A STUDY OF THE α↔γ TRANSFORMATION IN PURE IRON: RATE VARIATIONS REVEALED BY MEANS OF THERMAL ANALYSIS
Résumé
The α↔γ transformation in nominally high purity Fe is shown to occur with a stepped peak in differential thermal analysis on both heating and cooling at rates between 0.5 Kmin-1 and 10 Kmin-1. The composite peaks mark changes in the transformation rate. To endorse the findings, the instrumental output has been thoroughly analysed providing evaluations of time lags, suggestions for calibration and for the use of the derivative of the peak. The change in rate occurred in all sampled irrespective of their grain size (average values from 91 mm to 1100 mm). The rate of movement of the interface in the a«g transformation is estimated in between 4·10-5 ms-1 and 3·10-6 ms-1. The present results extend previous dilatometric work in which the rate variation was detected only for large grain size and low undercooling. Possible reasons for the variation in rate are outlined: local development of strain in the austenite due to lattice misfit with respect of the growing ferrite, formation of a ragged microstructure, pinning of the boundaries by impurity.
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