Chemical Composition and Crystal Structure of Oxide Inclusions Promoting Acicular Ferrite Transformation in Low Alloy Submerged Arc Weld Metal.

Abstract

High toughness are requested in the weld metals for offshore steel structures and steel line-pipes used at low ambient temperatures. Micro-alloying of titanium and boron effectively improves the toughness of low-alloyed weld metals with tensile strength ranging between 490 and 590 N/mm2. It is well known that refined intragranular ferrite or acicular ferrite nucleates on titanium containing oxides. However, there have been few reports on the chemical composition at local positions of these effective inclusions and their crystal structures.Two types of submerged arc weld metals were used ; one is a silicon-and-manganese type weld metal with a ferrite with aligned second phase and the other is a titanium type one with the acicular ferrite. The mechanical and metallurgical examination included the microscopic observation, Charpy impact tests of the welds and the characterization of oxides in weld metals with X-ray diffractions and analytical electron microscope.The following facts were clarified from the above investigations. The oxides in the titanium bearing weld metal are crystallized in a form of (Mn, Ti) (Al, Ti)2O4 with angularly rugged surfaces, while the oxides of a Si-Mn type are amorphous with smooth spherical shape. Titanium as low as 0.005 wt% in a weld metal satisfactorily crystallized oxides if titanium is included in oxides with aluminum and manganese.

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.…”

“…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. 23,24) In contrast, some researchers 25) have reported that the addition of titanium promoted the crystallization of inclusions, changing from amorphous manganese silicate to crystalline galaxite (MnO·Al2O3), and resulted in weld microstructure predominant with acicular ferrite.…”

Inclusion Formation and Microstructure Evolution in Low Alloy Steel Welds.

“…[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Based on these research, it is possible to design welding consumables that maximize the formation of acicular ferrite in welds and thereby obtain better strength and toughness. However, the extension of this knowledge to a wide variety of welding processes, consumable compositions, and shielding gases still eludes welding metallurgist due to the complex interactions between inclu-sion formation, solidification, and solid-state transformations.…”

Acicular Ferrite Formation on Ti-Rare Earth Metal-Zr Complex Oxides

“…[4][5][6] Many studies have been done to identify oxides that promote AF formation and to establish the mechanism of AF nucleation. [7][8][9][10][11][12][13][14] It is well known that Ti oxides have a positive influence on AF nucleation. 10,12,13,15,16) Researchers have pointed out that Ti2O3 incorporates Mn and forms a Mn depleted zone around the oxide, which increases the driving force for decomposition of the surrounding austenite matrix during cooling.…”