A New Technique for Intergranular Crack Formation in Alloy 600 Steam Generator Tubing

Abstract

For the integrity management of steam generator (SG) tubes, nondestructive evaluation performed using eddy current test (ECT) is necessary in the assessment. The reliability of ECT evaluation is dependent on the accuracy of ECT for various kinds of defects. For basic calibration and qualification of these techniques, cracked SG tube specimens having mechanical and microstructural characteristics of intergranular cracks in the field are needed. To produce libraries of laboratory-degraded SG tubes with intergranular cracks, a radial denting method was explored for generating inside diameter and outside diameter axial cracks by three-dimensional finite element analysis and experimental demonstration. The technique is proven to be applicable for generating axial cracks with long and shallow geometries as opposed to the semicircular cracks typically obtained by the internal-pressurization method. In addition, a direct current potential drop method with array probes was developed for accurate monitoring and controlling of crack size and shape. By these methods, long and shallow intergranular axial cracks more typical of actual degraded SG tubes were successfully produced.

Techniques for intergranular crack formation and assessment in alloy 600 base and alloy 182 weld metals

“…The reliability of the FEA simulation of DCPD monitoring was confirmed in a previous study on SG tubing materials [14]. The FEA simulation was performed on compact-tension specimens with various sizes of cracks, and the results were compared with Johnson's theoretical equation according to ASTM Method E 1737-96.…”

“…These methods have been demonstrated to generate mock-up cracks with high aspect ratios, i.e., crack length/depth ratios, like those typically obtained in NPP components. The loading schemes used in this study have the advantage of using online crack monitoring, which allows easier prediction of the location of stress concentration [14].…”

“…In order to characterize and calibrate these key techniques, a number of specimens that have intergranular cracks with mechanical, microstructural, and electrochemical characteristics similar to those found in the field are needed. Recently, an advanced technique was developed for the controlled production of simulated IGSCC mock-ups that possess the mechanical and microstructural features comparable to service-induced cracks in alloy 600 SG tubing, which usually has relatively thin walls [14]. In this study, the above technique is extended to relatively thick forged nickel alloy components, including the alloy 600 base and alloy 182 weld metals of PZR nozzles.…”