Fusion weldings of cast irons are very difficult to cause the cold cracking in the weld metal and heat affected zone. So, solid state bonding of ductile cast irons having the strength of 360, 390, 490 and 690 MPa and the ductile cast irons to a mild steel have been investigated using the diffusion welding technique at 730 to 980°C under restraint of test pieces or constant bonding pressures. The results obtained are as follows; (1) When the bonding of the cast irons were performed without a insert metal under the bonding temperature of higher than 820°C and for the bonding time of 1.8 ks, both graphites and ferrites precipitated at the bonding interface according to the transformation of γ→α+Gr on the cooling process. However, the fracture in the tensile test of the joints occurred in the base metal, and also in case of the bondings of the ductile cast irons using a insert metal of Ni foil of 10μm thickness, Cu foil of 10μm thickness or Fe foil of 100μm thickness the fracture in the base metal. Welding deformation of the joints was less than 1% in the diameter of the test piece. (2) The tensile fracture of the ductile cast irons bonded with the Ni foil at 820°C and for the bonding time of more than 0.24 ks under constraint of the test pieces or the bonding pressure of more than 0.8 MPa for 1.8 ks occurred in the base metal. (3) When the ductile cast irons and a mild steel were bonded by the diffusion welding using the Ni foil as the insert metal, the tensile fracture of the joint occurred in the base metal of the mild steel. However, in the bondings of the ductile cast irons and the mild steel with the Cu or Fe foil as the insert metal and without insert metal, the tensile fracture occurred at the bonding interface between the ductile cast iron and the insert metal. The tensile sterngths were about 400 MPa in all joints. (4) The phenomenon of the fracture at the bonding interface in the joints of the cast iron and the mild steel is considered that radial strain abruptly increases on the tensile axis because the hardness became discontinuity and the ductility of the cast irons is much smaller than that of the mild steel.
