Abstract
Fatigue strengths of diffusion bonded joints of spheroidal graphite cast irons and the spheroidal graphite cast iron to a mild steel have been investigated using the rotating bending fatigue test machine. The results obtained are as follows; (1) When tensile strength of the base metal of the spheroidal graphite cast iron was 500 to 800 MPa, the fatigue limit of these joints was a nearly constant value of 240 to 250 MPa though that of the base metal increased as the tensile strength of the base metal increased. (2) Fa-tigue strength at 105 and 106 cycles in the spheroidal graphite cast iron joint which was annealed to be changed the structure from ferrite+pearlite to all ferrite was almost the same as that of the base metal of the ferritic spheroidal graphite cast iron. However, the fatigue limit of the joint was lower than that of the base metal by 20 MPa. (3) Fatigue strengths at 105 and 106 cycles in the joint of the cast iron to the mild steel and after ferritizing of the joint were almost the same as that of the base metal of the ferritic spheroidal graphite cast iron. (4) When the joint of spheroidal graphite cast iron having the tensile strength of about 500 MPa was heat-treated at 950°C for 21.6 ks, the fatigue limit was improved up to that of the base metal.
