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