Effect of oxygen and titanium contents on the acicular ferrite formation in the bainitic type weld metal

Abstract

The effects of non-metallic inclusions on the as-deposited microstructure of bainitic-type weld metals were investigated in a series of gas-metal arc (GMA) welds produced with different shielding gases to obtain welds with different oxygen contents. For four different shielding gases, single run GMA bead-in-groove welds were made using ER100S-G grade wire which contained small amounts of aluminum and titanium. The acicular ferrite content in the weld microstructure was measured by a point counting method, and the inclusion characteristics were analyzed on the extracted particles or in thin foils with advanced analytical equipments such as SEM, TEM, STEM, EELSs or EDS. 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-type welds, with the exception that the maximum acicular ferrite occurred near 150 ppm oxygen. For a weld containing 150 ppm oxygen, which was the strongest weld studied, 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 no oxide layer was found in the inclusion surface. This resulted in a fully bainitic microstructure with the absence of acicular ferrite. 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, we conclude that the oxygen effect in the present welds is 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. The effect of titanium content on the formation of acicular ferrite has been studied in the fully bainitic welds having a nearly constant oxygen content. Analytical transmission electron microscopy analysis with thin foil specimens was carried out to investigate inclusion characteristics in terms of the crystal structure and the chemical composition of the constituent phases and then these results were related with the proportion of acicular ferrite measured under optical microscope. For a weld containing 0.002 wt.% Ti, Ti-Mn oxide layer containing manganese is present on the surface of amorphous inclusions and appears to be responsible for the formation of acicular ferrite in this weld. At the titanium concentration of ~0.02 wt.%, an appreciable amount of crystalline particles identified as Mn2TiO4 or MnTi2O4 is formed but these are imbedded in the amorphous matrix, not being able to play a dominant role in ferrite formation. Further increase in titanium content promotes the formation of Ti-containing oxides (either a mixture of Mn2TiO4 and MnTi2O4 or Ti2O3) and eventually results in the inclusion surface made with Ti-containing oxides. 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 are concluded to be a possible mechanism for acicular ferrite nucleation in the high titanium welds.