Abstract
Time-resolved and in-situ observations using synchrotron radiation X-rays successfully proved that a massive-like transformation, in which the γ phase was produced through the solid–solid transformation and partitioning of substitute elements such as Mn and Si at the δ–γ interface was negligible, was selected in the unidirectional solidification of 0.3 mass% C steel at a pulling rate of 50 µm/s. The massive-like transformation produced fine γ grains near the front of the δ–γ interface. The coarse γ grains also grew behind the fine γ grains along the temperature gradient. Distance between the δ–γ front and the advancing front of coarse γ grains was as short as 200 µm. Namely, the fine γ grains disappeared within 10 s owing to growth of coarse γ grains. In addition, the observation of the δ–γ interface confirmed that a transition from the diffusion-controlled γ growth to the massive-like γ growth occurred at a growth velocity of 5 µm/s. Thus, the massive-like transformation is dominantly selected in the carbon steel during conventional solidification processes.
Transmission images (left) and diffraction images (right) proved that diffusion-controlled growth of austenite phase was never observed and the massive-like
δ-
γ transformation was always selected during unidirectional solidification of Fe - 0.3 mass% C - 0.6 mass% Mn - 0.3 mass% Si at a pulling rate of 50
μm/s. The massive-like transformation is dominantly selected in conventional casting processes.
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