## 1 Introduction  
Dissimilar welded joints of austenitic to pearlicite steel are widely used in the manufacturing of elements for power equipment and pipelines at thermal power plants and nuclear power plants, in steam and gas turbines, and in pipelines at chemical and petrochemical plants. The use of such structures allows one to reduce significantly the cost of products, reduce their weight and sizes, and increase the reliability and durability of critical equipment. The production of dissimilar welded joints by fusion welding has a specific feature--when two or more materials are mixed in a liquid state in the weld pool, it leads inevitably to the presence of zones with highly gradient chemical composition in the welded joint. These zones are usually located near the fusion lines, and their width is determined by the welding method, as well as by the complex of physical processes at the interface between the solid and liquid phases [1]. In particular, in welded joints of dissimilar steels on different sides of the fusion line, decharburized interlayers and interlayers with increased carbide content may be observed [2, 3]. Decarburized and carbide interlayers are typical for dissimilar welded joints of low-alloy pearlicite steels to chromium-nickel austenitic steels [4, 5, 6]. For such welded joints having an austenitic structure in the weld seam, interlayers are formed on the fusion line of the pearlitic steel and the weld seam (a decarburized interlayer on the pearlitic steel side and a carbide interlayer on the weld side (see the scheme in Figure 1)). If such dissimilar welded joints are operated for a long time at high temperatures, the diffusion processes will proceed intensively, and as a result the interlayers will increase in size. The presence of such interlayers negatively affects the mechanical properties of the welded joint [7]. The decarburized interlayer, the size of which can reach several millimeters, has a ferritic structure and has lower strength characteristics, namely ultimate strength and yield strength. The presence of a hard but brittle carbide layer increases the risk of sudden fracture of the welded joint. This problem is especially relevant for dissimilar welded joints of power equipment at nuclear power plants. A large number of sudden destruction of combined steel joints during operation has led to the fact that in the regulatory document for the metal of power equipment (PNAE G-7-009-89L Instructions and standards in nuclear power engineering. Equipment and pipelines of atomic power systems. Welding and hardfacing. Main instructions), restrictions on the operation temperature and time for such joints were prescribed to prevent the growth of diffusion interlayers. The suddenness of such welded joints destructions caused by the difficulty of diffusion interlayers detection. To identify interlayers in dissimilar welded joints, non-destructive testing methods can be effective, which allow to investigate the weld joint structure throughout the entire thickness of the product; these are radiographic and ultrasonic testing methods. However, radiographic testing is sensitive to the defect orientation and does not provide the required sensitivity to identify the interlayer [8]. The most widespread method of non-destructive testing of dissimilar welded joints is ultrasonic testing. In a number of papers the effective use of ultrasonic testing of dissimilar welded joints in power engineering equipment is shown [9; 10; 11], but it is noted that inspection of dissimilar welded joints, as a rule, is carried out due to the complication of the inspection procedure and the development of specialized methods that take into account the design features of each welded joint [12; 13; 14]. Moreover, when using nondestructive testing methods, diffusion interlayers create structural noises that interfere detection of defects such as cracks, pores, or lack of penetration, which makes dissimilar welded joints a difficult object for diagnostics.