Abstract
The possibility of ago-hardening was studied in the spheroidal graphite cast irons when alloyed with 2.8∼4%Ni, 0∼0.6%Mo and 0∼2%Al. Test irons with several different alloying combinations were cast in sand molds of 30mm thickness keel blocks. Some irons were aged directly from as-cast state without any solution treatment. The others were isothermally transformed into banitic structure at 390°C in advance of aging treatment. The irons were most responsive to age-hardening when aged at 500°C and reached almost maximum hardness within 3 hours. The as-cast irons increased their hardness to some extent but larger aging effect was realized in the isothermally transformed irons. It was clearly shown that the banitic matrix iron is more sensitive to aging than the pearlitic or ferritic matrix. The largest aging effect occured in the presence of 2.8%Ni, 0.5%Mo and 1%Al. The iron with this alloying combination showed Brinell hardness number 271 in the isothermally transformed condition, which rose to 388 after aging treatment.
