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

Abstract

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.

Characterization of Inclusions Formed in Ti-containing Steel Weld Metals

“…This difference appears to be caused by the increased amount of (Mn,Ti)-spinel oxide and may explain why the 2Ti weld has a higher content of AF than the 1Ti weld ( Table 2). The inclusion characteristics of this weld have been reported previously, 14,15) thus this study simply reaffirms the previous results. The inclusions of this weld were composed mainly of two Ti-oxides having different Mn contents and Al-oxide along with a minimal amount of Mn-silicate.…”

“…These phases possessed different concentration of Mn, one of them being thought to be TiO. The present authors 14,15) also found such dual phase inclusions in the weld of 720 ppm Ti, but those phases were all identified as Ti 2 O 3 . In addition, these inclusions were characterized to be fully surrounded by a manganese-depleted zone (MDZ), which has been reported to be effective for AF nucleation in wrought steels.…”

“…Prior to the second welding, pure Ti fibers (0.3 mm in diameter) were inserted into the groove to change the titanium concentration of the second weld. 14) Using the same wire and shielding gas, single-pass bead-in-groove welding was performed with a rather high heat input (25 kJ/cm) to fill the groove and to melt the inserted Ti fibers. This resultant weld is referred to as the 'experimental weld'.…”

“…To minimize the base metal dilution, the welds were prepared using a two-step welding technique, which has been described previously. 14,15) First, five-pass buttering was performed using the gas metal arc welding (GMAW) process to fill the 10-mm-deep groove using ER 100S-grade wire and an Ar + 20%CO 2 shielding gas. After buttering, a 5-mm-deep V-groove was machined at the center of the buttering weld metal.…”

Role of welding flux in formation of weld metal during arc welding of high-strength low-alloy steels

“…When titanium oxide precipitates on the surface of refractory inclusion, then zones with reduced content of alloying elements, having high mobility in γ-phase, can be formed in adjacent areas of solid solution. The inclusions of given type are the most efficient centers of nucleation of bainite microstructure [9,10].…”