CATALYST EFFECTS IN HETEROGENEOUS NUCLEATION OF ACICULAR FERRITE

The present investigation is concerned with basic studies of the mecha nisms of acicular ferrite (AF) formation in low-alloy steel weld metal . It is confirmed experimentally that different types of orientation r elationships exist between AF and specific cubic inclusion constituent phases (i.e., gamma-Al2O3, MnOAl2O3, and TiN). Since the majority of these falls within the Bain orientation region, it is concluded that t he associated reduction of the energy barrier to nucleation is the pri mary cause for the ferrite nucleus to develop orientation relationship s with both the substrate and the austenite. Theoretical calculations show that about 12 pet of the inclusions will contain a cubic phase th at lies within the Bain region purely by chance if they are randomly o rientated in space. This intrinsic density of heterogeneous nucleation sites is sufficiently high to promote the formation of fine, interloc king AF laths in the weld metal during the gamma-to-alpha transformati on.