α to γ reverse transformation of bainite and martensite microstructures under various heating rates

The same coarse austenite (γ) grains as those before austenitizing emerge when a martensite and/or bainite steel with coarse grains is heated to an austenite region at a slow heating rate. This is called γ grain memory. In this study, ferrite (α) to γ reverse transformation behavior is investigated in order to clarify the mechanism of γ grain memory from the viewpoint of the roles of cementite and retained γ. Coarse γ grains and fine globular γ grains, which nucleate along the coarse γ grain boundaries, are formed when bainite and/or martensite steel is heated above the A_C3 temperature. The size distribution of γ grain is the same as that before heating. Coarse γ grains are formed by the growth, impingement, and coalescence of acicular γ grains, which corresponds to retained γ between laths. The γ grain memory is suppressed by decreasing the amount of retained γ and by increasing the amount of cementite before heating. 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. γ grain memory occurs when the growth and coalescence of retained γ dominates rather than the nucleation and growth of globular γ grains.