Impact characteristics of diffusion bonded joints of spheroidal graphite cast irons are affected by the graphite particles precipitated on the bond interface during ferritizing. This process is performed to improve the bond impact characteristicsafter bonding. To improve the toughness of the joints, it is necessary to decrease the graphites. Generally speaking, the graphites precipitate on bond interface, ferrite grain boundaries and nodular graphites in the base metal during ferritizing.Therefore, it is considered that the graphites precipitated during ferritizing are decreased by increasing of the nodular graphites in the base metal. So, diffusion bondings of spheroidal graphite cast irons having four grades of nodule count wereperformed. Effects of the nodule count of the cast irons and an insert metal on the bond impact characteristics have been studied. Main results obtained are as follows; (1) When the cast irons had a nodule count of 180/mm{sup}2 and were bonded without aninsert metal at 820℃ for 1.8 ks, the ratio of the absorbed 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/mm{sup}2 or 313/mm{sup}2 and werebonded at 900℃ for 1.8 ks, the absorbed energy ratios were more than 57% and these were nearly the ratios of the absorbed energy specified in JIS to that of the base metals. (2) When the cast irons had a nodule count more than 180/mm{sup}2 and werebonded 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/mm{sup}2 or 313/mm{sup}2 and were bonded at 820℃, their absorbed energy ratios increased with an increase of the thickness of Ni foil as an insert metal and that was reached to 100% when 50-μm thick Ni foil was used as an insert metal. (4) The area of graphite particles that precipitated on the bond interface during ferritizing decreasedwith 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 by decomposition of pearlite during ferritizing, the more the nodule count ofthe cast irons, the less the graphites.
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