Abstract: The sizes of non-metallic inclusions in weld metals for flux cored wires and the rule of their distribution are systematically studied. The chemical composition and the figures of nonmetallic inclusions in weld metals are analyzed with transmission electron microscopy and X-ray microanalysis. The results show that the non-metallic inclusions beneficial to nucleation and growth of acicular ferrite in weld metals are oxidation products of Ti, Mn, Si, Al and the sulphides of Mn and Cu. The percentage of Ni,Mn and Mo in the weld metals and welding energy input have little effect on the chemical composition,sizes and figures of non-metallic inclusions in the weld metals. As nucleation sites, the non-metallic inclusions can reduce the nucleation energy remarkably. Besides they have different thermal expansion coefficients from that of the substrate metals, which will introduce a high stress-stain field around the inclusions during the cooling process. These two factors are the main driving forces to the nucleation and growth of acicular ferrite.

Key words: flux cored wire, weld metal, non-metallic inclusion, acicular ferrite