Abstract:
SA-508 Cl-3 being High Strength Low Alloy (HSLA) steels are approved recommended materials for Nuclear Power Plant main components including Reactor Pressure Vessel (RPV), Steam Generator (SG), Reactor Coolant Pump (RCP) and Pressurizer(PRZ) due to good mechanical properties. SS-316 materials are also used commonly for main reactor coolant pipingalong with other applications due to high corrosion resistance. Welding of these two different materials is very critical due to the severe service conditions during the operation of NPP involving high temperature, pressure, corrosion, ductile to brittle transition etc. Welding of these two dissimilar metals can be carried out with simple and economical welding process such as shielded metal arc welding (SMAW) by taking proper precautionary measures and same were applied to join SA-508 Cl. 3 material and SS-316L by using ASME standard electrode SFA-5.11 ENiCrFe-3. Preparation of bevels for plates and extraction of test samples were carried out as per reference ASME Sec IX & ASTM-E-399 standard. After preparing the welding samples they were examined chemically, mechanically and microscopically to check the soundness and quality of weld. Fabrication and characterization of weld was carried out in order to assess the fracture behavior of dissimilar metal weld. Non-Destructive Testing (NDT) showed absence of any defects/ flaws in the weld. Fracture toughness test showed that weld metal has Fracture toughness Higher than SA-508 and lower than SS-316L Base Metals. Stereomicroscopy and Scanning Electron Microscopy (SEM) proved that fracture was ductile due to coalescence of micro voids. X-Ray Diffraction (XRD) and X-Ray Energy Dispersive Spectroscopy (EDX) results showed that there were no compound or second phases present in the weld. EDX Composition Line Profile showed diffusion of Fe at weld-base metal interface while other elements (Ni, Cr) showed sharp interfaces. Micro hardness results showed increased hardness in weld area. The weld has fulfilled all the Qualification requirements of International standards for construction of Nuclear Power Plants (ASME & RCCM). The fracture toughness is in the acceptable range but on lower side. This needs to be improved in future.
