| 초록 |
Effect of heating rate on microstructure of brazed joints with STS 304 Printed Circuit Heat Exchanger (PCHE),which was manufactured as large-scale( $1170(L){ times}520(W)){ times}100(T)$ , mm), have been studied to compare bonding phenomenon. The specimens using MBF 20 was bonded at $1080^{ circ}C$ for 1hr with $0.38^{ circ}C/min$ and $20^{ circ}C/min$ heating rate, respectively. In case of a heating rate of $20^{ circ}C/min$ , overflow of filler metal was observed at the edge of a brazed joints showing the height of filler metal was decreased from $100{ mu}m$ to $68{ mu}m$ . At the center of the joints, CrB and high Ni contents of ${ gamma}$ -Ni was existed. For the joints brazed at a heating rate of $0.38^{ circ}C/min$ , the height of filler was decreased from $100{ mu}m$ to $86{ mu}m$ showing the overflow of filler was not appeared. At the center of the joints, only ${ gamma}$ -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 $20^{ circ}C/min$ , diffusion amount of B was so small that liquid state of filler metal and base metal were reacted. But, for a slow heating rate $0.38^{ circ}C/min$ , solid state of filler metal due to low diffusion amount of B reacted with base metal as a solid diffusion bonding. |
