Weld Overlay Examination Using Ultrasonic Phased Array Technology

Weld overlay of piping was first used as a method to repair components of simple configurations which were known to contain cracking. An adequate ultrasonic examination of these geometries can be achieved by using a few conventional search units. Currently the application of pre-emptive weld overlays as a mitigation technique and/or to improve the examination surface condition for more complex configurations is becoming more common. These complex geometries may require several focused conventional transducers for adequate inspection of the overlay, the original weld, and the base material. Alternatively ultrasonic phased array probes can be used to simultaneously generate several inspection angles at various focal depths to provide better and faster coverage when compare to conventional methods. This technical update describes the qualification and use of ultrasonic phased array techniques for weld overlay examinations of piping. WELD OVERLAY BACKGROUND INFORMATION Weld overlay was first used in US power plants in the early 1980s as an interim method of repairing the welds of flawed piping joints. However due to EPRI research and development of effective ultrasonic (UT) techniques for inspection and corresponding requirements for conditioning the examination surface, the application of weld overlays was regarded as a long-term solution starting in the mid 1980s. This repair method involves the application of crack-resistant material on top of the original weld and base material. This process is designed to increase the load-carrying capability of the cracked pipe and to cause a compressive stress on the inside surface of the pipe to slow or inhibit crack initiation and propagation. After the new weld material is applied, the overlay is often machined smooth to allow for ultrasonic inspection. A pre-service inspection (PSI) is performed to detect fabrication flaws in the weld overlay before the repaired joint is returned to service. The original weld heat-affected zones (HAZ) at a depth of 25% of the original pipe thickness below the pipe-to-weld overlay interface is also examined ultrasonically both prior to returning the component to service and during service to detect and size flaws that may have propagated into the weld overlay. When the crack is considered to be through-wall only over its length a full structural weld overlay is engineered and applied which takes into account the strength of both the weld overlay and the unflawed portion of the pipe [1]. The examination volume for the pre-service and in-service inspection (ISI) for a full structural weld overlay can be found in figures 1 and 2 respectively. The application of a weld overlay typically increases the inspection interval of the pipe joint. In addition, there have been no reported cases of existing flaws propagating into the overlay material [2]. Figure 1 Full Structural Pre-service Inspection Examination Volume For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=70 6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components October 2007, Budapest, Hungary