A Many Splendored Thing
The Age of Iron dates back 3,300 years into the distant past. Spreading across the globe, depending on the knowledge of and skills in iron metallurgy, iron slowly replaced bronze from South Asia west to Africa by 1200 BC. Although evidence exists to confirm there had been attempts at iron working as far back as 3200 BC, the technology required to build furnaces capable of achieving melting points of 2,800°F for terrestrial iron did not become widely available until the times of the Trojan War. And it took another 1,600 years for the Chinese to be the first to mass produce cast iron for implements.
Cast iron begins as pig iron and includes iron/carbon alloys with a carbon content of 2% or more. The addition of Carbon (C) lowers Iron’s (Fe) effective melting point down to 2,200 – 2,300°F; by adding other elements to the mix such as Silicon (Si) of 1 – 3%, not only does this melting point vary, but different types of iron are produced, each with its own, unique characteristics. Re-melting pig iron with Iron, steel, limestone and carbon while removing contaminants (Phosphorous (P) and Sulfur (S)) produces a “melt” that is then cast/poured into its desired form.
Lowering that melting point and the ability to form it into a wide variety of engineering shapes has made cast iron an affordable alternative to steel. Although cast iron is brittle (except for malleable cast irons) compared to steel because of its molecular structure dictated by the alloy mix of its composition, it is relatively wear resistant and responsive to machining. Because of this, there are many types of cast iron, including grey cast iron, white cast iron, malleable cast iron and ductile cast iron.
The colors come from the description of the iron when it’s fractured, each type fracturing a different way, according to the molecular structure created by the combination of Iron, Carbon and Silicon atoms. These polycrystalline structures are usually either an iron carbide or graphitic matrix.
Grey Cast Iron is the most common of the four and has a chemical composition of
Carbon (C): 2.5-4.0%
Silicon (Si): 1.0-3.0%
Iron (Fe): 93.05-96.5%
Although it has less tensile strength and shock resistance than steel, Grey Cast Iron’s compressive strength is comparable to low-carbon steel. It has high solidification or graphitization potential, producing three different types of graphitic shapes: Flake, Compacted and Spheroidal.
White Cast Iron begins as Grey Cast Iron but with a lower percentage of Silicon in its chemical composition and a faster cooling rate, Carbon tends to slowly leach out of the melt, resolving the precipitate into cementite Fe3C instead of graphite. These large carbides provide hardness but not toughness throughout, so White Cast Iron is used for wear surfaces and not structural components.
If White Cast Iron is heat treated for up to two days at 1,740°F and then slowly cooled over another two days, the iron carbides transform into graphitic and ferritic spheroidal particles, creating Malleable Cast Iron. Because it begins as White Cast Iron, castings are limited by size but exhibit surface tensions equal to that of mild-carbon steel.
The most recent cast iron to be developed for commercial and industrial use is Ductile Cast Iron. It differs from the previous three because it’s neither the melt treatment nor the cooling rate that clearly determines Ductile Cast Iron. Instead, the composition is varied by adding 0.02-0.1% Magnesium (Mg) and 0.02-0.04% Cerium (Ce) to form spheroidal particles as the melt solidifies.
Like the majority of cast iron in use today, KML Bearing USA manufactures its mounted bearing units from Grey Cast Iron. With a compressive strength of low-carbon steel and castings that provide a solid base and reinforced bolt hole areas, KML Bearing’s mounted bearing units, from pillow blocks to two- and four-bolt flanges, are built to withstand their manufactured tolerances, and then some.
For a little stronger tensile strength, KML also manufactures a three bolt triangle flange from Ductile Cast Iron, providing a noticeably higher impact-resistant housing for special applications.
