Graphite Morphologies in Sintered Products of 1.7%C Cast Iron Swarf Powder from Nodular Cast Iron

The condition to obtain graphite nodules in sintered or powderforged products of cast iron swarf powder is that the carbon content must be less than the solubility in γ phase at a sintering temperature. This is because if all carbon atoms are dissolved in γ phase, pores in P/M products are spheroidized during sintering and supersaturating carbon atoms precipitate as graphite into spheroidal pores during cooling. The present paper confirms this condition using the swarf machined from nodular graphite cast iron. The swarf is pulverized into powder of around 100 mesh, and the carbon content is decreased by removing the graphite separated from the matrix down to 1.7%C which is excess of the solubility of γ phase. The powder was compacted at 600 MPa, sintered in H2 atmosphere at 1423 K for 30 min, powderforged, post-sintered in the range of 1123∼1423 K for 30 min, and air cooled. The results show that nodular graphite is never obtained irrespective of the nodular morphologies in the parent cast iron or of the presence of Mg when the powder contains carbon more than the solubility in γ phase, and that the tensile strength is lower than the parent metal owing to irregular morphology of the graphite.