Development of Ductile Irons for High-Temperature Applications: Characterization in the As-Cast and Oxidized States

Ductile iron alloyed with molybdenum and different levels of aluminium and silicon was cast to determine the proper combination of elements to increase the temperature range of operation. Four alloys containing 1.5wt.% molybdenum and different combinations of aluminium and silicon (i.e. 3.5Si-3.0A1, 4.5Si-2.0Al and 4.5Si-3.0Al) were cast at 1350, 1400 and 1450⁰C into step blocks. The effects of alloy chemistry, pouring temperature and casting thickness in the as-cast and oxidized conditions were studied. Results from the as-cast condition show that graphite morphology (i.e., size, count and sphericity) improved with lower Si/Al ratios and intermediate pouring temperatures. Higher silicon and lower aluminium contents accompanied by intermediate pouring temperatures reduced the onset of surface and sub-surface defects. Results from the oxidized condition show that maximum oxidation resistance was achieved in alloys containing higher aluminium and silicon contents. This also increased the critical α-ferrite to γ-austenite phase transformation temperature range.