Abstract
The authors have studied the tensile strength, the fatigue strength and the impact characteristics of the joints of the ferritic spheroidal graphite cast iron produced by the solid state bonding method and found that the mechanical characteristics of the joints were comparable to those of the base metal. With respect to practical uses, it is considered that the brazing method with a brazing sheet is easier than the solid state bonding for bonding of the cast iron. In this study, the ferritic spheroidal graphite cast irons were bonded with Ni-base brazing sheets using the high frequency induction heating apparatus. Those sheets were MBF 15 (Ni-Cr-Si-B system), MBF 30 (Ni-Si-B system) and MBF 60 (Ni-P system) in the thickness of 40μm. Furthermore, effects of bonding time, surface roughness of the surfaces to be bonded, and bonding atmosphere on the tensile strength of the joints were investigated. Main results obtained are as follows: (1) When the cast irons were bonded for 2 and 10 minutes with MBF 15 and MBF 60, the joints were fractured at the bonded zone on the tensile test. When the cast irons were bonded for 30 minutes, however, the tensile strength of the joints was nearly equal to that of the base metal. On the other hand, when the cast irons were bonded for 2, 10 and 30 minutes with MBF 30, all of the joints were fractured in the base metal on the tensile test. (2) When the surface roughness was 2.3μm, the tensile strength of the joints bonded with MBF 15 was nearly equal to that of the base metal. When the surface roughness was 13.9μm, however, all of the joints were tensile-fractured at the bonded zone because small shrinkages were formed at the center of the bonded zone. (3) The tensile strength of the joints bonded in argon was a little lower than that of the joints bonded in vacuum with MBF 15 and MBF 60. However, the strength of the joints bonded in air was decreased considerably because the oxides of Cr were produced in the center of the melting zone. On the other hand, when the joints were bonded with MBF 30 in air, in argon, and in vacuum, all of the joints were tensile-fractured in the base metal. (4) In the case of MBF 15 containing Cr, the high density zone of Cr was formed at the center of the melted brazing metal and Cr2O3 particles were produced by oxygen in air. By the final stage of the bonding, Si was accumulated near the Cr2O3 particles, and they were reduced by the Si, and the SiO2 particles were formed instead of the Cr2O3 particles. (5) When the brazing sheets having 5 levels of the Cr content were used for the bonding, the tensile strength of the joints was decreased with an increase of the Cr content.
