The effect of graphite properties on the dimensional change of Fe-2mass% Cu-0.8mass%C-0.75mass% zinc stearate was examined by using two kinds of natural graphite, flaky graphite of 0.5-35μm and amorphous graphite of 3-9μm. The compacts were sintered in endogas. Their sintering behaviors were correlated with the properties of graphite.
As the size of graphite decreased, the compacts shrank and the dimensional change varied according to 0.27 log(B/A)% when graphite was replaced from A to Bμm. Finely divided flaky graphite showed a rapid carburization, which led to the acceleration of sintering and the suppression of copper growth. When flaky graphite was divided to 0.5μm, however, the dimensional change did not obey above formula at all. In this case, the compact expanded at the temperature ranging from about 150 to 450°C and the carburizing rate markedly decreased above 970°C, with a large fraction of the graphite remaining after sintering. Amorphous graphite of 9μm also showed a similar expansion in the early sintering stage, but the carburizing rate did not decrease. Such behaviors led to a remarkable shrinkage by liquid generation due to the ternary monotecto-eutectic reaction above the melting point of copper.