Directional growth of pearlite in iron--carbon eutectoid alloys. [Rate]
Pearlite was transformed directionally by passing a zone of austenite through high purity Fe--C alloy bars of eutectoid composition. Using a small embedded thermocouple, the temperature gradient at both the austenitizing edge and the pearlite transformation edge of the austenitic zone was recorded. A sharp change in gradient occurred due to the change in thermal conductivity between the phases at each interface. The point of change in gradient enabled the temperature of both interfaces to be determined as a function of velocity. It was found that the undercooling of the pearlite transformation interface below the eutectoid temperature was quadratically dependent on the velocity of transformation. No superheating at the austenitizing interface was observed. Interlamellar spacing measurements of pearlite showed an inverse quadratic dependence of spacing on velocity. These two results are compared with theory. Volume diffusion of carbon through austenite adequately describes the kinetics of the decomposition of austenite to pearlite although extrapolated carbon diffusivities from high temperature austenite data are a factor of 2 to 4 too small. The interlamellar spacing of pearlite and the pearlite transformation interface temperature as a function of velocity are compared to other constant velocity transformation studies as well as isothermal investigations. Results of these comparisons indicate that pearlite forms via the same kinetics either isothermally or at constant velocity. The maximum rate at which pearlite can be forced to grow continuously under forced velocity conditions is shown to be approximately 100 ..mu..m/sec.
- Research Organization:
- Ames Lab., IA (USA)
- DOE Contract Number:
- W-7405-ENG-82
- OSTI ID:
- 7305757
- Report Number(s):
- IS-T-731
- Country of Publication:
- United States
- Language:
- English
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