Effects of inclusions and microstructures on impact energy of high heat-input submerged-arc-weld metals

Submerged-arc-weld metal made with variations of heat-input and consumables were examined to study the inclusions and their effects on the Charpy impact energy of the weld metal as a measure of the toughness. From the analysis, inclusions, of which diameters were smaller than 2 μ m, seemed to be effective for acicular ferrite nucleation and the fraction of acicular ferrite was in proportion to the number density of inclusions <2 μ m. However as the heat input increase, the average size of inclusions increased but their number density decreased. It was also observed that the grain sizes of polygonal ferrite with increased heat input. Accordingly, the decrease of the Charpy impact energy with the heat input is inevitable due to these microstructural changes. But, this does not mean that the acicular ferrite cannot be nucleated.