Impact characteristics of diffusion bonded jointsof spheroidal graphite cast irons are affected by the graphiteparticles precipitated on the bond interface during ferritizing.This process is performed to improve the bond impactcharacteristics after bonding. To improve the toughness of thejoints, it is necessary to decrease the graphites. Generallyspeaking, the graphites precipitate on bond interface, ferritegrain boundaries and nodular graphites in the base metal duringferritizing. Therefore, it is considered that the graphitesprecipitated during ferritizing are decreased by increasing of thenodular graphites in the base metal. So, diffusion bondings ofspheroidal graphite cast irons having four grades of nodulecount were performed. Effects of the nodule count of the castirons and an insert metal on the bond impact characteristics havebeen studied. Main results obtained are as follows; (1) When thecast irons had a nodule count of 180/mm2 and were bondedwithout an insert metal at 820℃ for 1.8 ks, the ratio of theabsorbed energy of the joints to that of the base metals(absorbed energy ratio) was less than 30. On the other hand,when the cast irons had a nodule count of 249/mm2 or 313/mm2and were bonded at 900℃ for 1.8 ks, the absorbed energy ratioswere more than 57 and these were nearly the ratios of theabsorbed energy specified in JIS to that of the base metals. (2)When the cast irons had a nodule count more than 180/mm2 andwere bonded with an insert metal of 10-μm thick Ni foil at 900℃ and 950℃, their absorbed energy ratios were about 100. (3)When the cast irons had a nodule count of 64/mm2 or 313/mm2and were bonded at 820℃, their absorbed energy ratiosincreased with an increase of the thickness of Ni foil as an insertmetal and that was reached to 100 when 50-μm thick Ni foilwas used as an insert metal. (4) The area of graphite particlesthat precipitated on the bond interface during ferritizingdecreased with an increase of the nodule count of the cast irons.This phenomenon became clear after using a simple model.Since graphite particles on the bond interface are precipitated bydecomposition of pearlite during ferritizing, the more the nodulecount of the cast irons, the less the graphites.
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