Sintering Atmospheres and DC Magnetic Properties of Iron

Abstract

The de magnetic properties of iron test-rings sintered under nineteen different atmospheres have been determined. High-purity iron powder rings were pressed to a density of 7·2 Mg m⁻³ and sintered in a laboratory tube furnace at 1120°C. The types of atmosphere studied were (i) hydrogen, nitrogen, and their mixtures, (ii) endothermic-base, (in) exo-endo, (iv) exothermic-base, and (v) nitrogen-base. Neither magnetic saturation induction B ₂₅ nor remanence was greatly affected by the type of atmosphere chosen, but both were decreased when the atmosphere was a carburizing one or when nitrogen pick-up could occur. The carbon content after sintering had about twice as much influence as nitrogen on B ₂₅ and remanence. Coercivity and maximum permeability were very strongly influenced by the type of atmosphere chosen. Nitrogen had the most potent influence on coercivity; carbon and nitrogen had about equal effects on maximum permeability. Compared with carbon and nitrogen, the levels of oxygen and sulphur and the grain size had, in general, secondary or insignificant influences on magnetic properties. A multiple regression analysis indicated that about 90% of the variation in magnetic properties due to atmosphere composition could be explained by the resulting chemical analysis and grain size of the sintered rings. Hydrogen and hydrogen/nitrogen atmospheres gave by far the best magnetic properties.