Investigation of the Peritectic Phase Transition in a Commercial Peritectic Steel Under Different Cooling Rates Using In Situ Observation

In situ studies of two separate events of the peritectic phase transition during continuous casting, the peritectic reaction and the peritectic transformation, were performed using high-temperature confocal scanning laser microscopy (HTCSLM). The interface migration velocities during the peritectic transformation at different cooling rates were analyzed in situ by measuring the migration distance of the interface vs time. Moreover, the solute distributions near the moving liquid/solid interface during the peritectic reaction and the peritectic transformation were predicted using the commercial software package diffusion-controlled transformation. The results revealed that the images of HTCSLM clearly recorded these two separate events: the peritectic reaction (L + δ → γ) and the peritectic transformation (L → γ and δ → γ). In the initial stage of the peritectic reaction, the austenite (γ) phase was observed to nucleate at the liquid/δ-ferrite (L/δ) interface and then grow along the periphery of primary δ phases. Upon further cooling, these emerging γ phases gradually isolated the liquid and primary δ phases. Subsequently, the laterally growth of the γ phase was regarded as the peritectic transformation. The growth rate of the γ phase was governed by the liquid to γ and δ to γ phase transformations. As the cooling rate increased, the peritectic reaction was observed to occur at lower temperature. Higher cooling rates enhanced the migration rates of the L/γ and γ/δ interfaces during the peritectic transformation. Meanwhile, an interesting massive transformation of δ into γ phase was observed to occur at a cooling rate of 60 °C/min. All primary δ phases were quickly covered by wrinkled γ phases in a short time. Based on the assumption of the solute incomplete diffusion in the liquid phase, the predicted results revealed that the enrichment of carbon near the L/δ, L/γ, and γ/δ interfaces increased as the cooling rate increased. An increase in the cooling rate exacerbated the carbon segregation of the interface during continuous solidification, causing a nucleation suppression of the γ phase. In turn, the increasing carbon enrichment accelerated the interface migration with the diffusion of large amounts of solute across the interface, causing an increase in the driving force for the peritectic transformation.