Abstract
Laser surface remelting at various velocities has been employed to study the selection of microstructures of high-purity Fe–C–Si alloys containing nominally 3.2 to 4.2 wt.% C and 1 to 3 wt.% Si. The microstructure of the remelted region consisted of metastable Fe–Fe3C eutectic (ledeburite) or austenite dendrites, with interdendritic eutectic. Furthermore, ledeburite presents two solid-liquid interface morphologies: planar and cellular. The competition between the austenite dendrites and the ledeburite eutectic as a function of solidification rate has been experimentally determined. The critical velocity which destabilizes the planar Fe–Fe3C eutectic with respect to primary austenite dendrites, was of the order of several mm/s and depends on the initial composition of the alloy. The critical velocity for the destabilization of the eutectic interface leading to two-phase cells was 0.44 mm/s, for the alloy containing 4.2 wt.% C and 1 wt.% Si. Theoretical calculations of the coupled zone have been performed using current microstructure selection models. The results of the simulation were then used to construct a microstructure map which was compared with experimental results.
The authors thank J.-D. Wagniére for technical assistance. The research of MSFL had been partially financed by the Swiss Government under a CFBEE Scholarship.
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Appendix: Thermophysical constants values*
| Constant | Symbol | Value | Unity | Reference: |
|---|---|---|---|---|
| Thermal conductivity | K | 44 | W m–1 K–1 | [19] |
| Thermal diffusivity | α | 8.9 · 10–6 | m2 s–1 | [19] |
| Densities at 24 °C | ρFe | 7400 | kg m–3 | [20] |
| ρFe3C | 7200 | kg m–3 | [20] | |
| Gibbs-Thomson coefficients | ΓFe | 1.9 · 10–7 | m K | [21] |
| ΓFe3C | 2.4 · 10–7 | m K | [21] | |
| Contact angle of the eutectic | θFe/θFe3C | 50/55 | Degrees | [21] |
| Solute diffusivities | D | 3 · 10–9 | m2 s–1 |
- *
The phase diagram data (e.g. liquidus slopes, partition coefficients and reference temperatures) were obtained directly from ThermoCalc Database [8] and vary for each alloy.
© 1998 Carl Hanser Verlag, München