Pulsating tension fatigue tests (R=0.1) and statical tension tests at room temperature to 500°C were performed on the as-cast and heat-treated materials of spheroidal graphite cast iron. The macro and microscopic observations of the fatigue-fracture surface of the specimens were carried out by SEM to interpret fatigue strength data.
The results obtained are summarized as follows:
(1) The tensile strength at elevated-temperatures decreased with the elevation of testing temperature. However, the effect of normalizing and austempering heat-treatment was remarkably noticed.
(2) The fatigue strength decreased with the elevation of testing temperature, which is the same as the tensile strength, but the effect of heat-treatment was noticed very slightly. The fatigue limit at 500°C for the bainitized material was σu=92MPa and for the annealed material σu=77MPa.
(3) Fatigue cracks appeared from internal defects such as small pinholes. Therefore, it is considered that the heat-treatment does not affect the fatigue strength so much.