Section size influences properties of ductile iron.

Ductile iron is section-sensitive, although not to the same degree as gray iron. Its properties are influenced by matrix microstructure, which, in turn, affects the section thickness of the casting and its chemical analysis.

Section thickness mainly affects solidification rate (time required for complete solidification from the beginning to the end of solidification) and casting cooling rate, through the austenite transformation range. In heavier sections, because of extended solidification times, magnesium fading will also be affected.

These two factors--solidification rate and cooling rate--influence nodule count, presence of carbides and the amount of ferrite and pearlite present.

Solidification Rates

Solidification time is a function of the casting volume and surface area, mold heat extraction rate, pouring temperature and the thermal properties of the solidifying metal. For a given molding medium and solidifying metal, solidification time depends on the volume-to-surface area ratio (V:SA). The smaller the V:SA ratio, the faster the solidification rates.

Solidification starts from the outside walls and proceeds inward. External corners solidify first and quickly. As solidification progresses, the molding medium is heated, reducing the heat extraction rate and leading to slower solidification at the casting's center. When a small amount of molding medium is surrounded by fairly large sections of metal, the molding mass is heated to a high temperature, reducing heat transfer from the solidifying metal.

If poured at higher temperatures, metal solidifies slower. The increased heat must be removed before the metal starts solidifying and the molding medium is heated to a higher temperature, reducing the heat transfer rate.

Ductile iron undercools before the bulk of the eutectic solidifies. The amount of undercooling increases when all other metallurgical factors are kept constant. When the eutectic undercooling temperature is low (below 2080F/1138C), iron is prone to chill carbide formation. Late, effective inoculation decreases the chill-forming tendency.

Thermal capacity and conductivity of mold material influence the heat transfer and solidification rates. Rates are faster in metal molds and the resultant ductile iron nodule count in a metal mold is extremely high (500-1000 m|m.sup.2~). As section thickness increases, solidification rate decreases, resulting in lower nodule counts.

Slow cooling rates promote segregation of elements such as manganese, chromium and molybdenum toward final freezing areas and tend to stabilize carbides and promote microporosity. Intercellular carbides and associated porosity are detrimental to dynamic properties, such as fracture toughness and fatigue endurance.

Cooling Rates

For a given chemistry and nodule count, the casting cooling rate through the austenite transformation range determines the final matrix structure. Faster cooling rates associated with thin sections promote pearlite formation; slower cooling rates favor ferrite formation. Faster solidification rates generally accompany faster cooling rates through austenite transformation.

Besides section thickness, the total mass of the casting--relative to the amount of medium (sand:metal ratio) and the nature of molding medium--influences the cooling rate differently from the solidification rate. When the total weight of metal poured is increased, molding materials are heated to a higher temperature as the metal solidifies and cools to the transformation temperature range. This slows down the cooling rate, providing longer times for ferrite formation. Schematic cooling curves in Fig. 1 show different solidification and cooling rate combinations and expected microstructures.

Nodule count is important in controlling the final matrix. For the same cooling rate, increasing amounts of nodules provides more centers for ferrite nucleation and growth, resulting in increasing amounts of ferrite. For this reason, late and multiple inoculation increase nodule count and ensure the maximum amount of ferrite.

The conflicting effects of section thickness, however, must be recognized. Thinner sections generally promote high nodule counts and have a tendency to increase the amount of ferrite. But they also increase the cooling rate, thus reducing the ferrite content. The final matrix structure depends on nodule count, cooling rate (section thickness) and chemistry (carbon equivalent and other alloy factors).