Residual Stresses in Austenitic Cladding

Reactor pressure vessels (RPV) are usually manufactured with austenitic cladding on their inner surface as a protection against corrosion from the primary circuit water environment. Thus, they are not included into the strength calculations of pressure vessels due to their lower strength properties and much smaller thickness in comparison with those of vessels as they are taken only as a corrosion layer. In the same time, due to different thermal coefficients and Young moduli, welding of austenitic cladding results in a high residual stresses in the cladding and also in the adjacent area in the base ferritic metal. These residual stresses as well as stresses resulted from the temperature field in the vessels represent necessary inputs into pressurized thermal shock calculations. WWER (Water-Water Energy Reactor = PWR type) reactor pressure vessels have relatively thick cladding — nominally 8 mm — made from two layers: first layer of 25/10 type welded by one pass while the second layer of 18/10/Ti typed is usually welded by three passes. The main part of the vessels was performed by strip welding with strips of 60 mm wide. Results of residual stresses measurements are given in the paper. Method with incremental milling of beams was used for the measurements and determination of residual stresses. Tests were performed on specimens in as-welded state and also after final heat treatment of the vessels, i.e. after several stress relieves including first hydrotest in shop. As residual stresses depends strongly also on direction of welding, beams were oriented in both directions — parallel and perpendicular to the welding direction. Results of these measurements are shown and discussed in the paper.