Formation Mechanism 上游充道大粤 SHANGHAI JIAO TONG UNIVERSITY 1.Austenite Nucleation -Sites Most favorite sites are the ferrite/cementite interfaces,because: Large carbon concentration gradient at the interfaces>composition of austenite Disordered structure>fcc structure formation through short-range diffusion ■ Interfacial energy of the ferrite/cementite interfaces lowered activation energy Similarly,boundaries of the pearlite colonies are preferred nucleation sites High heating rate large superheating >Critical nucleus size (? >Composition of nucleus (? >Sub-boundaries in ferrite become nucleation sites K.Chen 14K. Chen Formation Mechanism 14 1. Austenite Nucleation - Sites Most favorite sites are the ferrite/cementite interfaces, because:  Large carbon concentration gradient at the interfaces  composition of austenite  Disordered structure  fcc structure formation through short-range diffusion  Interfacial energy of the ferrite/cementite interfaces  lowered activation energy Similarly, boundaries of the pearlite colonies are preferred nucleation sites High heating rate  large superheating  Critical nucleus size (?)  Composition of nucleus (?)  Sub-boundaries in ferrite become nucleation sites