Abstract
The process of the formation of undercooled graphite in cast iron has recently been discussed again by many investigators as Morrogh and Williams1) Berman2), Shaw and Kondic3), Hultgren's group4) and Scheil5). The argument is that whether the undercooled graphite is a decomposition product of ledeburite or it solidifies directly from the melt.
The present paper dealt with the problem on the line and is composed of five experiments, that is, (a) an investigation of a solidification mechanism of Fe-C-Bi alloy, (b) chill test of commercial charcoal pig iron by additions silicon, (c) sand cast test of the pig iron with a little addition of bismuth, (d) graphitization of Fe-C-Si alloy just under the eutectic temperature, and (e) heating ledeburite successively to its formation from the melt.
(a) : Flake graphite structure in Fe-C alloy converted into undercooled graphite and inverse-chilled structure with increased bismuth. Thermal analysis and interrupted quenching was applied and it was to be noted that the formation of undercooled graphite was followed by the ledeburite formation.
(b) : Fracture of cast iron containing a certain amount of silicon had a dusky layer adjacent to chilled white zone, the constitution of which was coarse undercooled graphite with small amount of ledeburite. This was ascertained to be the graphitization product of columner ledeburite by interrupted quenching.
(c) : Fractures of sand cast specimens containing a small amount of bismuth revealed undercooled graphite both in outer and in central parts, but it seemed to be developed in a different way. Near the surface, ledeburites crystallized first and fine undercooled graphite deposited between them from the melt while the former decomposed to graphite which was quite similar to the latter. In the inner parts, fine undercooled graphite deposited from the melt with spherical crystallization fronts.
(d) : White iron was graphitized completely in a time comparable to that of the eutectic reaction on solidification when it had rather high silicon as more than 0.6% and heated just below the eutectic temperature.
(e) : When solidifying Fe-C-Te or Bi alloy was heated at the eutectic temperature immediately after ledeburite was formed, there appeared coarse undercooled lump, quasi-flaky and/or Widmannstätten graphite accompanying with residual acicular cementite.
It was concluded from the results mentioned above that the undercooled graphite generally solidified directly from the melt prior to ledeburite formation. But when the iron contained some graphitizing elements such as silicon, similar graphite could appear as a decomposite product of ledeburite. Strictly speaking, these were different and could be distinguished each other, but in some cases they had quite a resemblance which might make a confusion.
