Objective
Bi-metallic welds (BMWs) are a necessity within Pressurised Water Reactor (PWR) and Boiling Water Reactor (BWR) designs where stainless steel primary piping systems are connected to the various low alloy steel nozzles of the Reactor Pressure Vessel (RPV), Steam Generators (SG) and Pressuriser. One of the important BMWs is that of the so called Safe-End which provides the nozzle to piping transition with an attachment area of the same or similar metal as the piping. This procedure avoids the welding of dissimilar metals outside of the fabrication shop, thus eliminating the need for local post-weld heat treatment. (PWHT).
Recent international surveys have identified a prevalence of outer diameter (OD) cracking problems associated with BMWs, arising from either fabrication problems or surface corrosion induced mechanisms. At the same time, literature cases, dealing with analytical and/or experimental aspects of such conditions are rare. This results from the practical solution retained in most cases, of grinding out these cracks. Such procedures may, however, result in other problems, in that the repeated grinding of recumng indications may become life limiting in itself if minimum wall thickness requirements are violated. The problem of the integrity of cracked BMWs remains, therefore, an acute one. THE OBJECTIVE of the proposed project is to realise a signihcant contribution to the verification and development of analysis methods for the structural integrity assessment of cracked bi-metallic welds (BMWs).
The complexity of the problem results from the prevailing mixed-mode loading conditions, the variation in material constitutive equations across the weld zone, and the presence of a large residual stress field. Under these circumstances, the classical fracture mechanics concepts are difficult to apply.
The project is formed around the performance of two benchmark 4-point bend pipe tests, conducted on nominal 6" piping assemblies, each containing a femtic to stainless steel (A50#308/309ss-316ss) BMW. The weld will be notched and pre-cracked at the femtic / buttering layer, interface, to simulate piant experiences of the cracking behaviour of such components. A full range of analysis methodologies, including conventional fracture mechanics assessment methods, J approaches and "Local Approach" methods will be appiied to the detemmination of the critical load for initiation of the defect, the extent of crack growth and path followed by the crack through the weld. Particular consideration will be taken of residual stress fields, mixed-mode loading, and constraint effects. It is envisaged that, not only will this test provide a benchmark for comparison with results of these analyses, but through the provision of a full nuclear specification for the weid manufacture, the test result will also be of direct technological relevance to several of the project partners and the nuclear industry in general.
The project is designed as the fore-nunner to a complex full scale 14" BMW nozzle experiment, and Round-Robin assessment and NDE activity to be launched within the European NetNork for tihe Evaluation of Steel Components (NESCJ in 1997. Such a project will require considerable pretest evaluation, and this project proposal has been designed to provide an insight into the solution of some of the envisaged problems.
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Coordinator
77250 Moret-sur-Loing
France
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