Effect of Heating Rates on Microstructures in Brazing Joints of STS304 Compact Heat Exchanger using MBF 20

Effect of heating rate on microstructure of brazed joints with STS 304 Printed Circuit Heat Exchanger (PCHE), which was manufactured as large-scale(1170(L) × 520(W) × 100(T), mm), have been studied to compare bonding phenomenon. The specimens using MBF 20 was bonded at 1080°C for 1hr with 0.38°C/min and 20°C/min heating rate, respectively. In case of a heating rate of 20°C/min, overflow of filler metal was observed at the edge of a brazed joints showing the height of filler metal was decreased from 100μm to 68μm. At the center of the joints, CrB and high Ni contents of γ-Ni was existed. For the joints brazed at a heating rate of 0.38°C/min, the height of filler was decreased from 100μm to 86μm showing the overflow of filler was not appeared. At the center of the joints, only γ-Ni was detected gradating the Ni contents from center. This phenomenon was driven from a diffusion amount of Boron in filler metal. With a fast heating rate 20°C/min, diffusion amount of B was so small that liquid state of filler metal and base metal were reacted. But, for a slow heating rate 0.38°C/min, solid state of filler metal due to low diffusion amount of B reacted with base metal as a solid diffusion bonding. Key words: PCHE (Pring cicuit heat exchanger), Brazing, MBF 20, Boron diffusion, Microstructure