About the Heat Treatment of Steel
Without heat treatment of steel, the world of architecture would be flat, and industry would stand still. The strength of steel--an alloy of iron, carbon and other metals--is what allows skyscrapers to tower above masonry and wood frame buildings. Steel is the major component of machines that manufacture goods that make our lives comfortable. Steel doesn't always exhibit the right properties of strength necessary for construction and manufacturing. Heat treatment finesses the atoms within steel to remedy this problem.
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Features
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In solid form, molecules of pure iron fit together in a crystal lattice structure. Each crystal contains nine iron atoms. Eight are arranged in a cube shape, and one in the center of the cube. This arrangement of atoms lends ductility to iron, but it also makes iron relatively soft. When carbon is alloyed with iron to make steel, the carbon atoms situate themselves within the iron crystal so the alloy is harder than pure iron. At the same time though, carbon makes steel brittle and less ductile.
Potential
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In heat treating, the arrangement of carbon and iron atoms in steel is changed by alternately heating and cooling. Heating and cooling times, and carbon content affect the ultimate hardness and ductility of the steel. The most common heat treating method, quenching and tempering, illustrates the principles of heat treating.
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Considerations
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When steel is quenched, it is heated to a certain temperature calculated specifically for the chemical composition of the steel called a "normalizing temperature then cooled quickly, in a bath of water or oil. The heating causes carbon atoms to migrate to the center of the cube shaped iron crystals in the steel. Quick cooling causes the carbon atoms to be trapped in this arrangement, resulting in steel that is hard, but brittle.
Prevention/Solution
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Tempering of steel relieves some of the internal stress of quenched steel that is responsible for brittleness. Steel is tempered by reheating to a temperature carefully calculated below normalizing temperature. This causes the carbon molecules to move again within the metal crystal structure so that the steel regains some of its ductility.
Types
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When steel is manipulated while cold, as in hammering or bending, the crystal structure of the steel is altered, but not uniformly. The uneven distribution of carbon and iron atoms results in weak spots in the steel. To solve this problem, another heat treating process called annealing is used. In annealing, steel is heated and then cooled slowly. Heating causes the iron and carbon atoms to become evenly distributed. Slow cooling allows the carbon atoms to migrate out of the center of the crystal, restoring ductility to the steel.
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