## 1 Introduction  
Modern trends in improving industrial structures' strength lead to increased requirements for the applied structural materials and the quality of their welded joints. Weld structures made of dissimilar materials are used in cases where operating conditions (temperature, medium, mechanical load level) vary for different sections of the structure. Welding of dissimilar metals has a few features associated with the different chemical composition and mechanical properties of the materials being welded. During welding, subsequent heat treatment or long-time operation of dissimilar welded joints at high temperatures, diffusion processes occur in the fusion zones of the joint, due to which the formation of diffusion interlayers is often observed along the fusion lines [1]. Diffusion interlayers can be considered as welded joint microstructure defects, as their formation can cause premature failure of products under high-temperature operating conditions [2]. Acoustic emission (AE) is a nondestructive testing (NDT) method that makes it possible to detect defects in structures using elastic stress relaxation waves. Due to its sensitivity to changes in the microstructure parameters of the materials being tested, the AE method,unlike other NDT methods (such as X-ray diffraction [3], eddy-current testing [4], ultrasonic testing [5; 6]), allows detecting not only continuity defects but also microstructure defects. In previous research, the authors demonstrated the possibility of detecting diffusion interlayers in welded joints of pearlit to austenitic steel using the AE method under both static and cyclic loading [7]. Unlike cracks, which are characterized by a variety of shapes and an uneven nature of propagation, diffusion interlayers have a regular microstructure and are characterized by predictable parameters. For example, under cyclic loading, diffusion interlayers generate AE hits with a certain constant activity level, and AE hit amplitudes correspond to a power law [8]. This paper examines the practical application of the previously obtained results, namely, the detection of defects in dissimilar welded joints in field conditions against the technological interference background. The method for detecting microstructure defects proposed in this paper differs from known ones in that it attempts to provide more reliable and universal solutions for detecting diffusion interlayers in dissimilar welded joints of pipelines. The disadvantage of the AE method is the large number of factors affecting the diagnostic results. The diagnostic data depends on the AE source function, the waveguide transfer function, and the measuring system transfer function [9; 10]. In the case of oblong industrial structures testing, it is difficult to assess the influence of the AE source function and the acoustic waveguide parameters on the diagnostic data, so, as a rule, the defect detection criteria are formulated empirically by identifying differences in the AE data features obtained by comparison of defective and defect-free structural elements [11; 12; 13]. When using the empirical method of diagnostic criteria, it is difficult to assess the reliability and accuracy of defect detection [14; 15], and due to the close connection of the obtained defect detection method with the specific characteristics of the testing structure, it is impossible to extend such a solution to similar structure with other geometric parameters or with other operating conditions. However, in the problem under consideration, the AE data parameters predictability during the diffusion interlayer deformation and the certainty of its location (on the fusion line of the welded joint) reduce the AE data variability and uncertainty, which is a favorable factor that facilitates the task of AE monitoring system constructing.