Discussion
It was assumed that the transformation was complete when peak hardness was reached. The degree of transformation (Vf
) at various aging times was calculated using Equation 6. Equation 6
squares line fit of all the heats was 0.69 at 482°C (900°F) and 0.72 at 460°C (860°F) (Figure 11([a]). Hence, the heats show isokinetic behavior. These values are similar to the ‘n’ value for precipitation on dislocations.1 ue of ‘n’ obtained from previous study11
The val-
PH steel was 0.95 at 455°C (851°F) and 482°C (900°F) and 0.93 at 510°C (950°F). Mirzadeh et al.13
on wrought 17-4 , based on
The Avarami-Johnson-Mehl Equation (7) and Arhenius Equation (8) were used to determine the kinetic behavior. A plot of lnln(1/1-Vf
) versu ln time was constructed for the four
heats for determination of age-hardening exponent, n, and the activation energy, Q. Isokinetic behavior was assumed for the analysis where n ko and Q are assumed constant.
Vf = 1 – exp(–kt)n k = k0 exp(–Q/RT)
Equation 7 Equation 8
The age-hardening exponent ‘n’ obtained by a least
linear regression model of previous studies, reported ‘n’ values of 0.442-0.484 for a temperature range of 400°C to 551°C (752° to 1024°F). Equation 8 was used to calculate the activation energy (Q) for the four heats and the aver- age Q obtained by a least squares line fit was 235 kJ/mol (Figure 11([b]). If it is assumed that the initial hardening observed during aging is a result of a compositional mod- ulation as observed during spinodal decomposition14-15 then the value of Q may be directly related to the self dif- fusion of copper. The reported activation energy for dif- fusion of copper in austenite (Fe-.6Mn, .1Cu) are 255 kJ/ mole16-18
, and 247 kJ/mole.1 . Laik et al.20 reported a value
of 220 kJ/mole for the diffusion of copper in austenite. Mirzadeh et al.13
calculated the activation energy for pre-
cipitation of Cu in 17-4 PH steel as 262 kJ/mol based on a regression model of previous studies. A lower activation energy might be expected for the martensitic structure as a result of pipe diffusion in the dislocated microstructure.
Table 7. Chemical Composition (Weight Percent) Of The Features Marked In Figure 10
In Time, h
Figure 11. Kinetic evaluation: determination of (a) age-hardening exponent ‘n’ (b) activation energy Q. (a)
1/T (°K) (b)
International Journal of Metalcasting/Spring 10 67
InIn (1/(1-Vf)
In k
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