As heat treatments of metallic alloys generally require high temperatures, energy saving cycles are increasingly looked for. Castings offer the advantage over wrought products of possible hot shake out as they are just made.
The object of this study was to observe the structure of a SG cast iron resulting from hot shakeout right after the end of solidification, followed or not by a 1 000°C holding and subsequent heat treatment.
The austenite then presents the peculiarity of not having been re-generated by heating up from an initial structure as it is the case in conventional cycles.
This study has clearly showed the influence of composition heterogeneity within the solidification cell on the response to heat treatment. Along the solidification cell boundary, an increase in carbon and manganese contents was found while silicon decreased. Along the SG nodules, the opposite is true. Between these two zones, gradients vary. The presence of these three areas results in having three different CCT curves and, therefore, a varying response to heat treatment. This is even magnified by the section size of the sample because the cell size increases, but this is minimized by the decrease in quenching rate.
Structural differences observed in hot shake out samples and re-heated samples, both held or not at 1 000°C, and then cooled down, are directly related to these composition heterogeneities:
- hot shakeout cast iron shows a high hardenability which varies with the holding time at 1 000°C;
- after a ferritizing treatment, an homogeneous matrix is difficult to obtain from a hot shakeout cast iron;
- after austempering the structural heterogeneity of the cell boundary is greater for the hot shakeout cast iron;
- after the pearlitic transformation, the structure of the hot shakeout cast iron is similar to that resulting from conventional treatment.