Analysis of nonisothermal transformation kinetics; tempering of iron-carbon and iron-nitrogen martensites
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I.
INTRODUCTION
FOR the analysis of transformation kinetics a physical property (e.g., hardness, specific volume/length, electrical resistivity, enthalpy, magnetization) of the material subject to investigation can be traced as a function of time and temperature. In the past a number of methods have been proposed to evaluate kinetic reaction parameters from such data. For example, the Johnson-Mehl-Avrami (JMA) theory has been developed to describe the progress of solidstate reactions during isothermal annealing, although the limitations of this theory are generally recognized. 1 Recently attention was especially paid to the development of methods for the analysis of the kinetics of transformations during nonisothermal annealing. For example, adopting the JMA description also for nonisothermal behavior, it was shown in the case of a constant heating rate that kinetic reaction parameters as activation energies can be derived from the dependence on heating rate of the inflection-point and peak-maximum temperatures in dilatometry and calorimetry, respectively. 2'3 It has not been generally recognized, however, that a common basis for these methods can be given. As a result the specification of and the more severe assumptions inherent to a specific kinetic model, as for example provided by the JMA theory, are unnecessary. Such an integrated treatment is the more appropriate as it is not linked up with a certain experimental technique. In this paper first a short general description is given for kinetic analysis from experiments conducted in either isothermal or, in particular, nonisothermal fashion. E.J. M I T T E M E I J E R and B. M. K O R E V A A R , P r o f e s s o r s , LIU CHENG, Graduate Student, P.J. van der SCHAAF, Technical Assistant, and C . M . B R A K M A N , Senior Scientist, are with the Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands. Manuscript submitted May 29, 1987. METALLURGICALTRANSACTIONS A
The stimulus for this work arose in a project devoted to the analysis of the tempering kinetics of iron-carbon and iron-nitrogen martensites, in which most of the experiments are of nonisothermal nature. Although the tempering behavior of iron-carbon martensites has been investigated extensively in the past (cf. reviews presented in References 4 through 6), only a limited amount of corresponding knowledge exists on iron-nitrogen martensites. 7-12 In the present paper for the first time a comparison of the kinetics of tempering of iron-carbon and iron-nitrogen martensites is presented (it is remarked that even for the iron-carbon martensite the kinetics of tempering are still under discussionl3:4). For kinetic analysis of the sequentially occurring solid-state transformations, especially, dilatometric and calorimetric methods have been applied. In the last decades specific volume and enthalpy changes were only rarely measured in research on tempering (iron-carbon) martensite. However, in view of recent instrumental developments allowing high accuracy
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