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Section 6: Phase Changes in Solids
Unit 2 of this course presented some simple phase diagrams for water and carbon dioxide, showing the combinations of pressure and temperature at which these substances change between solid, liquid, gas, and supercritical phases. Phase changes can also occur within solids: As they undergo heat or pressure changes, the molecules' structure is altered, changing the material's properties. These shifts are known as "solid-solid phase changes."
Figure 13-10. Iron-Carbon Equilibrium Diagram
As iron and steel are heated, they undergo solid phase changes, with crystals within the metal changing form.
© Science Media Group, adapted from original image by U.S. Department of Transportation, Federal Highway Administration.
The historic practice of blacksmithing requires an extensive understanding of phase changes in solid metals, even though most blacksmiths never studied chemistry. Humans began working with iron more than 3,000 years ago, and iron was a centrally important material for many tools and applications until it was replaced by mass-produced steel in the latter half of the 19th century.
The first step in ironmaking is smelting iron from iron ore (iron oxide mixed with other trace elements), which was done for centuries by heating the ore with charcoal. Oxygen in the iron oxide combines with carbon from the charcoal to form carbon dioxide, leaving iron metal behind. As the iron metal is heated further, it absorbs carbon from the charcoal and forms different crystal shapes. Steel is an alloy of iron with less than 2 percent carbon. If the metal absorbs more carbon, it becomes cast iron, which contains about 2 to 4.5 percent carbon and is hard and brittle. Figure 13-10 shows some of the crystal structures that form in iron at different temperatures and carbon contents. Ferrite consists of iron molecules with no carbon attached, cementite is an iron-carbon compound (Fe3C), and pearlite is a mixture of ferrite and cementite.
Early blacksmiths did not have thermometers that could measure these high temperatures, but knew from practice that metal glowed in different colors as it heated, first turning red and then orange, yellow, and white. After smiths forged metal into a specific shape, such as a horseshoe, they often would let the metal cool to a certain point, then quench the object by plunging it into water or another liquid to cool it quickly. This process converted some atoms in the metal into hard, brittle crystals, giving the object strength. Today, advanced alloys may be quenched with oil, brine, or forced air.
Blacksmiths also developed many other techniques for manipulating the properties of alloys, which are widely used today in industry. Typically, they involve bringing the material to a certain temperature and holding it there for a specific length of time to form or dissolve certain crystals in the material. Examples include:
- Annealing: heating metal to a high temperature, then cooling it very slowly. During this process, small grains in the metal re-crystallize into large grains, which makes the metal softer and easier to form.
- Tempering: heating iron alloys to a lower temperature to reduce their hardness.
3Steel Recycling Institute, "Steel Recycling Rates," http://www.recycle-steel.org/Recycling%20Resources/Steel%20Recycling%20Rates.aspx.