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Electron beam welding of 6 kW class was applied to I-type butt welding of 18 mm thick as-cast plates of spheroidal graphite cast iron (FCD450 and FCD500) without preheating, using 0.5 mm plates of austenitic stainless steel (SUS304) and pure nickel as insert metals. Relations between mechanical properties of as-welded joints and porosity existence ratio or area ratio in fracture surfaces were examined. The mean hardness of the fusion zone with pure nickel insert metal was Hv250 to 350, being higher than that with SUS304, Hv140. The width of the hardened zone was larger with nickel insert metal than with SUS304. When graphite is present in the fusion zone, martensite is formed around its peripheries, causing a remarkable increase in hardness. The tensile strength of the welded joints was higher with SUS304 insert metal than with pure nickel, reflecting the difference in the amount of porosities. In the tensile tests of FCD500, the joint efficiency was 79 to 94%, joint strength increasing with an increase in percent elongation. Fracture in the impact tests occurred in the bond zones both with SUS304 insert metal (1, 2 pass welding) and pure nickel (1, 2 pass welding). Impact values exhibited a considerable scattering without regard to the existence of porosities. It was shown that the welded joints of FCD450 (SUS304 insert metal, 3 pass welding) have sufficient strength, because fracture in the fatigue tests occurred in the base metals.