Abstract: The rule of non-metallic inclusions distribution and the their sizes in HSLA steel weld metals for flux cored wires are measured, and the chemical composition and the figures of non-metallic inclusions in HSLA steel weld metals are analyzed with transmission electron microscopy and X-ray Microanalysis. The mechanisms of acicular ferrite nucleation in weld metal are systematically studied, The suitable percent of Ni, Mn and Mo in the weld metals can increase the percent of acicular ferrite in the weld metals microstructure. The percentage of Ni, Mn and Mo in the weld metals and welding energy input have little affect on the chemical composition, sizes and figures of non-metallic inclusions in the weld metals. 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. As nucleation sites, the non-metallic inclusions can reduce the nucleation energy remarkably. Besides they have different thermal expansion coefficients than 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: Acicular ferrite, Flux cored wire, Non-metallic inclusion, Weld metal