Fe-C-Si系白鋳鉄の第1段黒鉛化反応の解析について

抄録

The kinetic of first stage graphitization in white cast iron has been studied by Burke and Owen using the Johnson-Mehl equation, ξ=1−exp{−(kt)ⁿ}, and they have discussed the theoretical knowledge of the nucleation and growth of temper carbon. Then, most of the kinetic analyses have been based on their premises. But after wards, other authors put forward a question about estimation of the value of n of this method. It is considered that these disagreements are caused by the value of n varying with a starting point of graphitization. Therefore, this study was undertaken to determine accurately the value of n by modifying Johnson-Mehl equation as dξ⁄dt=nk{−ln(1−ξ)}^(n−1)⁄n(1−ξ), and also the activation energies were measured.
The results are as follows:
(1) An accurate value of n was not obtained by a double logarithmic plot of the Johnson-Mehl equation, log·log(1⁄1−ξ)=nlogt+const.
(2) According to the analysis by the modified method, the value of n gradually decreases during the graphitization and finaly becomes about 1.
(3) The activation energy gradually decreases until approximately 50% graphitization and thereafter remains nearly unchanged at 2.6×10⁵ J/mol.
(4) These results suggest that the latter half of reaction is controlled by the dissolution process of cementite.
(5) The meaning of the n value of time exponent proposed by Burke may have to be reexamined.