The technology of thermal welding of the circulation piping of NPPS containing the influence of ultrasound

Abstract

The authors propose a thermal welding technology with ultrasound treatment for NPP circulation pipe-lines. This technology can significantly increase the strength of welded connections by reducing residual stresses, grain size and welded joint degassing. Exposure to ultrasound increases the welding speed and reduces the current consumption, thus resulting in energy saving. The paper presents the results of theoretical and experimental studies of ultrasonic effects on the welded joints and heat-affected zone.As is known, bearing capacity of welded joints is significantly lower than that of the base metal. This is due to the emergence of internal and residual stresses in the process of welding which are added to operating stresses, thus resulting in the destruction of the weld joint metal.Currently, it is common practice to reduce residual stresses in welded connections of NPP circulating pipelines and equipment by means of the thermal tempering and deformation methods.Thermal and deformation methods may reduce residual stresses in the HAZ but do not eliminate structural instability and physical and chemical heterogeneity, resulting in internal stresses and microcracks in the weld metal.Specialists of the Obninsk Institute for nuclear power engineering have developed a technology for thermal welding with ultrasonic treatment during the welding process, as a result of which the metal structure becomes fine-grained and homogeneous. Internal stresses are excluded and residual stresses are relieved in the heat-affected zone.The role of individual factors of ultrasonic field in creating certain structural changes in the metal depends on the crystallization conditions. In different areas of the crystallizing melt, the effect of any of the ultrasonic field factors may dominate. For example, the dispersion of crystals may occur in the two-phase zone, and the acoustic streams and stirring may be only in the liquid phase. If the reduction of grain size and elimination of the columnar structure are due to the ultrasonic dispersion, the change in phase distribution and the dendritic process of elimination are mainly determined by changes in the temperature gradient in the melt and stirring. The reasons for dispersion are cavitation, viscous friction forces, oscillatory and radiation pressure. An increase in the rate of nucleation of crystallization centers is also associated with these parameters.