THE TECHNOLOGY OF THERMAL WELDING OF THE CIRCULATION PIPING OF NPPS CONTAINING THE INFLUENCE OF ULTRASOUND
The proposed technology is applicable to thermal welding of the NPP circulation piping using ultrasonics. This technology makes it possible to considerably increase the weld strength by reducing the residual stresses, reduce the grain size and weld degassing. Ultrasonic treatment increases the rate of welding current reduction resulting in electricity saving. The results of theoretical and experimental studies reveal the ultrasonic effects on the weld bead and heat-affected zone (HAZ). It is known that the bearing capacity of welded joints is considerably lower than that of the base metal. This is due to the welding process, internal and residual stresses formed at operating voltages, which leads to the destruction of metal of welded joints. Currently, residual stresses in welded connections of circulating pipelines and NPP equipment are reduced by the thermal tempering and deformation methods. The thermal and deformation methods can reduce residual stresses in the HAZ but do not eliminate the structural instability and physical or chemical heterogeneity, resulting in the formation of internal stresses in the weld metal and microcracks. The specialists of the Obninsk Institute for Nuclear Power Engineering developed the technology of ultrasonic and thermal welding, in which the metal structure becomes fine-grained and homogeneous, internal stresses are eliminated and residual stresses within the HAZ are removed. The role of individual ultrasonic factors in the creation of certain structural changes in the metal depends on the crystallization conditions. The effects of any of the ultrasonic factors may dominate in different areas of the crystallizing melt. For example, the dispersion of crystals can occur in the mushy zone whereas acoustic flows and mixing can take place only in the liquid phase. If the grain size reduction and the columnar structure elimination are due to the ultrasonic dispersion, the phase distribution changes and the dendritic elimination process are determined mainly by the temperature gradient changes in the melt and stirring. The dispersion is caused by the cavitation, viscous friction forces as well as oscillatory and radiation pressure. The same parameters determine the increase in the nucleation rate of crystallization centers.