<sup>1</sup> Weld Metal

In the LBW joint, the weld seam was formed by the solidification of partial base metal after high temperature melting. It was mainly composed of a certain amount of equiaxed crystal phases surrounded by coarse columnar crystal phases. The columnar crystal phases grew along the perpendicular direction to the side wall of the molten pool from the base metal to the weld seam, as shown in Figure 3. The width of columnar crystal phases is about 10 μm, and their maximum length can be greater than 200 μm. The mechanical properties of the joint significantly decreased because of the existence of the columnar crystal phases. The WM was mainly composed of ferrite (F) and bainite (B). The ferrite was mainly proeutectoid ferrite, which is an important component of columnar crystal phases, while the equiaxed crystal phases were almost entirely composed of polygonal ferrite phases. In addition, there were a few acicular ferrite phases and a certain amount of martensite phases. Acicular ferrite phases formed with some inclusions as the core and most martensite phases were distributed in the fusion line, at the root or top of the weld due to faster cooling rate.

**Figure 3.** Microstructure of WM of the LBW joint (**a**) Macro-photo in weld center area (**b**) Columnar crystal (**c**) Equiaxed crystal (**d**) Ferrite (F) and bainite (B).

 <sup>2</sup> Coarse-Grained Heat-Affected Zone

The CGHAZ is adjacent to the WM, and its higher peak temperature of the thermal cycle ranges from 1100 ◦C to the liquidus temperature. The austenite phases can stay in this temperature range for a long time and dramatically grow, and then form larger overheating phases during cooling, which can worsen the mechanical performance of the whole joint. The average grain size of the CGHAZ can reach about 50 μm; even 100 μm for the grains near to the fusion line.

The microstructure features of the CGHAZ are closely related to the heat input. When the heat input increases, acicular ferrite begins to gradually form in the CGHAZ. Acicular ferrite with low hardness and good toughness can significantly improve the impact behavior of the CGHAZ. If the heat input keeps increasing and the cooling rate decreases, polygonal ferrite phases will increase and acicular ferrite will decrease. Meanwhile, there is a small amount of pearlite that can possibly appear in this area.

Due to the lower heat input and faster cooling rate, there are more and more stripshape bainite, ferrite, and low-carbon martensite phases formed in the CGHAZ. Most of the strip-shape phases initially form from the original austenite grain boundary and grow toward the crystal at a certain angle; each strip maintains a certain phase relationship and a fine structure inside the strip. With lower heat input (9.6 kJ/cm) in the LBW, the internal phases in the CGHAZ are low-carbon martensite and a few bainite and ferrite, as shown in Figure 4.

**Figure 4.** Microstructure of the CGHAZ of the LBW joint (**a**) CGHAZ close to fusion line (**b**) Martensite and bainite (**c**) CGHAZ far from fusion line (**d**) Martensite and ferrite. F: ferrite; M: martensite; B: bainite.

 <sup>3</sup> Fine-Grained Heat Affected Zone

During the LBW process, for the FGHAZ, the peak temperature of the welding thermal cycle is between 1100 ◦C and AC3 temperature, and its mechanical properties performance is the best out of all the weld joints. The reason for this is that the microstructure in the FGHAZ undergoes a recrystallization and new grain refinement during heating and cooling; more fine grains can be obtained, which is equivalent to the base metal normalized. In general, most grain sizes of the fine grains are less than 10 μm. Observed under a high magnification microscope, the microstructure of the FGHAZ can be described by some finer martensite, bainite, and few fine carbides precipitated between ferrite blocks, as shown in Figure 5.

**Figure 5.** Microstructure of the FGHAZ of the LBW joint (**a**) Macro-photo of the FGHAZ (**b**) Microphoto of the FGHAZ (**c**) Ferrite and bainite (**d**) Carbides in the FGHAZ. F: ferrite; M: martensite; B: bainite.
