抄録
Fatigue strength of S.G. cast iron is considered to be influenced by matrix structure and graphite. The influence of pearlite content in the matrix structure was elucidated in the previous report and therefore the influence of graphite size is examined in this report. The chemical composition of the S.G. cast iron in this experiment was : C : 3.60%, Si : 2.75%, Mn : 0.24%, P··0.012%, Cr : 0.02%, Mg : 0.040%. Various graphite size was obtained by changing the solidification rate of the casting block. The fatigue strength and other mechanical properties are shown on table 1.From the results of the experiment the author abtained the following conclusions. 1) The tensile strength is hardly affected by graphite size. 2) The hardness is greatly affected by graphite size and the minimum value is found where the graphite diameter is 70μ for the ferrite matrix and 40μ for the pearlite matrix. 3) The elongation and impact value measured at room temperature increased with the increase of graphite size in the ferrite matrix but no change was observed in the pearlite matrix. 4) Rotation bending fatigue limit was obtained for fatigue strength. Two kinds of specimens were used: the unnotched specimen (with a diameter of 8mm) and the notched specimen (with a noch radius of 1mm). The fatigue strength decreased in proportion to the increase of graphite size, but became constant where the graphite diameter was over 40μ in the case of the unnotched specimen regardless of the matrix structure. This can be explained by regarding the graphites on the surface as notches. However, because the grain size in matrix structure changed in proportion to graphite size the influence of grain size must be also taken into consideration.
