Review of Peritectic Solidification Mechanisms and Effects in Steel Casting

Surface quality and castability of steels are controlled greatly by initial solidification. Peritectic steels suffer more from surface quality problems, including deep oscillation marks and depressions, crack formation, and breakouts than other steels. This paper reviews current understanding of the fundamental mechanisms of initial solidification of peritectic steels that lead to these problems. First, different empirical relations to identify peritectic steel grades from their alloy compositions are summarized. Peritectic steels have equivalent carbon content that takes their solidification and cooling path between the point of maximum solubility in δ-ferrite and the triple point at the peritectic temperature. Surface defects are related more to the solid-state peritectic transformation (δ-ferrite → γ-austenite) which occurs after the peritectic reaction (L + δ → γ) during initial solidification. Some researchers believe that the peritectic reaction is controlled by diffusion of solute atoms from γ phase, through the liquid, to the δ phase while others believe that γ growth along the L/δ interface involves microscale heat transfer and solute mixing due to local re-melting of δ-ferrite. There is also disagreement regarding the peritectic transformation. Some believe that peritectic transformation is diffusion controlled while others believe that massive transformation is responsible for this phenomenon. Alloying elements and cooling rate greatly affect these mechanisms.