The effects of titanium content on weld microstructure and inclusion characteristics have been studied using the two different bainitic welds fabricated to have similar oxygen content. Analytical transmission electron microscopy analysis with thin foil specimens was carried out to investigate inclusion characteristics focusing on the crystal structure and the chemical features of the constituent phases. Then, these results were related with the proportion of acicular ferrite measured under an optical microscope. For a weld containing 0.002 wt.% Ti, a Ti-rich oxide layer containing manganese is present on the surface of amorphous inclusions and appears to be responsible for acicular ferrite appreciably formed in the bainitic structure, ~50%. When the titanium content increases to 0.07 wt.% Ti2O3 inclusions were formed accompanying with manganese-depleted regions and eventually results in a significant increase in acicular ferrite content over 90%. Therefore, the manganese depletion developed along with the formation of Ti2O3 inclusions is concluded to be a possible mechanism for acicular ferrite nucleation in the high titanium welds.

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The effects of non-metallic inclusions on the as-deposited microstructure of fully bainitic weld metals were investigated in a series of gas-metal arc (GMA) welds produced with four different shielding gases to obtain welds with different oxygen contents. Single run GMA bead-in-groove welds were made using ER100S-G grade wire which contained minimal amounts of aluminum and titanium. It was found that the 'oxygen effect,' which is well known in ferritic welds as the maximization of acicular ferrite formation at intermediate oxygen levels of 200-300 ppm and is explained by the shift of CCT curves with oxygen content, took place in the present bainitic welds with the maximum acicular ferrite occurred near 150 ppm oxygen. For a weld containing 150 ppm oxygen, all inclusions were extensively covered with a thin layer of titanium oxide. As the oxygen content decreased to about 100 ppm, a thick sulfide shell surrounded the aluminum oxide core and resulted in a fully bainitic microstructure. On the other hand, for welds containing more than 250 ppm oxygen, less than 50% of inclusions were covered with a titanium-oxide layer, and the coverage rate was substantially lower than for welds of 150 ppm oxygen. This led to a mixed microstructure of acicular ferrite and bainite. Therefore, the oxygen effect in the present welds could be concluded to be not due to the shift of the CCT diagram but to the fact that the inclusion phases vary with oxygen content. A new model based on these facts is suggested.

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