Abstract
Fatigue tests under two different axial loading conditions, which were completely reversed stress (R=-1) and pulsating tension stress (R=0) conditions, 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 and their fatigue strength was investigated. The results obtained are summarized as follows:
(1) The tensile strength of the heat-treated material varied largely from 455N/mm2 for the annealed specimen to 920N/mm2 for the bainitized specimen. However, the difference due to the amount of added Mn was found to be little.
(2) The fatigue strength of the as-cast material for R=0 was found to be much lower than, that for R=-1, and their fatigue limits were σw=172N/mm2 for R=-1 and σu=114N/mm2 for R=0, which is about 35% lower than the former.
(3) The fatigue limits of the annealed, normalized, and bainitized specimens for R=0 were in a comparatively narrow range of σu=110∼123N/mm2. No specific influence by heat treatment or by the amount of added Mn was recognized on the fatigue strength of these specimens.
(4) Fatigue cracks appeared from such internal defects as small pinholes. Therefore, it is considered that the heat treatment or the amount of added Mn would not affect the fatigue strength of these materials so much.
