Malleable cast iron is made by smelting scrap steel and cast iron and carefully controlling the cooling process. It is created through annealing and can be either white-heart or black-heart iron. The ISO has a classification system for malleable iron based on tensile strength and ductility.
Iron is the second most common metal on earth. It’s used to make everything from the hulls of huge ships to the decorative railing on the steps of a house. Malleable cast iron is a form of cast iron that is easier to work with than pure iron. It is made by smelting scrap steel and cast iron and then carefully controlling the cooling of the mixture for many hours. This results in an iron that is very hard but not brittle.
The method of creating malleable iron dates back to 1720, when the French scientist and metallurgist RAF de Re’aumur published the first technical paper on this type of iron. To make it, metallurgists must first make white cast iron. White cast iron is a very hard but very brittle type of iron made by melting steel and cast iron together in an air furnace.
Cast iron is not pure iron. It is actually a combination of iron ore, coal and limestone fused together under intense pressure. It cools into a high carbon iron material. The high carbon content makes cast iron brittle, which is why it is not used without further treatment.
White iron castings become malleable iron through a process called annealing. The castings are melted and then kept at a controlled temperature for a certain period of time. They are then left to cool very slowly for about 24 hours. Controlled cooling rearranges the chemical structure of the metal, allowing it to maintain its toughness without remaining brittle.
Differences in the rate and temperature of cooling can lead to two different types of malleable iron. White-heart iron is lighter in color than black-heart iron. It also differs in chemical composition. Whiteheart has a pure ferrite surface with a pearlite, tempercarbon and ferrite core, while blackheart is primarily ferrite. Pearlitic malleable iron is a type of blackheart iron that includes pearlite, which is a blend of ferrite and cemetite.
The International Organization for Standardization has published a classification system for white heart and black heart malleable iron known as ISO 5922-2005. The first character is a letter indicating whether the cast iron is whiteheart, blackheart or pearlite. The following two numbers list the tensile strength of iron. Tensile strength is how much a sample can be stretched before it deforms significantly. The last two numbers in the designation list the minimum elongation or ductility of the iron sample.
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