Since the dilatometer curves indicate the irreversible increase and unusual changes in dimension, thermal behaviors of cast iron can not be understood merely by thermal expansion and phase transformation. However, the behaviors can clearly be explained by taking the migration of carbon in cast iron as a factor of analysis, in addition to the other factors. Formerly, a new mechanism of growth in cast iron was suggested by the present author by regarding the migration of carbon as the main source of growth. The present investigation is intended to analyse the dilatometer curves based on the same concept of argument.
In cast iron the graphite is distributed separately in the continuous matrix. In this model of structure, the matrix must expand with the increase of carbon and contraction of graphite must simultaneously accompany, so far as the solubility of carbon in the matrix increases. In such process, as contarction of inner graphite would not restrict the expansion of outer matrix, the total volume should at least nominally change and some porosity must exist within the graphite. These relations and changes in volume can easily be explained by performing some calculations.
Under the process of cooling, precipitation of graphite is seen and the dilatometer curves usualy show a nature of irreversibility. This change means a redistribution of graphite. These phenomena can also be explained by some calculations based on migration of graphite carbon. In conclusion, a thermal cycle of growth in cast iron includes two steps: the porosity formation followed by dissolving and the redictribution of graphite caused by precipitation. And after this cycle porosity remains irreversibly. By applying the concept obtained in this manner concerning the growth in austenite region, one can well understand the nature of the sawteeth-like curve and the change of thermal expansion during the cyclic heating. Further, the growth in critical range can also be explained theoretically. Because, carbon migrates from graphite to matrix when the ferritic matrix transforms to austenite, and under the process of cooling carbon also migrates following the reverse process in heating, so far as the austenite changes into ferrite by graphitization in the critical range. Usually ferrous metals contract in the critical range of heating and expand in cooling, but in cast iron the ferritic matrix transforms into austenite without contraction, and austenite dose not expand in the following process of cooling in its transformation into ferrite by direct graphitization. These changes can properly be explained in the same mathematical formulation, because the dilatometer curves of cast iron in critical range also indicate nominal changes.
As a result of the present investigation, it is recognized that the porosity formation by carbon migration is an essential phenomenon in the growth of cast iron. Therefore, a consideration on this problem was performed from a viewpoint of physical metallurgy to conclude that carbon may migrate by a flow of vacancies and porosity may take place at the diffusion zone supersaturated with vacancies within the graphite crystal, but a flow of vacancies scarcely occurs in the process of precipitation. Graphite must precipitate at the surface and porosity must remain within the graphite. Irreversibility in the diffusion mechanism of graphite carbon is an important metallurgical problem in the growth of cast iron.