Abstract
In order to investigate the mechanism of abnormal expansion observed during delubrication of Fe-Cu-C-Zn-st compacts above 500°C, two extreme conditions were selected. Delubricating condition at a dew point of -35°C, a heating rate of 10°C/min and a holding time of 600°C×60 min in endothermic gas, in which specimen has shown the largest expansion, was compared with the same condition in argon, in which no expansion has been observed.
Abnormal expansion was not caused by vapour pressure of zinc stearate nor deposition of soot but by a lot of fine cementites formed on the surface of iron powders in compacts. The carbon for cementite is supplied by Boudouard reaction and decomposition of methane in endothermic gas. The de-composition residue of zinc stearate, such as zinc oxide plays an important role as a catalyst in this process. The cementite is directly formed as equilibrium phase, because the carbon solubility in ferrite phase is little.
