Studies on the Sintering of the Metallic Powder of the Iron Family. II : The Effect of the Atmosphere on the Sintered Metallic Compact

The properties of the sintered product of metallic powder are controlled by the physical, chemical and mechanical properties of the powder and also by the sintering condition. As sintering conditions, the molding pressure, temperature and atmosphere may be mentioned. Because the effects of the pressure, temperature and time on the sintered product are very important, that of the atmosphere has not been considered much. Its role is liable to be considered as preventing the oxidation and reducing the surface oxide film. But when the same powder is sintered at the same condition only changing the atmosphere, the properties of the sintered products sometimes change extremely.(1) It is not, therefore, reasonable to think the role of the atmosphere as being the same as stated above. R. Kieffer and W. Hotop(2) reported that when hydrogen, used as the atmosphere, contained large amounts of water vapor, more active superfine powder of the metal would be produced through the oxide according to the reversible reaction of (1), so the shrinkage of the compact would become large. Metal+H_2O⇄Metallic Oxide+H_2……(1) In the study on the magnetic property of sintered iron powder, the auther has found that the magnetic property is largely improved by mixing a certain amount of water to the sintering atmosphere. So the following experiments was conducted. The role of the sintering atmosphere (here, water and air in hydrogen) on the magnetic property of the sintering product was first confirmed. Then the mechanical property and shrinkage of the sintered product were measured by properly changing the amount of water in hydrogen as the sintering atmosphere. The activation energy in sintering was calculated and that water in the atmosphere positively accelerated the sintering was confirmed and its mechanism was considered. Also a clear result on the effect of water content in hydrogen was obtained