<sup>4</sup> Two-Phase Zone

The range of the TPZ in the LBW joints is wider (especially in the center of the joint), and its peak temperature of the welding thermal cycle is between AC3 and AC1. During this temperature range, only partial base metal can obtain sufficient heat to finish phase transformation and form complex phases based on sorbite and the partial transformation structure.

It can be seen from Figure 6a,b that there are some obvious striped microstructures in the TPZ. The reason for these striped microstructures is due to interlacing ferrite and other phases along the deformation direction when heating. The appearance of striped microstructures makes the steel anisotropic, and has a negative effect on the mechanical properties of the steel. In the base metal, there are few striped microstructures can be less obviously seen. However, striped microstructures can be easily found after welding because the secondary cementite of the austenite with partial phase transformation in the TPZ precipitates along the original strip direction and forms duplex phases of ferrite and cementite. These striped microstructures can be seen more clearly due to their own fine structures, making most of the incomplete phase transformation turn black after corrosion.

The striped microstructures in the TPZ are shown in Figure 6c,d, under the scanning electron microscope. The black part is similar to the tempered sorbite duplex phase, which is mainly composed of striped ferrite, carbides, and few polygonal ferrite. However, there are more granular carbides among the ferrite phases, and black striped microstructures increase with the precipitation of carbides, which results in a finer microstructure and a stronger black color. The white part of the striped microstructures is mainly irregular tangled ferrite bands and a few precipitated carbides. Compared with the black part, this part looks white and its hardness is lower due to obvious ferrite characteristics; the reason is that when carbon-poor austenite reaches the austenitizing temperature, there are more proeutectoid ferrite precipitated, and then ferrite and cementite are precipitated, which results in the formation of some complex ferrite morphologies.

**Figure 6.** Microstructure of the TPZ of the LBW joint (**a**) Segregation band in the TPZ (**b**) Microstructure of the segregation band in SEM (**c**) Microstructure in black part (**d**) Microstructure in white part. F: ferrite.
