## 2 Materials and Methods
### AE Source Characteristic  
Dissimilar welded joints of grade 20 steel (carbon steel with a ferrite-pearlite structure) and 12 Kh18N10T steel (austenitic stainless steel) were the objects of the research. The welded joints were fabricated from sheets of grade 20 steel and 12 Kh18N10T steel with a thickness of 3 mm. The chemical compositions of the 12 Kh18N10T and grade 20 steel are provided in Table 1. The sheets were cut into strips 1000 mm long and 200 mm wide and were welded together using MIG welding (Figure 2a). Figure 1: Proposed algorithm of data classification flowchart. Welding was performed with a double-sided butt joint, using Sabaros O101 wire as the filler material. The chemical composition of the filler wire is provided in Table 2. The use of Sabaros O101 wire provides the formation of an austenitic microstructure in the weld metal, which is optimal in terms of its mechanical and operational properties. Additionally, the chromium content in the wire contributes to the formation of carbide and decarburized diffusion interlayers during the heat treatment of the welded joints. The preparation of specimens for tension tests was carried out by cutting the welded sheets with a laser. The shape and sizes of the test specimens were selected in accordance with the recommendations of tension test standards and the specific requirements of the AE tests being conducted. The width of the specimen gauge section was set at 20 mm to securely attach an AE sensor with a diameter of 15 mm to its surface. The weld was in the center of the specimen gauge section. The width of the grip section was set at 50 mm to prevent plastic deformation of the 12 Kh18N10T steel in the grip area during tension testing (Figure 2b,c). To form diffusion interlayers, which typically occur in the welded joints during long-term operation under high temperatures, a few of the weld specimens were subjected to subsequent heat treatment to simulate the effect of long-term operation under high temperatures. Heat treatment was conducted in a Nabertherm P180 furnace at a temperature of 650 \({}^{\circ}\)C with different holding times (1 h, 5 h, and 25 h). Different holding times were necessary to form diffusion interlayers of different thicknesses. Increasing the heat treatment duration and temperature intensifies the diffusion of carbon from grade 20 steel into the weld metal, resulting in the thickening of the diffusion interlayers in the area of the fusion line--both carbide (in a weld metal) and decarburized (in grade 20 steel). The \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**C** & **Si** & **Cr** & **Ni** & **Mn** & **Fe** \\ \hline
0.10 & 0.50 & 19.0 & 9.0 & 6.0 & Base \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of the Sabaros O101 filler wire for MIG welding, % wt. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Steel Grade** & **C** & **Si** & **Mn** & **Ni** & **S** & **P** & **Cr** & **Cu** & **Ti** & **Fe** \\ \hline
12 Kh18N10T & max 0.12 & max 0.8 & max 2 & 9–11 & max 0.02 & max 0.035 & 17–19 & max 0.3 & 0.4–1 & bal. \\
20 & 0.17–0.24 & 0.17–0.37 & 0.35–0.65 & max 0.3 & max 0.04 & max 0.035 & max 0.25 & max 0.3 & max 0.08 & bal. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions of grade 20 steel and 12 Kh18N10T steel, % wt. Figure 2: Welded sheets made of grade 20 steel and 12 Kh18N10T steel (a), specimen scheme (b), and specimen after cutting (c). average values of interlayer thicknesses are presented in Table 3. The microstructure of welded joints after heat treatment is shown in Figure 3.  
The samples were tested for cyclic tension by zero-triangular cycles with a maximum stress of \(\sigma_{\text{max}}\) = 200 MPa, which corresponds to the elastic region of the weld joint tensile stress diagram. The tests were carried out on an Instron 5982 testing machine with a frequency of about 0.2 Hz.