Effect of Inclusion Size on the Nucleation of Acicular Ferrite in Welds.

Abstract

Low-carbon steel weld with a high density of oxide inclusions prepared using 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 ferrite morphology associated with inclusions, a total of 282 inclusions observed under TEM could be classified into two groups, i.e. the non-nucleant and the nucleant. Experimental results showed that the group of inclusions acted as nucleant were appreciably larger in size compared with those of non-nucleant resulting in the increased probability of nucleation with the increase of inclusion size, even though the chemical and structural natures appeared to be the same. The group of nucleant-inclusion was further divided into two types depending on the degree of nucleation, which was evaluated by the number of ferrite lath nucleated. Statistical analysis performed on inclusion size indicated that the larger the inclusion size is the more ferrite laths could be nucleated. Those laths nucleated from a large single inclusion have grown in many different radial directions and mostly had a different 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. Thus 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 were unlikely to be operated in the present weld metal system.

On the Capability of Nonmetallic Inclusions to Act as Nuclei for Acicular Ferrite in Different Steel Grades

“…The upper size limit for 99 pct of inclusions is marked with a cross for each case. In the literature, several authors [33][34][35][36] described a boost in nucleation potential with the increasing inclusion size, which is in good accordance with earlier findings of Ricks et al [37,38] However, it should be noted that in many cases also, a lower and an upper critical sizes for active inclusions are defined. The lower critical value is the minimum size beneath which particles are too small to act as active nuclei.…”

“…Above 1 lm, the probability curve flattens out. Huang et al [34,39,40] who considered inclusions up to 7 lm, observed 1.5 lm as the upper critical value but described a sharp decrease in the nucleation probability above 4 lm, resulting in inert behavior of inclusions larger than 6.5 lm. Similar results were gained by Song et al [41] and Wang et al [42] who found inclusions with sizes ranging from 1 to 3 lm and 1 to 2 lm, respectively, as the most appropriate nuclei.…”

“…This value was determined to be 0.3 to 0.5 lm in various studies. [34,39,40] Ricks et al, [37,38] Lee et al [34] and Mu et al [33][34][35][36] investigated inclusions smaller than 1.5 lm and defined 1 lm as the upper critical value. Above 1 lm, the probability curve flattens out.…”

The Nature and Consequences of Coherent Transformations in Steel.

“…This particular example appeared in the heat-affected zone of a weldment in a steel that had been "salted" with oxide particles. 38) These particles are intended to maintain fine grain size in the heat-affected zone by providing a dense array of ferrite nucleation sites for retransformation of the austenite in the HAZ.…”

Influence of Oxide Particles and Residual Elements on Microstructure and Toughness in the Heat-Affected Zone of Low-Carbon Steel Deoxidized with Ti and Zr

“…This is because the microstructure is influenced not only by the particle size, but also by the particle surface composition consisting of nucleation sites such as in the Tirich phase. It has been reported [18][19][20][21][22][23] with respect to the inclusion size that a certain minimum inclusion size is required for intragranular ferrite nucleation and that inclusions smaller than this size are not effective as nucleation sites.…”