Influence of Ti on non-metallic inclusion formation and acicular ferrite nucleation in high-strength low-alloy steel weld metals

Variation in the Chemical Driving Force for Intragranular Nucleation in the Multi-pass Weld Metal of Ti-Containing High-Strength Low-Alloy Steel

“…After re-machining a 5 mm deep V-groove at the buttering welds, the single-pass bead was deposited using a gas metal arc welding process with a heat input of approximately 2.0 9 10 3 kJ/m. In our recent study, [22] the spinel (predominantly MnTi 2 O 4 ) and ilmenite (a solid solution of MnTiO 3 and Ti 2 O 3 ) were confirmed to be formed at the inclusion surface, and these phases have the ability to develop the MDZ near the inclusions in as-deposited weld metals with a Ti content in the range of 0.007 to 0.1 wt pct. Thus, two types of chemical compositions were prepared by controlling the Ti content in order to make the two specimens, respectively, have spinel and ilmenite at the inclusion surface.…”

“…The aforementioned inclusion phase identifications agreed well with our previous detailed investigation. [22] The simulation results are compared with the experimental results as follows. The corundum phase, which was expected to form first during solidification of the liquid oxide, was not observed in the experiments.…”

“…[21][22][23] Considering the fact that the width of the MDZ in weld metals is on the order of 10 nm, the failure to detect the MDZ by earlier researchers is likely attributable to conducting energydispersive spectrometry (EDS) analysis with too-wide intervals. Actually, Ricks et al [5] conducted Mn-profile analysis with intervals higher than 50 nm.…”

“…[10,11] Importantly, many researchers have identified the factors that improve the inclusion potency for acicular ferrite nucleation. These include (1) lattice coherency between the inclusion and ferrite, [12][13][14][15] (2) an increase of the chemical driving force due to the depletion of austenite-stabilizing elements (such as Mn) in the austenite adjacent to the inclusion, [12,[16][17][18][19][20][21][22][23] and (3) strain energy due to the difference in the thermal contractions between the inclusion and austenite. [24,25] In early studies, acicular ferrite was defined as intragranularly nucleated Widmansta¨tten ferrite, [5] but it was described as intragranularly nucleated bainite by later researchers.…”

“…Furthermore, the Mn-depleted zone (MDZ) was recently shown to contribute to the intragranular nucleation in high-strength low-alloy (HSLA) steel welds. [21][22][23] A multi-pass welding process is used to join thick plates in order to prevent the deterioration of the base metal by minimizing the size of the heat-affected zone. However, in contrast with single-pass weld metals, multi-pass weld metals contain a large amount of reheated regions in each weld bead (except for the last bead), which are reheated to a temperature above Ac 3 via thermal affection from the following beads.…”