Effect of non-metallic inclusions on microstructure of weld beads deposited in submerged arc welding

Abstract

Submerged-Arc Welding (SAW) is one of the most important welding processes applied in the fabrication of modern engineering structures. During the deposition of molten steel, which is protected against oxidation by agglomerated flux layer, the microstructure of the weldment undergoes considerable changes because of the heating and cooling cycle directly related to the welding process employed. Several studies have shown that acicular ferrite provides an optimum combination of strength and toughness in steel weld metal. The flux formulations are prepared using mineral compounds, such as oxides and silicates, and it is possible to increase the content of acicular ferrite by higher quantity of intergranular nucleation sites. So, dispersed non-metallic inclusions can promote the formation of acicular ferrite during phase transformation, at the expense of other undesirable weld microconstituents such as allotriomorphic ferrite and Widmanstätten ferrite. The aim of the present study is to investigate the effect of inclusions on the mechanism of acicular ferrite formation through an extensive light microscopy analysis of SAW steel samples prepared with different heat input levels, where parameters such as size and volume fraction of inclusions are related with weld microstructure and the nucleation of acicular ferrite.