Solidification characteristics and feeding process characteristics of ductile iron

Nodular cast iron and gray cast iron have great differences in solidification characteristics, mainly in the following aspects:

(1) The eutectic solidification range of ductile iron is wide. During eutectic solidification of gray cast iron, the end of flake graphite is always in contact with liquid iron, so the eutectic solidification process is faster. Because the graphite ball is surrounded by austenitic shell in the later stage of growth, its growth needs to be carried out through the diffusion of carbon atoms, so the solidification process is slow, so that it requires the formation of new graphite nuclei on the new graphite heterogeneous core at a greater degree of supercooling to maintain the eutectic solidification. Therefore, there is a wide liquid-solid coexistence area on the cross section during the solidification of ductile iron, and its solidification mode has the characteristics of congee solidification. This makes the feeding of nodular cast iron difficult during solidification.

(2) The graphite core of ductile iron is much. After spheroidization and inoculation treatment, the graphite core of nodular cast iron is much more than that of gray cast iron, so the size of its eutectic clusters is much finer than that of gray cast iron.

(3) ductile iron has a large eutectic expansion force. Due to the fact that the graphite is quickly surrounded by austenitic shell during the eutectic solidification process of ductile iron, the expansion caused by the volume increase during the growth of graphite cannot be transferred to the liquid iron, resulting in a larger eutectic expansion force. When the mold stiffness is not high, the resulting eutectic expansion will cause shrinkage defects.

(4) The volume change of ductile iron in the solidification process can be divided into three stages: the liquid shrinkage of liquid iron after pouring into the cast and cooling to the eutectic temperature, the volume expansion caused by the precipitation of graphite spheres during the eutectic solidification, and the volume shrinkage during the cooling process after the solidification of liquid iron.