低熱膨張球状黒鉛鋳鉄の引張, 衝撃特性及び破壊靭性に及ぼすNi 量とCo量の影響

抄録

  Several characteristics such as thermal expansion, strength, ductility, and toughness were examined in thermal expansion, static tensile, dynamic Charpy impact, and fracture toughness (J-integral) tests for two series of 1.8%C-1%Si low thermal expansive spheroidal graphite cast iron (LE-FCD) samples. Co addition series was 33%Ni- X%Co (X ＝ 0, 3, 6, 9 and 12) and Co substituent series was (36-Y) %Ni-Y%Co (Y ＝ 0, 4, 6, 8 and 12). The coefficient of thermal expansion at 373K of the LE-FCD samples in both series decreased with increasing Co content except the 24%Ni-12%Co sample which contained about 50% martensite in austenite matrix. The tensile and impact tests were carried out below room temperature using LE-FCD samples of the Co addition series. The tensile strength and elongation at 77K were found to be large owing to the transformation-induced plasticity (TRIP) of martensite. On the other hand, the impact values decreased with decreasing temperature. It is thought that the TRIP effect seen in the dynamic impact testing does not occur at all. From SEM observation, fracture surfaces tested in the temperature range from 123K to 573K appear not in the brittle but in ductile mode. The fracture toughness J_IC of the LE-FCD samples was more or less the same as or larger than those of fully ferritic spheroidal graphite ductile irons.