| 摘要 |
PURPOSE:To prevent the lowering of strength due to the rise of temperature by the procedure in which a nodular cast iron containing Ni is subjected to a hardening treatment ranging the austenitic zone to the Ms point, followed by a subzero treatment and a tempering treatment. CONSTITUTION:The molten cast iron, containing 1.7 to 3.8 wt% C, 0.8 to 3 wt% Si, and 0.3 to 2 wt% Mn, is added 6 to 12 wt% Ni, and then graphite is spheroidized by using a Fe-Si-Mg spheroidizing agent. Very shortly thereafter, the molten cast iron is inoculated with Fe-Si. The molten nodular graphite cast iron is cast into a sedired mold and then after solidification is taken out of the mold. Then, it is hardened at a temperature ranging the austenitic zone to the Ma point and subjected soon to a subzero treatment with liquid nitrogen. After being allowed to stand in the air, it is then heated at 150 to 400 deg.C for 30 min, cooled by air, and tempered. The treatment gives the cast iron a tensile strength of more than 140 kg/mm<2> even at high temperature of 300 deg.C. |
