Casting of Ductile Iron
Cast iron essentially is an iron-carbon alloy. The percentage of carbon in cast iron is greater than 2%. Depending on the casting requirements, other important elements like silicon, manganese, sulphur, phosphorus, etc. can be added to the cast iron which modify the structure and properties of the resulting alloy markedly.
Ductile iron casting refers to a process in which magnesium / cerium (as an alloy of magnesium / cerium) is added to cast iron. It reacts with the sulfur and oxygen in the molten iron and changes the way the graphite is formed. Ductile cast iron (also called as spheroidal graphite cast iron or nodular cast iron) has been widely used as a main industrial material in place of high-grade cast iron. While most varieties of cast iron are comparatively brittle, ductile iron castings are much more ductile due to the inclusion of nodular graphite. Solidified castings of ductile iron contain nearly perfect spheres of graphite. Because of the addition of a miniscule percent of magnesium or cerium to the graphite, ductile cast iron has fewer discontinuities in the structure of the alloy.
Ductile Cast Iron – the Process
The base iron used for ductile cast iron typically has high carbon equivalent value. About 0.04% magnesium is added to the liquid iron. Since magnesium boils at a relatively low temperature, there is a violent reaction due to the high vapour pressure of the magnesium at the treatment temperature. It causes violent agitation of the liquid iron. A substantial amount of magnesium is also lost in vapor form. In the process, oxides and sulphides are also formed in the iron, resulting in formation of dross on the metal surface. The dross has to be removed as completely as possible before casting of the iron. It is also pertinent to note that after treatment, residual magnesium in the liquid iron oxidizes continuously at the metal surface, causing loss of magnesium which may affect of the graphite spheroids. It is therefore essential to choose a quality conscious manufacturer of ductile cast iron in order to retain its properties. Once the process of adding magnesium is completed properly, the iron is inoculated and cast. The graphite then precipitates in the form of spheroids. While casting ductile iron, it is very important to keep the impurities out. Oxygen and sulphur reacts with magnesium, affecting the spheroidal shape of the molecules. There are various ASTM / DIN and other international standards for casting of ductile cast iron. These different grades are obtained by manipulating the microcrystalline structure around the graphite spheroid.
Composition of Ductile Iron Castings
Cast or grey iron is an alloy characterized by its relatively high content of carbon flakes (2% to 4%). In contrast, the carbon in ductile iron is in the form of spherical nodules. The formation of such nodules is achieved by the addition of 'nodulizers' like magnesium or cerium into the castings melt. Due to its inherent properties, such nodules resist the creation of cracks and augment its ductility. That is why this process is called as ductile iron casting. In the as-cast condition, the matrix will consist of varying proportions of pearlite and ferrite, and as the amount of pearlite increases, the strength and hardness of the iron also increase. Ductility and impact properties are principally determined by the proportions of ferrite and pearlite in the matrix.
The mechanical properties of ductile iron are controlled by the presence of graphite nodules. The different grades of ductile iron castings are produced by obtaining different matrix microstructures in the Iron. Alloying elements may be added to enhance as cast properties of ductile iron. In some special cases heat treatment can be employed to achieve different properties. The grades of ductile iron castings are based on the mechanical properties of the casting.
Differences in Casting of Ductile Iron and Grey Iron
There are two important differences in the casting characteristics of grey iron and ductile iron. Unlike grey iron, ductile iron is a dross forming alloy. The residual magnesium which is needed to ensure nodular graphite formation rapidly oxidizes whenever the liquid metal is exposed to air. If care is not taken, a magnesium silicate dross is formed which may give rise to defects at or just below the casting surface, usually on the upper surfaces of the castings. It is therefore a common practice to filter ductile iron casings through ceramic filters. The second difference as compared to grey iron is the need to feed ductile iron castings to ensure minimal shrinkage defects. Ductile irons always have a high carbon equivalent, so the volume of graphite precipitated duding solidifications should ensure completely sound sections. Only the strongest moulds (e.g. sodium silicate bonded moulds or lost foam moulds) will withstand the pressures resulting from the graphite precipitation.
Advantages of Ductile Cast Iron
- Since it is malleable, ductile cast iron improves the safety and reliability of process equipment. The improved mechanical properties of the ductile cast iron also make it resistant to corrosion. In general, many of the properties of the ductile cast iron are superior or at par with cast iron and even cast steel. Cast iron costs about 30 – 35% lower than cast steel. It also has a better wear resistance and vibration damping as compared to cast steel. In addition, because of a larger concentration of free carbon and higher silicon content, graphitic (gray and ductile) cast iron has the greatest fluidity and the least shrinkage of any ferrous metal.
- It takes experience and sophisticated infrastructure to manufacture high quality ductile cast iron. Very few companies in India possess such an infrastructure. Qualitas Exports is one such manufacturer of ductile cast iron and machined components.
- It is estimated that ductile iron production accounts for about 35 to 40% of all iron castings in countries like US, India, Germany, UK, France, China, etc.
- Ductile cast iron finds applications in crankshafts, brake calipers, hubs, calves, water pipes, pipe fittings and other parts where flexibility is desired.