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 graghite/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 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 t ensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of the graphite and is increased with increasing j oining temperature. The position of fracture in a bending test specimen cor responds approximately to that of the maximum tensile thermal stress. A par t of the thermal stress is released with the deformation of SUS304 steel an d graphite. The bending strength of the joint increases with decreasing res idual tensile stress on the surface of graphite. Relaxation of thermal stre ss depends on the amount of carbon atoms which diffuse into SUS304 steel, T his may be related to the changes in plasticity of SUS304 steel and graphit e, that is, the changes in the amounts of Cr23C6 precipitated in SUS304 ste el and point defect introduced in graphite.