Solid State Bonding of Graphite to S45C Steel

Graphite was bonded to S45C steel by using RF-induction furnace, and bending strength, microstructure and carbon distribution of graphite/S45C steel joint were investigated. Thermal stress produced in the joint was estimated with an X-ray stress analysis and a finite element method. On the basis of these results the influence of thermal stress on bending strength of the joint was examined in connection with the diffusion of carbon atoms.Austenite phase contributes greatly to solid-state bonding of graphite/steel system. Therefore good diffusion bonding becomes feasible when joining temperature exceeds eutectoid transformation temperature A1. Axisymmetric thermoelastic finite element analysis suggests that the increase in carbon content near the interface of S45C steel has no influence on maximum tensile stress induced on the surface of graphite. Bending strength of the joint increases with decreasing residual stress on the surface of graphite. Relaxation of thermal stress is increased to a maximum when the amount of carbon atoms which diffuse into S45C steel comes to an optimum value. Lattice defects introduced with the Kirkendall effect seem to be pertinent to stress relief.