Influence of Oxygen Content on Microstructure and Inclusion Characteristics of Bainitic Weld Metals

Abstract

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.

Effect of Ti Addition on Weld Microstructure and Inclusion Characteristics of Bainitic GMA Welds

“…In addition, this phase was confirmed to be purely amorphous through the SAD analysis. This type of amorphous inclusions has been reported by many researchers not only in the welds nearly free in titanium 22,25,33) but also in the titaniumcontaining welds. 21,34) Of great importance in this inclusion is the formation of titanium-rich layer on the inclusion surface.…”

“…Not all inclusions exhibited the titanium-rich layer, but only four out of ten inclusions examined were partly coated with titanium-rich layer. Considering the previous findings that both amorphous and sulfide phase are not effective for acicular ferrite, 6,22,25,30,35,36) such a titanium-rich layer seems to play a favorable role for the formation of acicular ferrite in this weld. Thus, further examination on the chemical nature of this layer was performed.…”

“…As described in the previous report, 22) a two-step welding was performed to minimize the base metal dilution. The first weld was to deposit five buffering layers in the 10 mm deep V-groove machined on the steel plate.…”

“…Subsequently, X-ray diffraction analysis was carried out on the electrolytically extracted inclusions of Ti-B containing weld metal and showed that a large proportion of inclusions comprised with TiO, which in turn was believed to be a phase responsible for acicular ferrite nucleation. 16) Since then, many other workers [17][18][19][20][21][22] have observed the presence of titanium-rich phase on the inclusion surface when the welds contained titanium to some extent and this phase was reasonably considered to be TiO, TiN or Ti (C,N). Recently, researchers in Osaka University identified the titanium-rich surface layer found in the weld of 0.02 wt.% titanium as being titanium mono-oxide (TiO) conclusively by detailed electron diffraction analysis.…”

Characterisation of local grain size variation of welded structural steel

“…The differences were attributed to carbonitride precipitation, higher dislocation density and the lath bundle size of acicular ferrite. In general, acicular ferrite is beneficial for mechanical properties and besides tensile properties it improves impact toughness and decreases the transition temperature [49,50]. Thus, methods are sought after to increase the volume fraction of acicular ferrite in welds [49,51].…”