Welding and maintenance operations on aged austenitic-ferritic materials

Some primary system loop components (pipes, elbows, pump casings, valves) can be made from cast austenitic-ferritic stainless steel. These types of steel are liable to age at operating temperature, which leads, in particular, to reduced ductility. With regard to operations to replace primary system components (steam generators, elbows, pipe runs), the installation of new components on existing systems raises the problem of welding on aged materials and the subsequent strength of such welded joints under operating conditions. The various procedures implemented during replacement and maintenance work on these components can be perfected and demonstrated by testing either removed, cast austenitic-ferritic stainless steel components, representative of the severest aging conditions found in French nuclear power plants, or materials aged in the laboratory. Tests concerning the replacement of primary system components focus on two main issues: a) Machining aged materials and b) welding together new and aged materials. In addition, in situ samples can be taken of cast austenitic-ferritic stainless steel components in operation, as part of in-service aging monitoring programs. This type of sampling is intended to provide insight into the real inservice aging of the component. Several types of maintenance operations are carried out: - Taking in situ material samples from cast, austenitic-ferritic stainless steel components: - Taking cup-shaped samples using a mechanical tool. - Taking block samples using an electrical discharge machining (EDM) tool on parts remaining inside the structure. - Cutting up aged components for examination. - On-site welding operations: - Repairing casting defects on aged materials. - Welding new components to aged components. The various maintenance operations will be discussed, together with the examinations performed to demonstrate the harmlessness of welding onto aged material, and to determine the mechanical properties of welded joints. (authors)