Ductile cast iron is a carbon and iron alloy. If the percentage of the carbon is lower than 1.7% is steel and if it is higher, cast iron. The 1.7% limit has a lot of metallographic sense, because there is a limit in which the solidification process generates austenite.
There are plenty types of foundry. The most general definition could be ” iron and carbon alloy that allows the formation of a eutectic at the time of solidification” (a process in which two solid phases are formed). It is important to point out that the eutectic temperature is the lowest at which the liquid melt can be maintained, so the melting is carried out at temperatures lower than those of steel. This allows the energy consumption to be lower than steel one, and allows the mouldability of the material to be much greater.
Among the most common foundries there are the gray iron and the ductile cast iron (as well as many other variations).
The gray iron (image on the right) is the one in which the precipitation of the carbon takes place in the form of sheets. It is a material with good mouldability and resistance to the abrasion but that, nevertheless, turns out to be fragile.
On the other hand, the ductile cast iron (image on the right) precipitation of carbon occurs in the form of spheres. It is a material with good mouldability and very ductile. The material is also known as nodular cast iron or GS cast iron.
The carbon form in the metal matrix is crucial. At the previous picture, some lines of tension have been drawn. In the gray iron they are concentrated and easily generate break points, whereas in the case of ductile or nodular cast iron they do not accumulate with the same density. This means that the material can withstand large stresses and deformations before breaking: this is what “ductile” means. It is an essential property when water is supplied under pressure.
The element that causes that carbon crystallization occurs in nodules is the addition of approximately 0.04% magnesium (Mg) to the liquid melt. This procedure was discovered and developed since the middle of the 20th century. Magnesium is added to the liquid melt in the process called “conversion”, which is strongly exothermic, and is carried out under controlled conditions.
An orientative composition of the ductile iron is the following:
Iron Fe 93,2% a 94,3%
Carbon C 3,3% a 3,4%
Silica Si 2,2% a 2,8%
Manganese Mn 0,1% a 0,5%
Magnesium Mg 0,03 a 0,04 %
Phosphorus P 0,005% a 0,04%
Sulfur S 0,005% a 0,02%