固溶化熱処理した24%Ni-12%Co系球状黒鉛鋳鉄のマルテンサイト変態に関する臨界冷却速度

抄録

  Thermal expansion characteristics and static mechanical properties of 24%Ni-12%Co-2%C-1%Si low thermal expansive spheroidal graphite cast iron were investigated by varying the amount of martensite by changing cooling rates from 1173K of solution treatment temperature. The microstructure of the as-cast sample consisted of graphite and matrix of 50% martensite and 50% austenite, and its coefficient of thermal expansion was about 6 × 10^-6/K at 373K. The coefficient of thermal expansion of the sample, which was subjected to solution treatment, dropped to about 4 × 10^-6/K owing to inverse-transformed austenite from martensite. The amount of martensite changed with the cooling rate and it increased when cooled slower than air cooling. The critical cooling rate for martensitic transformation was determined to be about 1 K/s from observation of microstructure and change of coefficient of thermal expansion. The coefficient of thermal expansion at 373K of the samples with higher amount of inverse austenite became lower because the martensite suppressed Invar effects which contribute to low thermal expansive characteristics. Therefore, the coefficient of thermal expansion at 373K depended on the amount of martensite. The tensile strength and hardness of the samples increased with increase in the amount of martensite and tensile characteristics was found to be greatly affected by change with the amount of martensite. However, as elongation of the samples was 2 to 5% regardless of the amounts of martensite or austenite, materials characteristic included poor ductility.