## 4 Discussion  
The AE signature obtained for the specimens of the parent metals (09G2S and 12 Kh18-N10T steels) are consistent with the fundamental works [32] and [33] and do not contradict the general theory, according to which the AE upon deformation is proportional to the rate of the dislocation density increase. Therefore, the onset of plastic deformation corresponds to an intense accumulation of dislocations, and an increase in AE hits rate. At the stage of strain strengthening, when the dislocation density approaches the critical level, the AE hits rate decreases. AE data obtained by tensile defect-free dissimilar welded joints form a repeatable signature consisting of two peaks of AE hits rate corresponding to the onset of plastic deformation of each of the parent metals and two periods of acoustic calm corresponding to their strain hardening (Figure 5a). In general, this AE signature is a combination of a data obtained during deformation of the steels being welded. The effect of acoustic calm during the deformation hardening explained in detail in [34]. The character of AE data during the tension of defect welded joint specimens is also repeatable. For the specimens with a lack of penetration, AE hits rate have a two local maxima, the first of which corresponds to the initiation of plastic deformation of 12 Kh18N10T steel, and the second corresponds to the destruction of a defective welded joint (Figure 6). For the specimens of welded joints with the presence of a diffusion layer, the AE hits rate has an additional maximum at a stress value corresponding to the ultimate strength of the  
Figure 11: Scatter plot between the amplitude of AE signals and the median frequencies (**a**) for defect-free weld joint, (**b**) for weld joint with diffusion interlayers, (**c**) for weld joint with lack of penetration. ferrite phase (Figure 8a). Therefore, it can be assumed that an additional maximum of the AE hits rate is an indicator of a decarbarurized diffusion layer. This fact is also confirmed by the results of local deformation using the digital image correlation method; simultaneously with the maximum of the AE activity, a local maximum of deformation is observed in the region of the decarbarurized diffusion layer (Figure 8c,d). For early detection and identification of damage to a dissimilar welded joint, it is advisable to consider the period of acoustic calm, when the change in the AE data for defect-free specimens is almost stationary. For defect-free samples, this period is characterized by a scattered distributed accumulation of damage, while the distribution of time intervals between adjacent hits corresponds to an exponential law (Figure 9a). In the presence of a lack of penetration or diffusion layers, the exponential law of distribution of time intervals is violated, which is confirmed by the Pearson's goodness-of-fit test (Figure 9b). The distribution function of time intervals is used as an indicator of the destruction process by a few authors [35]. The correspondence of the function of time intervals to the exponential distribution in [36], as well as in this paper, is a sign of scattered accumulation. A more obvious indicator of the presence of a defect is the appearance of correlated AE signals during the period of acoustic calm. For welded joints with lack of penetration, the appearance of correlated signals with a long duration and a narrow-band spectrum is characteristic; in the presence of diffusion interlayers, most AE signals recorded during the period of acoustic calm are correlated short signals with burst form and broadband spectrum (Figure 10). The presented result agrees with [37], in which it was shown that the appearance of a defect is accompanied by a shift of the AE signal spectrum towards low frequencies compared to the spectra of signals corresponding to the plastic-deformation process. A more obvious diagnostic result can be obtained using the scatterplot between the amplitudes and median frequencies of the AE signals (Figure 11).