Role of the particle-matrix interface on the nucleation of acicular ferrite in a medium carbon microalloyed steel

The inclusions responsible for the nucleation of acicular ferrite in a medi um carbon microalloyed steel have been studied using optical and transmissi on electron microscopy. The acicular ferrite microstructures for such studi es were obtained by isothermal treatments following austenitization at 1250 degrees C. The samples were maintained for different times at 450 degrees C and then water quenched, permitting observation of the evolution of the m icrostructure. Intragranular nucleation is enhanced by the presence of MnS inclusions covered by a CuS layer leading to an improvement in the global e nergetic balance for the transformation. This better energetic balance is o btained by replacing a high-energy interface between inclusion and austenit e with a lower energy interface between inclusion and ferrite. The result i s a fully acicular microstructure. (C) 1999 Acta Metallurgica Inc. Publishe d by Elsevier Science Ltd. All rights reserved.