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 changes of microstructure and hardness near the joining interface of SUS304 steel, were investigated. Thermal stress induced in the joint was estimated with a finite element method. On the basis of these results the influence of thermal stress on the bending strength of the joint was examined.Good solid state bonding becomes feasible when the joining temperature exceeds 923 K. Axisymmetric thermoelastic finite element analysis suggests that maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of the graphite, increasing with increasing joining temperature. The position of fracture in a bending test corresponds approximately to that of the maximum tensile thermal stress. A part of the thermal stress is released with the deformation of SUS304 steel and graphite. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of thermal stress depends 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, that is, the changes in the amounts of Cr23C6 precipitated in SUS304 steel and point defect introduced in graphite.