MACHINE GTAW AMBIENT TEMPERATURE TEMPER BEAD TECHNIQUE FOR Nl-BASED WELD OVERLAY REPAIRS OF NUCLEAR COMPONENTS - ASSESSMENT OF HYDROGEN CRACKING BY IMPLANT TEST

摘要

Repair by welding of a metallic component may be required during original fabrication, following in-service inspection or during planned maintenance operations. In some instances a post-weld heat treatment (PWHT) is required. Codes/standards that cover the repair of or alterations on nuclear components, such as Section XI of the ASME Boiler & Pressure Vessel code or the French RSE-M nuclear code for in-service inspection, include provisions for waiving the need for a PWHT by using a welding technique referred to as 'temper bead', but in the case of in-service repairs, the practical challenges may make any heating cycles, including pre or post-heating, undesirable. So, starting from the existing temper bead procedures, repair solutions that do not require preheat, PWHT or post-heating, have been developed, particularly in the nuclear sector, referred to as 'ambient temperature temper bead' (ATTB). One of the most common applications of the ATTB technique is the deposition of full structural weld overlays (FSWOLs), made with Ni-based consumables on ferritic base materials, to repair piping components and nozzles susceptible to primary water stress corrosion cracking (PWSCC) and to mitigate the risk of further cracking. The ATTB technique is currently permitted for the repair of nuclear power plant components by the ASME code via Code Cases N-638-8 and N-740-2, provided the machine gas tungsten arc welding (GTAW) process is used, and it has been successfully applied on various occasions. However, the technique is currently not permitted by the RSE-M code. As the current rules in the RSE-M codes were established following experimental programs based on implant testing, the same type of tests would be expected by the French nuclear regulator to justify any changes. This paper presents the results of a research project carried out to explore the possibility of justifying the introduction of ATTB using the machine GTAW welding process to deposit austenitic weld overlays into the RSE-M code, based on implant testing. Externally loaded tests according to ISO 17642-3 (Implant test) were carried out to determine the effect of different preheat and stress levels on the cold cracking susceptibility in the parent metal. Test loads representative of actual residual stresses were determined through a review of published literature on the residual stresses associated with ATTB FSWOLs. Test pieces were obtained by depositing single beads with Inconel 52 filler metal (AWS A5.14 ERNiCrFe-7) on ASME SA 508 Grade 4N substrate. Conclusions are presented with regard to the possibility to waive preheat or to reduce the minimum preheat temperature specified for temper bead repairs, with respect to the current RSE-M code requirements. Recommendations for further work required to support a request for modification of the RSE-M code are provided.