## 2 Material Descriptions  
The material used for the specimens was DH36 steel (certified by American Bureau of Shipping). Specimens tested in this study were welded with the carbon dioxide arc welding process. The filler metal was CHE50 (J507) which was referenced to ISO 2560:2009 [22]. The chemical compositions of DH36 steel and filler metal are shown in Tables 1 and 2. The name of the welding power source was CHAMP MIG 400 (ADOR). The height of weld toe was 6 mm. Other main parameters of the welding procedure are given in Table 3. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{3}{*}{**DH36**} & \multicolumn{8}{c}{**Chemical Compositions, \%**} \\ \cline{2-9}  & **C** & **Si** & **Mn** & **P** & **S** & **Als** & **Cr** \\  & 0.17 & 0.20 & 1.16 & 0.018 & 0.0034 & 0.023 & 0.02 \\ \cline{2-9}  & **Nb** & **V** & **Ti** & **Mo** & **Ni** & **Cu** & **Ceq** \\  & 0.015 & 0.002 & 0.017 & 0 & 0.01 & 0.02 & 0.37 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions of DH36 steel. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{3}{*}{**CHE50**} & \multicolumn{8}{c}{**Chemical Compositions, \%**} \\ \cline{2-9}  & **C** & **Si** & **Mn** & **P** & **S** & **Cr** & **Ni** & **Mo** & **Cu** \\  & 0.24 & 0.35 & 1.5 & 0.045 & 0.05 & 0.3 & 0.3 & 0.3 & 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical compositions of filler metal. The metallographic samples, cut from the welded joints, are shown in Figure 1a,b. A welded joint can be divided into the welded zone (WZ), fusion zone (FZ), heat-affected zone (HAZ), and base metal (BM) [23]. As shown in Figure 2a,b fatigue failure of the specimen was predominantly located in the HAZ. Therefore, it was essential to study the microstructure of the HAZ. In this study, the microstructures of the T-welded and cruciform welded joints specimens were examined using SEM, and the various zones of the welded joints were classified. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Unit** & **Value** \\ \hline Preheat temperature & \({}^{\circ}\)C & 100 \\ Filler diameter & mm & 1 \\ Welding current & Amps & 120 \\ Arc voltage & Volts & 22 \\ Heat input & KJ/cm & 15 \\ Welding process & Gas metal arc welding (GMAW) \\ Shielding gas & Carbon dioxide \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Welding conditions. Figure 1: The metallographic samples: (**a**) T-welded joints and (**b**) cruciform welded joints. Figure 2: The profile of T-and cruciform welded joints: (**a**) T-welded joints and (**b**) cruciform welded joints (BM: Base Metal, FZ: Fusion Zone, WZ: Weld Zone, HAZ: Heat Affected Zone). In the welded zone, a large amount of bainite was observed, and it contained mixed martensite-austenite phases (an M/A island) (M/A island) [24]. During the formation of the M/A island, while ferrite was precipitated from the supercooled austenite, carbon was also formed in the supercooled austenite, which had not yet undergone the transformation and was gradually enriched. Carbon-rich austenite was formed because the carbon content was not sufficiently concentrated to reach the level of precipitated carbide. The carbon-rich austenite was mostly transformed into martensite during cooling, and a small amount of austenite remained, thereby forming an M/A island [25], as shown in Figures 3a and 4a. The fusion zone refers to the boundary zone between the weld and base metal and is also called the semi-melting zone. It is the zone where the weld metal transitions to the heat-affected zone in the welded joint [26]. The zone was very narrow, with the completely-melted weld zone and completely-infusible heat-affected zone on both sides, and the fusion zone was in the transition zone from the weld metal to the base metal in the zone close to the weld [13], as shown in Figures 3b and 4b. The grains in this zone were coarse, and the chemical composition was very complex [13]. The zone of the base metal mainly consisted of flaky pearlite and quasi-polygonal ferrite [13], as shown in Figures 3d and 4d.