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

The same coarse austenite (gamma) 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 (alpha) to austenite (gamma) transformation or gamma grain memory. In this paper, alpha to gamma transformation behavior is investigated in order to clarify the mechanism of abnormal alpha to gamma transformation from the viewpoint of the roles of cementite and retained gamma. (1) Coarse gamma grains and fine globular gamma grains that nucleate along the coarse gamma grain boundaries are formed when bainite or martensite steel is reheated above the A(C3) temperature. The size distribution of gamma grain is the same as that before reheating. (2) Coarse gamma grains are formed by the growth, impingement, and coalescence of acicular gamma grains that corresponds to retained gamma between laths. (3) Abnormal alpha to gamma transformation is suppressed by decreasing the amount of retained gamma and by increasing the amount of cementite before reheating. These results suggest that alpha to gamma transformation behavior is governed by competition between the nucleation and growth of newly formed gamma from the dissolution of cementite and the growth and coalescence of retained gamma. Abnormal alpha to gamma transformation occurs when the growth and coalescence of retained gamma dominates rather than the nucleation and growth of globular gamma grains.