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We think that the dimensional accuracy of sintered machine part can be controlled mainly by regulating the relative density of green compact (Dg) and sintering shrinkage and by adding post-treatment such as repressing (cold forging), grinding or cutting in some cases, even if the compact is designed to considerably be densified by sintering. This opinion is based on the fact that such idea have been adopted since about 15-20 years ago in the fabrication of cemented carbides cutting tools and also in MIM or PIM process, where Dg in both cases is as low as 60% and the relative density of sintered compact (Ds) is 100%; the sintering shrinkage is as large as about 14%. Then, we thought that the promotion of the densification is valuable to increase the mechanical properties of sintered machine parts and thus investigated the conditions in the first step for obtaining high Ds as much as possible for coarse powder (45-150μm) of Fe which is the base metal of the parts.The results obtained were as follows. (1)As for Fe coarse powder compact, Ds was nearly equal to Dg when the compact was sintered in vacuum or H2 gas for 3.6-14.4ks at 1573K. Sintering for 14.4ks at 1743K was necessary for obtaining Ds of 97% even in case of Dg=90%. (2)The addition of a small amount (2-4mass%) of Mo could enhance the densification; the compact with Ds of 96-98% was obtained by solid phase sintering at 1573K for 3.6-14.4ks, under which condition pure Fe compacts hardly densified. This enhancement was attributed to the considerable increase of diffusivity of Fe atoms due to the change in the crystal structure of Fe from fcc to bcc which is caused by the addition.

Effects of Sintering Conditions and Mo Addition on Sintering Densification of Fe Coarse Powder.