Melting iron ore in a furnace makes it stronger and more workable. Foundry workers pour the molten metal into molds and allow it to cool and solidify, producing cast iron. The furnace temperature and materials added or "alloyed" to the molten iron determine the finished product's exact characteristics. Ductile and malleable iron are both formed by casting the ore, but there are a number of differences between these two types of metal.
You can hammer a malleable material into a different shape without causing it to crack or fracture. You can draw a ductile material out into a long thin strand, such as a wire. When it comes to iron, these terms are a little misleading. Both malleable and ductile irons respond well to hammering, and both can be drawn out into strands, though the latter can be drawn further.
The earliest forms of cast iron, known as "gray" and "white," proved very brittle and prone to crack under stress. Malleable iron, a significant improvement, was easier to shape and far less brittle. It remained the dominant form of cast iron until 1943, when Keith Dwight Millis of the International Nickel Co. research laboratory developed ductile iron, an even stronger and more flexible metal.
To produce malleable iron, foundry workers melt iron ore and rapidly cool it by pouring it into molds. They reheat the castings to around 1,700 F for up to 100 hours, before allowing them to cool slowly. During this tempering or "annealing" process, the workers add hematite ore. Ductile iron is simpler to make. As foundry workers melt the iron ore, they add cerium, sodium or magnesium. The resulting alloy is cast in molds and allowed to cool.
Malleable iron's annealing process causes small, irregularly shaped particles of carbon to form. They give the metal more strength and flexibility than earlier cast irons. In ductile iron, the materials added during the melt help form more regular spheres of carbon. This spheroid composition gives the metal fewer internal flaws, making it stronger and allowing it to be bent, twisted and elongated more successfully than malleable iron.
When malleable iron is cast into a mold, it shrinks considerably as it cools. Foundry workers must top off the mold with molten ore known as a "feeder," in order to achieve the desired shape. Ductile iron shrinks much less than malleable iron as it solidifies in the mold. This lack of shrinkage makes ductile iron less prone than malleable iron to internal defects and weak points caused by the casting process.
Metal workers can draw out a 2-inch test sample of ductile iron by 18 to 30 percent more than its original length. By contrast, malleable iron elongates by only 10 percent. Ductile iron has a higher "yield strength" than malleable iron, meaning it can bear greater loads and higher temperatures. However, according to the Kirkpatrick foundry, malleable iron is the best choice when you want to cast thin sections or plates.
Ductile iron uses less feeder metal. It also requires less heating than malleable iron and so uses less energy. These considerations make it much cheaper for manufacturers to produce ductile iron than malleable iron.
Both types of iron are strong, durable and good for machining into new shapes. Ductile iron is used to make pipework and automotive parts such as crankshafts, truck axles and wheel hubs. Malleable iron is well suited to situations where it has to be flattened and shaped by hammering. Metal workers often use it for "wrought iron" items, such as brackets and gates.
British writer Martin Malcolm specializes in children's nonfiction. His books include "A Giant in Ancient Egypt" and "Poetry By Numbers." His schoolkids' campaign for the Red Cross won the 2008 Charity Award. A qualified teacher, he has written for the BBC and MTV. He holds a Master of Arts in English from the University of London.