フェライト球状黒鉛鋳鉄の破壊じん性及びその遷移挙動に及ぼす黒鉛形状の影響

抄録

  Influence of graphite shape on fracture toughness and its transition behavior of ferritic cast iron was studied by testing four kinds of specimens whose graphite shapes were spheroidal, compacted vermicular and flake. Fracture toughness testing was carried out by means of an electrical potential method using 15mm thick CT test pieces at temperatures between 20°C and −150°C. At 20°C, when the matrix is ductile, elastic plastic fracture toughness J_IC increases as graphite nodularity increases. However, at −150°C, when the matrix is very brittle, J_IC becomes the highest in CV graphite. In case of spheroidal or CV graphite, a remarkable transition behavior in J_IC takes place in a very low temperature range due to change of fracture mode to brittle from dectile. The higher the nodularity is, the higher the temperature at which transition behavior begins is. The tendencies mentioned above are considered to result mainly from the differences in effective sectional area, multiplicity in stress conditions and in cushioning effect by graphite.