抄録
Generally, spheroidal graphite cast iron is superior in heat resistance to normal gray cast iron with flake graphite. The use of spheroidal graphite cast iron at elevated temperatures has been restricted for many years to temperature pu to 350°C. Long-term stress-rupture values are most important for estimating heat resistance of materials for turbine parts, pressure vessels application and so on. The spheroidal graphite cast iron has great strength and ductility as carbon steel, and yet is inferior to it in elevated temperature creep-rupture properties. For higher temperature services, therefore, plain carbon or alloy steels have been used. But creep-rupture properties of spheroidal graphite cast iron can be improved by alloying.
Ferritic spheroidal graphite cast irons, FCD 40, and two alloyed, ferritic and ferritic-pearlitic spheroidal graphite cast irons, one of which contains 1% molybdenum-1% nickel and another 1% molybdenum-0.3% chromium, were tested for a period of about 10,000 hours at temperatures up to 550°C. From a Larson-Miller master rupture curve, creep-rupture properties were compared on various materials, including these spheroidal graphite cast irons, low carbon steel and 0.5% molybdenum-alloyed steel, of which data were obtained from the literature. The rupture stresses for long-term 105-hrs were estimated, and influences of the ferritizing annealing on creep-rupture properties were investigated.
The results of the experiment were summarized as follows :
(1) Alloyed Spheroidal graphite iron indicated better stress-rupture properties than low carbon steel or FCD 40. Molybdenum-chromium-alloyed, ferritic-pearlitic cast iron was best of them all and its properties approximately equaled to that of the 0.5% molybdeum-alloyed steel. The 105-hrs stress-rupture values at 450°C were estimated to be approximately 15kg/mm2.
(2) After the creep rupture test there were no microstructural changes, that is, residual pearlite in the ferritic-pearlitic, molybdenum-chromium-alloyed one did not decomposed and was stable at temperatures up to 550°C.
(3) For ferritizing annealing, full annealing including high temperature treatment is more desirable from the view point of ductility rather than from that of creep-rupture properties.
