# Assessment of the Structural State of Dissimilar Welded Joints by the Acoustic Emission Method
## 4 Discussion  
Features of AE data during the cyclic loading of dissimilar welded joints without diffusion interlayers, as well as during static tension, correspond to the inhomogeneous nature of the object under study. At stress \(\sigma_{\text{max}}\) equal to 200 MPa, the weld and base metal experience elastic deformation (Figure 4). AE hit generation in this case is non-intense; the average AE hit rate is 0.05 AE hits per cycle. This can be explained by the small number of mobile dislocations, which are sources of AE, as well as the absence of material structural elements that can cause failure at a given stress. The obtained result fits the theory of AE [23] and does not contradict the fundamental studies [16]. A more detailed analysis of the AE data showed that the parameters of the AE hits, such as the amplitude and rise time, are mostly random and have a considerable scatter of values (Figure 6a). The lack of ordering parameters indicates the scattered nature of the destruction [24; 25]. The load range \(\sigma_{\text{max}}\) = 250-300 MPa corresponds to the beginning of plastic deformation of steel 12 Kh18N10T, which is manifested by more intense AE generation with the appearance of high-amplitude AE hits up to 80 dB. The correspondence of the AE data to the process of plastic deformation of austenitic steel is confirmed by the location results (Figure 5d), i.e., the indications of AE events located in the parent metal area from the side of steel 12 Kh18N10T. The scatterplot (rise time vs. amplitude) at \(\sigma_{\text{max}}\) = 300 MPa acquires an ordered form due to the grouping of indications into horizontal lines corresponding to  
Figure 11: Frequency histogram of AE hits’ rise times for defect-free welded joint (**a**) and for welded joint with diffusion interlayers (**b**). Figure 12: Typical AE signal waveforms of a specimen with diffusion layers at \(\sigma_{\text{max}}\) = 300 MPa (**a**) and \(\sigma_{\text{max}}\) = 400 MPa (**b**). AE hits with different amplitudes, but equal rise time values. AE hit rise time depends both on the type of AE source and its location; therefore, AE hits with the same rise time are most likely emitted by the same localized AE source. Analysis of the AE signals waveform showed that the AE signals belonging to the same cluster on the scatterplot have a similar waveform, with a cross-correlation coefficient of 0.6-0.8. A typical AE signal waveform is shown in Figure 7. The burst nature of this signal, relatively high amplitude and a specific waveform make it possible to assume that the process of twinning during plastic deformation of austenitic steel was recorded [26]. AE activity increases and the AE source position occurs over the entire surface of the specimen with predominant localization in the parent metal from the side of 09G2S steel when the maximum stress is 400 MPa. The scatterplot (Figure 6c) for a given stress value is a set of horizontal lines of AE hits with equal rise times. The results of cluster analysis of AE signals show the presence of many AE signals with similar waveforms. This AE signature can be explained by the several active AE sources and characterizes the process of plastic deformation of the parent steel 09G2S. Tests of specimens of dissimilar welds with diffusion interlayers have shown that the presence of carbide and decarburized interlayers can be determined based on AE data. Thus, at close to operational stress \(\sigma_{\text{max}}\) = 200 MPa, AE hit generation is 20 times more active, and the average value of the AE hit rate turns out to be 20 times higher than for defect-free specimens (Figure 9a). The results of cluster analysis showed that an increase in AE hit rate occurs due to the generation of AE hits of similar waveform (Figure 12); in this case, a cluster is formed on the rise time vs. amplitude scatterplot and a distribution of rise time acquires some specific changes, indicating the presence of diffusion interlayers.