## 2 Materials and Methods  
Both the 316L austenitic stainless-steel sheets and the B610 CF high-strength steel plate for dissimilar welding were in their annealed states. The chemical compositions of the materials are listed in Table 1. Due to the special structure of metal multi-layer bellows, multi-layer 316L steel sheets with a thickness of 1.5 mm and one-layer high-strength steel with a thickness of 8 mm were welded. As shown in Figure 1, comparative experiments of V-groove multi-layer multi-bead welding using GTAW and laser welding with filling wire were carried out. The filler wire in dissimilar welding is 316L austenitic stainless steel with a diameter of 2.5 mm in GTAW and a diameter of 1.2 mm in laser welding. The equipment used includes a manual GTAW system and a manual laser welding system with a wire feeder. An ITG-500 AP welding machine (Institute of Intelligent Manufacturing, Guangzhou, China) with a maximum voltage of 30 V and a maximum current of 500 A, and a DoF-HLW-2000 laser (Institute of Intelligent Manufacturing, Guangzhou, China) with a maximum power of 2000 W were used in the manual GTAW and laser welding experiments, respectively. The wire feeding nozzle was fixed at the front of the laser head or arc torch, and the wire was fed into the molten pool on the surface at an angle of 30\({}^{\circ}\) with respect to the workpiece surface. In order to avoid oxidation, argon was used as a shielding gas during the whole welding processes. Prior to welding, the workpiece surface was cleaned with ethanol. Firstly, the adjacent plates of multi-layer bellows were welded, as depicted in Figure 1c. And then, the 316L multi-layer bellows and the one-layer high-strength steel were welded by V-groove multi-layer multi-bead welding. The detailed welding parameters are given in Table 2. The V-groove angle was 60\({}^{\circ}\), and the root butt clearance was 1.5 mm.  
In order to detect the internal defects of the welded joints, a Doppler PHASCAN II (Guangzhou Doppler Electronic Technologies Incorporated Company, Guangzhou, China) ultrasonic device was used for defect inspection after welding. The cross-sectional samples and tensile samples were taken by wire electric discharge machining. The metallographic samples were ground with sandpapers of different mesh sizes, and polished with a 0.5 \(\upmu\)m diamond paste, and then etched with a solution (5 g CuCl\({}_{2}\) + 5 g HCl +100 mL alcohol). A DM3000 Optical Microscope (OM, Leica GmbH, Wetzlar, Germany), a QUANTA 200 Scanning Electron Microscope (SEM, FEI, Rock Hill, SC, USA) and an energy dispersive spectroscopic (EDS) were used to characterize the microstructure. The microhardness test was conducted using an HV-1000A Vickers instrument (Laizhou Hu  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Materials** & **C** & **Mn** & **P** & **S** & **Si** & **Cr** & **Ni** & **Mo** & **Fe** \\ \hline
316L & \textless{}0.03 & \textless{}2 & \textless{}0.045 & \textless{}0.03 & \textless{}1 & 16–18 & 10–14 & 2–3 & Balance \\ B610 CF & 0.09 & 1.57 & - & - & 0.31 & 0.21 & - & - & Balance \\ \hline \hline \end{tabular}
\end{table}
Table 1: The chemical composition of 316L stainless steel and B610 CF high-strength steel (wt.%).  
Figure 1: The schematic diagrams of welding processes and analysis: (**a**) GTAW, (**b**) laser welding, (**c**) V-groove welds, (**d**) the size of the samples for tensile tests.  
Laizhou, China) with a load of 300 g and an adjacent position spacing of 1 mm. According to the GB/T228.1-2021 standard [30] in Figure 1d, the tensile tests were carried out using an Hb250 electromechanical universal testing machine (Zwick/Roell GmbH, Ulm, Germany) at ambient temperature with a strain rate of 10 mm/min. Three tensile specimens with a gauge length of 80 mm were prepared for each welding process. After the tensile tests, the fracture morphology of the tensile specimens was observed using SEM.