# Assessment of the Structural State of Dissimilar Welded Joints by the Acoustic Emission Method
## 2 Materials and Methods  
In this study, heterogeneous welded joints of 09G2S (pearalitic class) and 12 Kh18N10T (austenitic class) steels were used. To produce welded joints, pairs of plates made of 09G2S and 12 Kh18N10T steels with dimensions of 250 mm \(\times\) 300 mm \(\times\) 3 mm each were welded together by argon arc welding with a non-consumable electrode Sbaros SW146 filler wire with a diameter of 1.2 mm. The chemical composition of these steels and a filler material is presented in Table 1; the mechanical properties, determined by tensile tests, can be found in Table 2.  
To make a defect-free welded joint, welding of the plates was carried out in compliance with the recommended arc welding technology parameters. As a result, a welded joint with full penetration and austenitic structure of the weld metal was obtained (Figure 1a). No diffusion interlayers were observed near the fusion lines (Figure 1b). The structure of 09G2S steel near the fusion line was pearliti, while the weld metal was austenitic. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **12 Kh18N10T** & **09G2S** & **Sbaros SW146 Wire (Reference Values)** \\  & **Steel** & **Steel** & **Sbaros SW146 Wire (Reference Values)** \\ \hline C & \textless{}0.12 & \textless{}0.12 & 0.10 \\ Si & \textless{}0.8 & 0.5–0.8 & 0.80 \\ Mn & \textless{}2 & 1.3–1.7 & 8.5 \\ Ni & 9–11 & \textless{}0.3 & 8.5 \\ S & \textless{}0.02 & \textless{}0.04 & – \\ P & \textless{}0.035 & \textless{}0.035 & – \\ Cr & 17–19 & \textless{}0.3 & 18.5 \\ Cu & \textless{}0.3 & \textless{}0.3 & – \\ Ti & 0.4–1 & – & – \\ Fe & \textless{}67 & balance & balance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of welded steels and a filler wire, wt %. Figure 1: Welded joint of steel 09G2S and 12 Kh18N10T (**a**) and fusion line from the side of the steel 09G2S (**b**). To obtain diffusion interlayers, some welded joints were submitted to post-weld heat treatment in Nabertherm P180 furnaces according to the following regime: heating to 650 \({}^{\circ}\)C and holding for 5 h and still air cooling. This heat treatment simulated long-term operation of these welded joints at high temperatures. Due to the presence of chromium and manganese in the composition of the SW146 filler wire, diffusion layers were formed in the near-weld zone. The structure of the heat-treated welded joint is shown in Figure 2.  
Due to the carbon diffusion from 09G2S steel into the fusion line zone, a decarburger layer with a ferritic structure was formed. The thickness of such an interlayer was 500-700 \(\upmu\)m (Figure 2b). In addition, near the fusion line on the side from the weld, a carbide interlayer formed due to the carbon diffusion into the weld. The thickness of the carbide layer was 10-70 \(\upmu\)m, and 30 \(\upmu\)m on average (see Figure 2b). Figure 2c shows a photo of the fusion line from the side of steel 09G2S, the distribution of carbon in this area and the chemical composition of the carbide interlayer, obtained by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results indicate high carbon content in the interlayer formed as a result of carbon diffusion during additional heat treatment. Figure 2: Welded joint of 09G2S and 12 Kh18N10T steels after PWHT: weld (**a**) and cross-section of the fusion line from the side of 09G2S steel (**b**); SEM images (**c**) and EDX analysis results (**d**) of carbide interlayer. Flat specimens were made for mechanical testing with a width of the gauge section of 20 mm from the obtained welded joints. Tensile tests were carried out on an Instron 5982 testing machine with a constant crosshead speed of 2 mm/min. Cyclic tensile tests were carried out on an Instron 8801 testing machine; loading parameters were set as sinusoidal tensile cycles with a frequency of 1 Hz and a cycle load ratio \(R\) = 0.05; the value of the maximum cycle stress \(\sigma_{\text{max}}\) varied from 200 to 400 MPa based on the specimen type.