Transition from liquid to solid state.
The direction of solidification of technical cast iron materials is outside-in since the heat released due to overheating and the solidification heat must be dissipated to the outside. As a general rule, solidification begins when the material reaches the liquidus temperature and ends when it reaches the solidus temperature. The range between liquidus and solidus temperature is referred to as solidification interval; however, not all eutectic alloys have a solidification interval. In pure metals and eutectic alloys the liquidus temperature is equal the solidus temperature. During solidification, volume contraction (solidification shrinkage) occurs; the deficit in volume resulting thereof must be compensated by post-feeding of molten metal.
Consequently, important influential variables with regard to solidification are thermal conductivity and thermal capacity of the casting material, range of the solidification interval, the solidification temperature and occurring temperature difference, and thermal conductivity and thermal capacity of the molding material.
On the basis of the obtained depth of the solidification area, the following statements can be made:
High thermal conductivity and thermal capacity of the molding material; this means conditions as they are present in an ingotlead to a steeper temperature gradient than molding materials with lower thermal conductivity and thermal capacity corresponding to a sand mold. Consequently, the depth of the solidification range is considerably greater than in ingotcasting or when using chill elements in the effective sections. Thermal conductivity of the casting material has a greater effect with metal molds. Thus, the higher thermal conductivity of the casting material entails a greater solidification space due to the flatter temperature gradient. For example, aluminum and copper alloys have greater solidification space than steel. This effect is intensified by the fact that the solidification temperature of steel is higher than that of aluminum or copper. A greater solidification interval, for example resulting from the phase diagrams of the respective cooling conditions, also results in a greater solidification space. This statement must be viewed in reference with the ratio between melting heat and specific heat.
Additional influence is imposed by the effect of crystallization conditions resulting from undercooling effects (or through modification of the crystallization rate). In this scenario, trace elements and additives, equally also slightest changes in the chemical composition, may have a significant effect.
The solidification sequence of real alloys consequently results from the interaction of the factors specified above. In view of the positive influence of the solidification sequence, it is also possible to deduce suitable measures from this context. For improvement of post-feeding, reduction of the depth of the solidification area is reasonable while simultaneously achieving directional solidification from the casting section to be fed towards the feeding casting section. An effective measure may be the use of molding materials with better heat dissipation capacities if implementation of an alloy with smaller solidification interval or a different solidification morphology is not possible.
In general, extraction of heat has a main influence on the solidification sequence (solidification type) in addition to the type of alloy and its position in the phase diagram taking into account the solidification interval. The crystallization conditions of the alloy (Balance of nuclei and Crystal growth conditions) act as interference factor.