種々の基地組織を有するレアアースレス球状黒鉛鋳鉄の疲労強度

抄録

  Spheroidal graphite cast iron is widely used for various purposes due to its excellent mechanical properties. As the spheroidization ratio of graphite influences the mechanical properties, rare earth is used as a graphite nodularizer to improve spheroidization. However, given that the supply of rare earth mainly relies on imports from overseas, there is need for the development of spheroidal graphite cast iron with reduced rare earth. For this reason, this study investigated the fatigue strength of spheroidal graphite cast irons without using rare earth.

  Specimens of spheroidal graphite cast iron containing rare earth and those not containing rare earth were used. The matrices were all ferrite, pearlite ratio 30%, and all perlite. Fatigue test was carried out using a plane bending fatigue test machine. The stop condition of fatigue test was after 10⁷ time repetitions or fracture of specimen. The load stress of specimens that did not fracture was taken to be the fatigue strength.

  The results of the fatigue test showed that the σ_w of specimens not containing rare earth was reduced compared to those containing rare earth for each matrix. Casting defects existed inside the specimens. The effects of casting defects on fatigue strength were evaluated using a √area parameter model, stress intensity factor range ΔK, and threshold stress intensity factor ΔK_th. As a result, it was found that the difference between the ΔK and ΔK_th of specimens not containing rare earth increased, resulting in reduced fatigue strength.