Nodular Graphite Formation in Pore-containing White Cast Iron Sintered by means of Direct Electric Resistance Heating

抄録

Using an electric resistance sintering aparatus, 2–5%C cast iron powder compacts containing flake graphite were heated very rapidly to melt the central part of the compacts. During rapid cooling, the pores were frozen in a white cast iron matrix. The pore-containing white cast irons were annealed at 873–1223 K for 1.8 ks under dry hydrogen flow, and then cooled in air or in a furnace. Nodular graphite 5∼10 μm in diameter was distributed depending on the distribution of pores which existed before annealing. However, in the pore-free specimens remelted at 1723 K for 0.6 ks, no nodular graphite was formed. The crystal structure of the nodular graphite was polycrystalline, with the c-axis of all crystallites oriented in a radial direction, similar to that of conventional nodular graphite crystallized in a melt. These results indicate that the nodular graphite can be formed by annealing of the pore-containing white cast iron made from any kinds of cast iron powder irrespective of carbon centents. This is because carbon atoms undissolved in austenite are decomposed from cementite and precipitate as graphite on the surface of pores and then fill them up during annealing.