Abstract
Cast iron for high temperature use grows by repeated heating and as a result weakens and fails easily. Another characteristic change due to growth is the decrease of thermal conductivity. The decrease of thermal conductivity adversely influences the thermal efficiency of an apparatus. Moreover, the change of temperature gradient within the material due to the decrease of conductivity will increase thermal stress and weaken the iron indirectly by thermal shock and thermal fatigue. Thermal conductivity of grown cast iron was measured with rod specimens set horizontally in still air with one end contacting the heat source. Axial temperature distribution by natural cooling was measured and the relative value of conductivity was calculated. For the experiment, six specimens of grown flake graphite cast iron were prepared by cyclic heating to 950°C in air. The linear growth of specimens was from 2% to 9% by heating of 10 and 50 cycles respectively. Specific weight, hardness and microstructure were alse examined. Results may be summarized as follows; 1. The thermal conductivity of cast iron decreases continuously with growth, and it was about 50% of as-cast iron when the liner growth reached 8% by 40 cycles of heating to 950°C. 2. With more than 50 cyles of heating, the decrease in thermal conductivity slowed down and showed a tendency of saturation. 3. The decrease in thermal conductivity of grown cast iron is mainly due to the porosities caused by growth, and the graphite flakes changing to porosities acting as barriers for thermal current. 4. Changes in microstructure such as ferritization and graphite-redistribution may also have some effects on the thermal conductivity. 5. On the other hand, since the conductivity of a specimen held at 950°C for 25 hrs. in air was not remarkably reduced, the influence of oxidation will not be as important as repetitive heating.
