鋳肌を有する薄肉球状黒鉛鋳鉄の引張および疲労強度特性評価

抄録

  With increasing demands for the weight reduction of cast iron products in various industries, advanced processing techniques to make thin-wall ductile cast iron without chill structure were developed. This study investigated the tensile and fatigue properties of the thin-wall ductile cast iron and effects on the casting surface. Thin-wall ductile cast iron specimens of 2mm in thickness were prepared. The tensile test and fatigue test were carried out using test pieces with machined and casting surfaces. The tensile strength of the test pieces with casting surface was smaller than that of the machined test pieces. The measured thickness including the casting surface is thought to be larger than the effective thickness which supports the load. The authors estimated the effective thickness and it was approximately 0.25mm smaller than the thickness of the test pieces with casting surface. The S-N diagram indicated that the fatigue limit of the test piece with the casting surface was 217MPa, which is smaller than that of the machined test piece. Most of fracture origins were the concavity of the casting surface, and shrinkages and dross were also observed. The fatigue limit evaluated with the test pieces whose fracture origins were the concavity of the casting surface, was 252MPa. The surface roughness profile was evaluated by the two parameters in the altitudinal and horizontal directions, and the fatigue limit was estimated from the maximum concavity that was obtained by extreme statistics. Estimated fatigue limit was 259MPa, and it is close to the fatigue limit of the test pieces whose fracture origin was the concavity of casting surface. The authors also proposed a method to estimate the size of the fracture origin by roughness parameter.