X@=1-exp(-kt") 
where: X(t) - the transformated fraction, 
k - rate constant dependent on temperature; 
n - exponent of the reaction. 
1.0 vn Tor 
0,9-, —e--T_=300 C | . ; 
RX 08- —A—T_ = 350 Ci ~ “= 
oe ve i 
06: ae : 
0,5 t rd 
, 0,4- 1 : 
* : ) 1 f 
il of 
4 ¥ 
~ 01 a 
004 ~ per yp TTT TUG Va 
Q 5 10 15 20 25 30 35 40 45 50 55 bu 4 - u 
The maintained time at the isothermal level, T, [min] 
Fig. 1: Transformated fraction curves at Tj, = 300 and 350° C, for different maintaining times 1;z, at Phe 
the isothermal level fo 
Le 
In order to determine “k” and “n”, the natural logarithmic expression was used: we 
log [-log (1-X)] = (nlog k + logloge ) + nlogt se 
Values of “n” and “k” determinated from the slopes and intercepts of the linear regression lines are i i 
listed in table 1. The obtained equations from the linear regression adjustment are: m 
Yıo= -6,73507 + 2,10337*X, R?= 0,974; Yıso = -6,57897 + 2,06804*X, R? = 0,974; 
Table 1: Values of “n” and “k” for the formation of bainite 
EE Ve nm k[1/s] 
_ 300 2,1033 48,36 x 10° 
| 350 2,0680 4955x107 
The kinetics of austenitization of S.G. cast iron, can be described by the Johnson-Mehl-Avrami 
equation with exponent “n “= 2,06 - 2,11. 
According to Liu (2), if the “n” exponent is between 1 and 2,3, the transformation is interfacing 
controlled. 
At the same maintaining time in the isothermal level, the transformation process is different in the 
each maintaining isothermal temperatures. The bainitic reaction rate "k" increases when the 
isothermal temperature increases from 300 to 350° C 
Determination of the activation energy “Q” 
For the determination of the activation energy “Q”, it was used the Arrhenius equation: 
k=A¢e RT [1 /min] 
379