108 Metallurgy-High

Metallurgy FOCUS Question How do people change metals to create useful objects?

Medieval Metallurgy For thousands of years, people have made useful objects out of metal. One type of metal is called bronze. During the Bronze Age, people made many useful and beautiful things with bronze. People used it to make cooking tools and spears. Bronze is a mixture of two other metals. It is made of copper and tin. Mixing metals together or adding other ingredients can create a stronger metal. This kind of metal is called an alloy. The Iron Age came later. Iron is common and is found in Earth's crust. It weighs less than bronze and is slightly stronger.

Strength from the Ground Today, people use steel to make bridges, buildings, and cars. Steel is an alloy made of iron and carbon. Steel is much stronger than either bronze or iron. People dig, or mine, to find iron on Earth's surface or underground. Enormous machines mine iron and other metals. The iron is found in special rock called iron ore. The ore must be crushed into smaller bits after it is hauled away. But that's not all that has to happen in order to get steel.

The Smelting Process The iron ore must be melted to pull out the iron. First, coal is heated to 1,315 degrees Celsius (2,400°F). The heat removes gases from the coal. It turns the coal into something called coke. Burning coke helps remove the carbon found naturally in iron ore. Machines lift the iron ore and coke to the top of a blast furnace. Limestone is added to help remove other materials from the iron. Hot blasts of rising air melt the ore. The metal falls down through the furnace. Iron collects in a container at the bottom. Anything that is not iron rises to the top of the container and is removed. Next, the melted iron is placed in an oxygen furnace.

Oxygen is blown onto the iron to remove extra carbon. This process changes the iron into steel. The amount of carbon left in the iron determines which type of steel is made. The liquid steel is poured into a container called a ladle. The giant ladle lifts the hot, drippy steel to the top of another machine. The steel is poured into a mold, called a cast. The steel is sprayed with water until it cools and becomes solid. The steel is reheated and flattened into sheets. People can make useful items from these sheets.

Types of Steel The kind of steel you end up with depends on what you add to or remove from the iron during the smelting process. Mild steel is iron with 0.25 carbon. It is strong and flexible. Building supports, parts of ships, and car bodies are made of mild steel. Carbon steel is iron with 1.5 carbon. Nails made of carbon steel can puncture brick or concrete. The steel used in spacecraft is a mixture of iron and titanium. Iron mixed with manganese makes steel used in machinery and helmets. Both of these alloys are light and strong. Word Wise Stainless steel is a special blend of iron, nickel, and chromium. It does not easily stain or rust in water as regular steel does. Many tools for cooking and eating are made of stainless steel.