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.