and for 24 hours, instead of at 1150°C for one hour. Different powders, however, now show quite different permeabilities, which could be explained by a different inclination of the powder to form spheroidal pores. It is shown that coining after high temperature sintering brings the permeability values back to the curve of the disk-shaped flat pores. The effect of the increase in permeability at higher temperature cannot, therefore, be due to a purification process, but must be attributed to a change in the pore shape.