Effect of inclusion size on the nucleation of acicular ferrite in welds

Low-carbon steel weld with a high density of oxide inclusions prepared usin g a experimental metal-cored wire has been examined to study the effect of inclusion size on the formation of acicular ferrite, and to understand the role of inclusion in the nucleation of ferrite lath. Depending on the ferri te morphology associated with inclusions, a total of 282 inclusions observe d under TEM could be classified into two groups, i.e. the non-nucleant and the nucleant. Experimental results showed that the group of inclusions acte d as nucleant were appreciably larger in size compared with those of non-nu cleant resulting in the increased probability of nucleation with the. incre ase of inclusion size, even though the chemical and structural natures appe ared to be the same. The group of nucleant-inclusion was further divided in to two types depending on the degree of nucleation, which was evaluated by the number of ferrite lath nucleated. Statistical analysis performed on inc lusion size indicated that the larger the inclusion size is the more ferrit e laths could be nucleated. Those laths nucleated from a large single inclu sion have grown in many different radial directions and mostly had a differ ent crystallographic orientation from those of adjacent ferrite laths. As a result of this study, it is demonstrated that larger inclusions are indeed more potent nucleation sites when compared with those of smaller size. Thu s it could be concluded that the provision of the inclusion surface as for the inert surface for the heterogeneous nucleation of acicular ferrite lath would be the principal role of inclusions playing in the weld metal of low alloy steels. Other possible mechanisms were also considered, but they wer e unlikely to be operated in the present weld metal system.