Tube expansion and diffusion bonding of 316L stainless steel tube-to-tube sheet joints using a commercial roller tube expander

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 to support tube-to-tube sheet joints of large coil-wound heat exchangers (CWHE) made from 316L stainless steel. In contrast to traditional diffusion bonding where pressure is applied constantly during heat treatment, pressure is applied here initially prior to heat treatment using a commercial three roller tube expander. Processing parameters of this novel approach are presented in this study. Mechanical pull-out tests were conducted to evaluate if the manufactured joints are sufficiently bond to guarantee safe operation of the device. It was found that diffusion bonding followed tube expansion increases the pull-out force nearly threefold from a mean of 3.4 kN to 9.2 kN at which level the tube ruptures so that higher loads could not be measured. Tube-to-tube sheet joints fabricated using tube expansion and diffusion bonding met the engineering requirement, tube rupture against mechanical pull-out, set for this study.