No abstract

No abstract

No abstract

The high-temperature heat treatment of the welded joints of Du300 austenitic pipelines on straight sections and lead-aways, welded joints of Du300 pipelines to pipe connections for equipment and fittings and welded joints in the zones of construction walkways is described. It was implemented in the welded joints in a real Du300 pipeline and full-scale mockup of a Du300 pipeline and the pipe connections of the intake manifold in an operating power-generation unit of the Leningradskaya NPP with an RBMK-1000 reactor, which was shut down for midlife repairs. The technology suppresses the sensitization of the metal in the near-joint zone to below the proneness toward intercrystallite corrosion cracking under stress (coefficient of potentiodynamic reactivation below 1%) and decreases the residual stresses by 70-80%. Cracking of Welded Joints in Austenitic Pipelines.In the investigation of intercrystallite corrosion cracking of stressed welded joints in Du300 austenitic pipelines in NPPs with RBMK-1000 reactors, it was assumed that defects develop as a result of the presence of tensile (residual weld) stresses, whose magnitude could reach the yield point of the material, and erection (bending) stresses, possible sensitization of the metal in joint zone and the corrosive action of the coolant.The compensatory measures, consisting in compression of the welded joints, austenitization at 1050°C, high-temperature heat treatment at 900°C and repair welding, slow the development of discontinuity flaws occurring predominately in the metal in the joint zone along the fusion line [1,2]. The zone near the joint enters into a more extended zone affected by heat. It is characterized by microstructure with variation of the grain size, the presence of an interface between the cast and deformed metal, higher (compared with the main metal) stress levels and a common phenomenon, viz., inadequate resistance of the near-boundary sections of grains to electrochemical corrosion, which consists in a reduction of the austenite dissolved in the solid solution below 12% owing to the formation of chromium carbide of the type Cr 23 C 6 .Each compensatory technology has its own advantages and disadvantages. Heat treatment of the entire welded joint of Du300 pipelines is most effective in eliminating the residual stresses and sensitization of the metal in the zone around the joint. The welded joints in the bottoms of the distributing group headers were subjected to austenitization, for which they were heated to 1050°C, soaked for 1 h and cooled in air or under heat insulation. The main difference between the heat treatment proposed for welded joints in Du300 pipelines and that used previously for the bottoms of distributing group headers is heating to 900°C, soaking for 1 h and cooling in air. Since the heat treatments were done at different temperatures, the processes occurring during soaking are fundamentally different [2]. During austenitization, high-chromium carbides dissolve, and as a result sensitization is suppressed and the residual stresse...