Influence of Inclusion Chemical Composition on Weld Metal Microstructure

摘要

The effects of nonmetallic inclusions on weld metal microstructures were investigated. The inclusions were extracted from niobium microalloyed steel weld metal specimens, and examined with light and electron microscopic techniques. An EDS (ENergy Dispensive Spectroscopy) system was used to determine the chemical composition of the inclusions. Correlation between weld metal and inclusion composition was established. Aluminum, titanium, sulfur, and iron were the most important elements in the inclusions that affect the final weld metal microstructure. Manganese/silicon ratio was also found to affect the amount of oxygen and acicular ferrite in the weld. The state of deoxidation, as indicated by the amount of iron oxide present in the inclusions, actually determines the recovery of alloying elements and the amount of oxygen in the weld pool. It also determines the chemical composition of the nonmetallic inclusions. Inclusions with high aluminum content tend to cluster together forming larger particles while pure silica or silicate particles are small and well disseminated in the weld metal. This explains the different inclusion size distributions observed in the weld specimens of different oxygen concentration. Consequently, the prior austenite grain size will be different resulting in different amounts of acicular ferrite and grain boundary ferrite.