Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 203 Water with 40% glycol at –12°F (–24°C) consists of one phase, a liquid. In fact, Figure 10-2 shows that at –12°F (–24°C) or higher, every composition of 40% or more glycol is a single-phase (liquid) solution of water and glycol. Below the red transition line in Figure 10-2, the solution forms a mixture of solid ice crystals and liquid. You could pour the slushy mixture through a sieve, keeping the ice crystals behind in the sieve. The ice-and-glycol mixture consists of two phases, solid ice and liquid solution. At the surface boundary of this volume, the properties change abruptly. The boundary from a solid ice particle to liquid glycol solution, for example, is an abrupt change from solid ice to liquid solution. The red line in Figure 10-2 is a phase boundary, in this case between entirely liquid and liquid plus solid. Below –69°F (–56°C), the ice and glycol mixture becomes completely solid, another phase transition. Just as in the water-glycol system, iron and carbon form different phases at different temperatures and compositions. Some areas of the iron-carbon composition-temperature map, or phase diagram, contain single-phase solutions, and other areas contain mixtures of multiple phases. To understand what happens in steel during processing, you need to understand the phase diagram of the iron- carbon system and the transitions between areas. Other metal alloys have their own phase diagrams, which are discussed in Chapters 15–22. 10.2 The Iron-Carbon Phase Diagram Pure iron forms a body-centered cubic crystal structure, called ferrite, at room temperature, and face-centered cubic iron, called austenite, above 1673°F (912°C). Above 2541°F (1394°C), iron again forms a body-centered cubic crystal structure, called delta iron, and at 2800°F (1538°C), it melts to become liquid iron. At one atmosphere pressure and above 5184°F (2862°C), iron becomes a gas. That makes for a total of five possible phases of iron. With only one variable, temperature, these transformations can be shown on a single axis, Figure 10-4. Adding carbon to the iron changes the temperatures at which the phases form, just as adding antifreeze to water changes the freezing temperature. The compositions and new temperatures for these phases has a great deal to do with the formation of different microstructures, and hence the final properties. Figure 10-5 is a map of these phases, showing temperature from room temperature up to 3000°F (1650°C) on the y-axis, and composition from pure iron (0% carbon) to cementite (Fe3C, 6.67% carbon) on the x-axis. The transition temperatures for pure iron appear at the far left. For the metal phase diagrams in this book, the y-axis will always be temperature, using both Fahrenheit and Celsius scales. The x-axis will always use weight percent concentration of the second element in the alloy. That is, if a block of graphite (which is pure carbon) weighing 1 pound (0.454 kg) is mixed into a ladle holding 99 pounds (44.905 kg) of pure iron, the 100-pound (45.359 kg) alloy will contain 1% carbon in iron. Goodheart-Willcox Publisher Figure 10-4. Pure iron has three solid phases, one liquid phase, and one gaseous phase. The transitions between the phases are marked on the temperature scale. °F °C Boiling point (1 atm.) Gaseous phase Liquid phase Delta phase 2800 1538 2541 1394 1673 912 72 22 –460 –273 Austenite phase Ferrite phase 5184 2862 Melting point Ferrite-austenite Room temp. Absolute 0 Austenite–delta Copyright Goodheart-Willcox Co., Inc.