Ab Initio Study of Weldability of a High-Manganese Austenitic Twinning-Induced Plasticity (TWIP) Steel Microalloyed with
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Overview of TWIP-B steel with generation of welding buttons by GTAW. a) Welding condition 1, and b) Welding condition 2. Scanning Electron Microscopy-Electron Dispersive Spectroscopy (SEM-EDS) During the segregation study in the FZ of weld buttons by spot chemical analysis, it was found the following results: 0.36 Mn, 0.18 Al, 0.32 Si and 1.08 C (wt%) of average chemical composition for welding condition 1; and 0.6 Mn, 0.02 Al, 0.22 Si and 0.49 C (wt%) of average chemical composition for welding condition 2. In this case, condition 2 showed more stability of elements than condition 1, particularly Al, Si and C. On the other hand, Mn showed less variation in condition 1. The distribution of the alloying elements (Mn, Al, Si, and C) through the welding buttons of the TWIP-B steel shows variations in the FZ, maintaining this behavior for the two welding conditions. Differences in chemical composition of the solidified metal is known as segregation phenomenon, and particularly in welding this is an important phenomenon resulting from the solute redistribution, producing heterogeneities in physical, chemical and mechanical properties of steels [13]. The elemental mapping carried out in the FZ of the TWIP-B steel, allows qualitatively determine the Mn segregation produced by the welding process, as shown in Fig. 2. In this case, it is clear that Mn and Si tend to segregate toward the dendrite grain boundaries, while Al and C show a decrease in the interdendritic areas. On the other hand, it can be observed the presence of particles which are distributed along the dendrite grain boundaries. Some of these particles were identified as AlN (see Fig. 2a). Reyes-Calderon et al. [12] reported the presence of particles precipitated in TWIP steels microalloyed with B, which were identified by carbon replica technique in transmission electron microscopy (TEM) as AlN (approximate size 400 nm-3 µm), BC (0.5 µm-1 µm) [7].
Figure 2. Elemental mapping of chemical composition in the FZ of TWIP-B steel. a) Image of the analyzed area, and b) Mn distribution. Vickers microhardness test Fig. 3 shows the Vickers microhardness profiles of TWIP-B steel under different welding conditions. Welding condition 1 exhibited a higher microhardness in the FZ reaching up to 300 HV25, with softening in the HAZ and slight hardening in the base metal (MB). Condition 2 exhibited higher softening in the HAZ, being this area where lower microhardness values were registered, as shown in Fig. 3(b). In general, the two welding conditions exhibit lower microhardness values than the solubilized condition used as base material for welding buttons (average microhardness of ≈ 332 HV25). Changes in mechanical properties both FZ and HAZ are explained in terms of microstructural changes caused by welding process. In the case of the FZ, microhardness reduction is attributed to the formation of dendritic microstructure [11, 13], while in the HAZ it is attributed to grain growth. Saha et al. [14] reported hardness values significantly lower in weld points than in the base
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