A new technique for intergranular crack formation on alloy 600 steam generator tubing

For the integrity and life management of nuclear steam generators, the detectability and reliability of eddy current test (ECT) are of prime importance. ECT qualification requires a number of cracked mockup specimens, having physical and micro-structural characteristics of intergranular cracks in the field component. Manufacturing of tube specimens with long-and-shallow intergranular cracks that represent field cracks in SG tubes is involved. While it is desired, techniques to control the crack shape during the specimen manufacturing was not established. In our new technology, a radial dent loading was applied on a sensitized Alloy 600 tube, in corrosive environment at room temperature, to introduce axial intergranular cracks with high aspect ratios, in good agreement with prediction by finite element analysis, as shown below. By experiment and hardware development, the radial denting method is shown to be suitable for the production of cracks with controlled aspect ratio. The aspect ratio can be controlled by combined application of this method and traditional internal pressurization method. In order to reduce amount of tube materials and time, direct current potential drop (DCPD) method was installed to monitor crack size and shape online so that the loading can be stopped at desired point. A set of miniaturized array probes was developed for application to the production of ID crack and/or OD crack. Tube specimens that are cracked at room temperature are then exposed in high temperature water environment in order to produce representative oxide film on all surfaces including crack plank. This is important as electromagnetic ECT response of cracks can vary with different oxide chemistry and thickness that in turn depend on water chemistry. PWR primary or secondary water chemistry with controlled ECP and pH is applied in a refreshed autoclave system