Microscopic observation of inclusions contributing to formation of acicular ferrite in steel weld metal

Abstract

The strength and toughness in low carbon low alloy steel welds is improved by the refinement of microstructures resulting from the formation of acicular ferrite. The behaviour of the nucleation and growth of acicular ferrite has been extensively studied and the weld metal of refined acicular ferrite is practically used in industry. It is known that inclusions in weld metal strongly contribute to the nucleation of acicular ferrite. The present authors made dynamic observations of the nucleation and growth of acicular ferrite at inclusion sites using a high temperature laser scanning microscope. In the steel weld metal of a Ti–B system, the formation of acicular ferrite is reported to be controlled by the weight ratio of aluminium and oxygen (Al/O ratio) which is an index of oxygen potential after the completion of deoxidisation by aluminium. The proper control of the Al/O ratio results in the formation of MnAl2O4 oxide of a spinel structure whose capability of the acicular ferrite nucleation is governed by a specific lattice misfit between nucleated ferrite and this oxide. It is considered that the addition of titanium is essential because titanium assists the formation of MnAl2O4. 8 However, there are reports depicting that there is no specific lattice orientation between ferrite and oxides of a spinel structure. As to the role of titanium, the researchers reported that it brings about catalysis or acts as the first nuclei for acicular ferrite nucleation. In spite of the fact that the acicular ferrite production technology is practically used in industry, the mechanisms of the acicular ferrite nucleation have been not completely clarified yet and thus, the development of nucleation control technologies for a further improvement of weld metal toughness is hindered. In the present study, the inclusions were formed by varying Al/O ratios. Thus formed inclusions were directly sliced into thin foils and crystallographic analyses were performed using a transmission electron microscopy (TEM). The formation of a multiphase type of inclusions and the existence of titanium enriched layers on the interface between the inclusions and nucleated ferrite were clarified by an electron diffraction analysis and an energy disperse X-ray spectroscopy (EDS). Experimental