Solid state bonding of graphite to SUS304 steel

Graphite was bonded to SUS304 steel under a compressive stress of 25 MPa in a vacuum over a temperature range from 873 to 1023 K using a RF-induction furnace. The bending strength of the graphite/SUS304 steel joint, and chang es of microstructure and hardness near the joining interface of SUS304 stee l, were investigated. Thermal stress induced in the joint was estimated wit h a finite element method. On the basis of these results the influence of t hermal stress an the bending strength of the joint was examined. Good solid state bonding becomes feasible when the joining temperature exce eds 923 K. Axisymmetric thermoelastic finite element analysis suggests that maximum tensile thermal stress is induced at a distance of 0.64 mm from th e joining interface on the surface of the graphite, increasing with increas ing joining temperature. The position of fracture in a bending test corresp onds approximately to that of the maximum tensile thermal stress. A part of the thermal stress is released with the deformation of SUS304 steel and gr aphite. The bending strength of the joint increases with decreasing residua l tensile stress on the surface of graphite. Relaxation of thermal stress d epends on the amount of carbon atoms which diffuse into SUS304 steel. This may be related to the changes in plasticity of SUS304 steel and graphite, t hat is, the changes in the amounts of Cr23C6 precipitated in SUS304 steel a nd point defect introduced in graphite.