Effects of chemistry, density and size distribution of inclusions on the nucleation of acicular ferrite of CMn steel shielded-metal-arc-welding weldments

Abstract

The influence of inclusions on the nucleation of acicular ferrite of six as-deposited CMnTi welds prepared by the shielded-metal-arc-welding process was studied. The microstructure of the welds, the inclusion size distribution, the density and the average chemical composition were characterized. It was found that a titanium-rich phase which partially covers the inclusions is responsible for the nucleation of acicular ferrite. It is estimated that a titanium content of about 39 wt.ppm in the weld metal is necessary for the formation of this titanium-rich phase. However, even if the presence of the titanium-rich phase in the inclusions is necessary, it is not the only factor affecting the nucleation of acicular ferrite (AF). After correcting for the effect of austenite grain size, it was found that the efficiency of the inclusions for the nucleation of AF is related to the total external surface area of the titanium-rich phase covering the inclusions per unit area of weld metal. This is in turn related to the density, the size distribution and the titanium level of the inclusions.