Nature of inclusions in steel weld metals and their influence on formation of acicular ferrite

Abstract

The chemistry and structure of weld metal inclusions has been studied. Four submerged arc welds which utilized plate and consumables to cover a range of oxygen and deoxidant contents were examined. Analysis of the inclusions was carried out on carbon extraction replicas in a Philips 400T scanning transmission electron microscope, fitted with an energy dispersive analyser. Two major types of inclusion were found. With weld metal aluminium approaching the stoichiometric ratio with oxygen, the inclusions were crystalline and had a spinel structure at the centre with a discontinuous, polycrystalline, titanium-rich phase on the surface. With weld metal oxygen high compared with the stoichiometric ratio with aluminium, inclusions were glassy and essentially manganese silicate in composition, again with areas of a polycrystalline, titanium-rich phase on the surface. The interinclusion spacing varied little with weld metal oxygen content in the range 0·0268–0·0858 wt-%. The spacing was found to be of a similar order to the acicular ferrite grain size. The titanium-rich surface phase in all the welds was of fcc structure with a lattice parameter of 0·42 nm, which suggests a mixture of TiO and TiN, possibly rich in TiO. The spinel phase was also fcc and had a composition between galaxite (Al₂O₃MnO) and γ-alumina. Both these phases have a low lattice misfit with ferrite. A low lattice misfit of the inclusion surface layers with ferrite coupled with closely spaced inclusions would seem to be key factors in the development of an acicular ferrite weld metal microstructure. MST/543