## 2 Materials and Methods  
Dissimilar welded joints of 09G2S (low-alloyed pearlitic steel) to 12 Kh18N10T (chrome-nickel austenitic steel), obtained using argon-arc welding with a filler wire, have been investigated. Chemical compositions of steels are presented in Table 1.  
To obtain the joints, pairs of plates with dimensions of 250 mm \(\times\) 300 mm \(\times\) 3 mm each were welded to each other. A total of three pairs of plates were welded. Argon-arc welding was carried out using a Sabaros SW146 filler wire with a diameter of 1.2 mm. The chemical composition of the wire is shown in Table 1[30; 31]. Welding of the plates pair No.1 was carried out in compliance with the correct arc welding technology at the recommended welding parameters. As a result, a defect-free welded joint with full penetration was obtained (Figure 2a). The weld metal structure was austenitic. The plates pair No.2 was welded deliberately at low welding current to obtain defects of the \"lack of penetration\" type in the welded joint. A welded joint with lack of penetration is shown in Figure 2b. Welding of the plates pair No.3 was carried out in the same way as for the plates pair No.1, in compliance with the correct technology of arc welding on the recommended parameters of the welding mode. After welding, the joint was subjected to subsequent heat treatment in Nabertherm P180 furnace according to the mode (heating to 650 \({}^{\circ}\)C + holding for 5 h + cooling in calm air). This heat treatment simulates the long-term operation of these welded joints at high temperatures. Due to carbide-forming elements (chromium and manganese) in the SW146 wire, diffusion interlayers were formed during additional high-temperature heat treatment. Intense diffusion processes took place on the fusion line \"weld seam--09G2S steel\". The structure of the heat-treated welded joint (plates pair No.3) is shown in Figure 2c. Due to diffusion of carbon from 09G2S steel into the weld seam, a decarburized ferritic layer is formed in the 09G2S steel near the fusion line. The thickness of this interlayer was 500-700 \(\upmu\)m (Figure 2d). In addition, in the weld seam near the fusion line, a carbide interlayer was formed due to the interaction of carbon diffused into the weld with carbide-forming elements in the weld metal. The thickness of the carbide interlayer is 10-70 \(\upmu\)m, and 30 \(\upmu\)m on average (see Figure 2d). \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & **C** & **Si** & **Mn** & **Ni** & **S** & **P** & **Cr** & **Cu** & **Ti** & **Fe** \\ \hline
12 Kh18N10T steel & \(\leq\)0.12 & \(\leq\)0.8 & \(\leq\)2 & 9–11 & \(\leq\)0.02 & \(\leq\)0.035 & 17–19 & \(\leq\)0.3 & 0.4–1 & \(\sim\)67 \\ \hline
09G2S steel & \(\leq\)0.12 & 0.5–0.8 & 1.3–1.7 & \(\leq\)0.3 & \(\leq\)0.04 & \(\leq\)0.035 & \(\leq\)0.3 & \(\leq\)0.3 & - & Balance \\ \hline Sabaros SW146 wire & 0.10 & 0.80 & 8.5 & 8.5 & - & - & 18.5 & - & - & Balance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of welded steels and filler wire, wt.%. Thus, three types of welded joints were prepared for research: a defect-free joint (plates pair No.1), a joint with lack of penetration (plates pair No.2), and a joint with diffusion interlayers (plates pair No.3). From the obtained welded joints, specimens for static tension test were prepared. Tensile tests were carried out on an Instron 5982 testing machine with a strain rate of 2 mm/min. Tensile tests were also carried out on flat specimens made of parent metals--09G2S steel and 12 Kh18N10T steel (specimens without welded joints). AE signals were recorded during the tests using the A-Line 32D industrial system. The measuring path consisted of resonant sensors GT200 (Global test LLC, Sarov, Russia) which have resonance frequency 180 kHz and preliminary amplifiers of the electrical signal PAEF-014. The study of the waveform and spectra of the AE signals was carried out using broadband sensors SE 1000 (Acoustic Technology Group, Score Atlanta Inc., Chicago, IL, USA).