Diffusion welding of compact heat exchangers for nuclear applications

Diffusion welding is a useful technique for assembling plate-type heat exchangers. The present work examines the diffusion welding characteristics of alloys of interest in advanced energy systems, including Alloy 800H (∼45Fe-35Ni-20Cr), Alloy 617 (∼55Ni-22Cr-12Co-9Mo-2Fe), Alloy N (∼71Ni-17Mo-7Cr-5Fe), and Alloy 242 (∼67Ni-25Mo-8Cr). A diffusion welding procedure was developed for the Gleeble thermomechanical testing machine, which allowed the critical welding parameters for diffusion welding to be explored efficiently. Stacks composed of many sheets were also welded in a vacuum hot press. Composition profiles across the finished joints were measured with SEM X-ray spectroscopy, and compared with models developed with Thermocalc/DICTRA to evaluate such modeling as a tool for rapid parameter development, and agreement was generally good. Appropriate surface treatments were found to be critical to achieving good mechanical properties, which were typically >90% of base metal properties. Diffusion welds in these alloys led to fully integrated grain structures across the previous joint. In addition, slight differences in diffusion with crystallographic direction were noted.