Development of Engineering Parameters for Low Pressure Diffusion Bonds of 316 SS Tube-to-Tube Sheet Joints for FHR Heat Exchangers

Diffusion bonding is a solid-state welding technique to join metallic and non-metallic materials. Due to geometrical considerations, fabrication and possible materials choices diffusion bonding was chosen here for tube-to-tube sheet joints of large coil wound heat exchangers for Fluoride salt cooled High temperature Reactors (FHRs). In this work the processing parameters for these critical nuclear component manufactured out of 316l stainless steel are presented and the bonded areas are investigated using optical microscopy (OM) and scanning electron microscopy (SEM). In addition mechanical tests were conducted (pull out testing) to evaluate if these joints are sufficiently bound to guarantee a safe operation of the device. The detailed joining parameters are reported and recommendations for future fabrications with the physical restrictions of a large heat exchanger are made.