Grain Size of Acicular Ferrite in Ferritic Weld Metal

Morphologically, the acicular ferrite (AF) microstructure developed in ferritic weld metal has been widely described as having an “interlocking structure” or “basket weave structure,” following its fine nature of the optical microstructure revealed by 2 % Nital etching solution. However, the grain or effective grain size of AF, which controls the fracture unit, is not well understood. In this study, we attempted to develop the morphological grain structure of AF by using a newly-developed electro-chemical etching technique and then determined the crystallographic grain structure using EBSD analysis with misorientation angles. Both of these grain structures were then compared to the cleavage facet size measured from the brittle fracture surface. As a result, the morphological grain boundaries revealed by electro-chemical etching were found to match the crystallographic boundaries developed by EBSD. It was also demonstrated that the crystallographic grains with tolerance angles of 15° or higher, i.e., high-angle boundaries, were similar in size to the cleavage facets. From these results, it was concluded that the effective grain size of AF can be determined by EBSD and can be estimated by the electro-chemical etching technique.