Abnormal α to γ Transformation Behavior of Steels with a Martensite and Bainite Microstructure at a Slow Reheating Rate

Abstract

The same coarse austenite (γ) grains as those before austenitizing emerge when a martensite or bainite steel with coarse grains is reheated to an austenite region at a slow reheating rate. This is called abnormal ferrite (α) to austenite (γ) transformation or γ grain memory. In this paper, α to γ transformation behavior is investigated in order to clarify the mechanism of abnormal α to γ transformation from the viewpoint of the roles of cementite and retained γ. (1) Coarse γ grains and fine globular γ grains that nucleate along the coarse γ grain boundaries are formed when bainite or martensite steel is reheated above the AC3 temperature. The size distribution of γ grain is the same as that before reheating. (2) Coarse γ grains are formed by the growth, impingement, and coalescence of acicular γ grains that corresponds to retained γ between laths. (3) Abnormal α to γ transformation is suppressed by decreasing the amount of retained γ and by increasing the amount of cementite before reheating. These results suggest that α to γ transformation behavior is governed by competition between the nucleation and growth of newly formed γ from the dissolution of cementite and the growth and coalescence of retained γ. Abnormal α to γ transformation occurs when the growth and coalescence of retained γ dominates rather than the nucleation and growth of globular γ grains.