# Assessment of the Structural State of Dissimilar Welded Joints by the Acoustic Emission Method
## 1 Introduction  
Heterogeneous welded joints are widely used in industry to optimize application and engineering requirements with economic considerations. For instance, dissimilar welding is frequently applied for the joining of pipelines at thermal and nuclear power plants, assemblies of steam and gas turbines, as well as technological pipelines at chemical and petrochemical enterprises. This technology can significantly reduce the material costs and the weight of the construction. However, the majority of dissimilar welded joints have complex microstructure due to the differences in mechanical, chemical and thermophysical properties of the welding materials. Moreover, there is a high level of residual stresses in such joints, which cannot be removed by post-veld heat treatment (PWHT). The complex structural and mechanical states of the welds interfere with the detection of defects and discontinuities when using conventional physical methods of control. As a result, dissimilar welded joints become a difficult object for diagnostics. Besides typical defects such as cracks, undercuts and lack of penetration, the formation of specific defects also can be observed. In most cases, they may be associated with an abrupt change in the structure between the weld metal and the heat-affected zone in dissimilar welded joints. When dissimilar joints are obtained by fusion welding, diffusion zones are formed near the fusion lines. If the welded joint is made from steels of different structural classes, then the process of carbon diffusion will take place. In this case, decarburized interlayers and layers with a high content of carbides (carbide interlayers) are often located in welded joints [1, 2]. If such dissimilar welded joints are operated for a long time at elevated temperatures, diffusion processes will proceed intensively throughout the entireservice life. This will lead to the constant increase in interlayer size. The presence of such large interlayers will negatively affect the mechanical properties of the welded joints [3; 4; 5]. As a rule, a decarburized interlayer, the size of which can reach several millimeters, has a ferritic structure and low strength characteristics--ultimate tensile strength (UTS) and yield strength (YS). The presence of a hard carbide layer commonly increases the risk of sudden failure due to the great brittleness. Therefore, when conducting a non-destructive test of a joint, it is important to identify both structural defects and also zones with a sharp gradient in structure/chemical composition, e.g., diffusion interlayers [6]. Conventional NDT methods do not provide required sensitivity when testing dissimilar welded joints due to pronounced heterogeneity and anisotropy of the physical properties. The most sensitive method is ultrasonic testing. The use of ultrasonic testing for dissimilar welded joints of reactor equipment is noted in several works [7; 8].Essentially, testing is carried out using special equipment and techniques that take into account the design features of each welded joint [9; 10]. However, the coarse-grained structure leads to a high attenuation of ultrasonic vibrations in the austenitic steels, and the complex structure of a dissimilar welded joint displays a high level of interference [11]. Even though ultrasonic testing provides a fairly reliable detection of cracks, pores, cavities and lack of penetration, its sensitivity is not enough to detect structural defects, such as carbide and decarburized interlayers. In this paper, we present information about the acoustic emission (AE) method application for diagnosing dissimilar welded joints of steels from different structural classes. The AE method is based on acoustic elastic wave generation when a material undergoes structural changes, for example, because of crack formation. The AE method is highly responsive to defect detection and can be applied to determine defect growth.