There is a general agreement that the graphite nodule number in black-heart malleable cast iron is an important ferritization rate-determining factor. The authors reported in a previous paper that the rate of ferritization in spheroidal graphite cast iron can be explained from the distribution or graphite nodules and the diffusion of carbon through the ferrite shell around the nodules, but they did not directly indicate the effect of graphite nodule number on the ferritization. In this paper, the effect of graphite nodule number on the rate of ferritization during isothermal annealing in black-heart malleable and spheroidal graphite cast irons were evaluated mathermatically and experimentally.
In the direct transformation of black-heart malleable and spheroidal graphite cast irons, the isothermal annealing time t, to get the same level of ferritization is inversely proportional to Nv
2/3 or to N
A, where N
V and N
A denote the graphite nodule number per unit volume and the graphite nodule number per unit area, respectively. In the indirect transformation, the effect of graphite nodule number on the rate of ferritization during 720°C isothermal annealing in black-heart malleable cast iron is almost the same as that in direct transformation. But in spheroidal graphite cast iron the effect of nodule number becomes less as the annealing temperature is lowered. An increase in silicon content of the cast iron causes a marked increase in the growth rate constant of the ferrite layer.
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