220 Section 3 Ferrous Metallurgy • Steel cooled rapidly to an intermediate temperature, such as 600°F (320°C), and held at this temperature for an hour will form a bainite microstructure, which is much finer than pearlite. It has high strength yet retains some ductility and toughness. • Tempering a steel that has been quenched to form martensite improves the toughness of the steel by “rounding” the edges of the martensite needles, relieving the internal stresses caused by the martensite transformation from austenite and converting the body-centered tetragonal (bct) martensite to extremely fine cementite particles in ferrite. • Soaking a carbon steel slightly below the eutectoid, or A1, temperature for many hours converts the cementite from platelets to spheroids and reduces the resistance of cementite to dislocation motion and bulk deformation. Thus, this spheroidized microstructure is more formable than a pearlite platelet microstructure. Review Questions Answer the following questions using the information provided in this chapter. Know and Understand 1. Pure iron forms _____ (bcc structure) at room temperature. A. austenite B. bainite C. ferrite D. martensite 2. True or False? Adding carbon to iron changes the temperatures at which phases form. 3. True or False? At 2800°F (1540°C), iron becomes a gas, one of five possible phases of iron. 4. True or False? Any liquid iron-carbon alloy with 2.14% to 6.67% carbon will form cast iron upon cooling to room temperature, made of ferrite and large cementite particles. Summary • As discussed in Chapter 4, a phase consists of a volume of material with uniform properties. A solution is one phase, while a mixture consists of two or more phases. • A microstructure consisting of particles in a metal matrix is made up of two or more phases—it is a mixture. The importance of a microstructure lies in the form taken by those phases. • The iron-carbon phase diagram shows which phases are present at different steel compositions and temperatures, when the metal is held at temperature for up to a few minutes to stabilize. • In addition to carbon content, high soak temperature, and lower hold temperature, the cooling rate between the higher and lower temperature is a major cause of different microstructures. • Three types of cooling—moderate, rapid, and interrupted—can be demonstrated by different cooling paths on an isothermal transformation (IT) diagram, and are clearly differentiated by the different microstructures produced by each cooling path. • Steel cooled at a slow to moderate rate, or cooled rapidly and held at 1300°F to 1000°F (700°C to 540°C), develops a pearlite microstructure consisting of ferrite and cementite platelets. • Cooling and holding a plain carbon steel at 1300°F (700°C), or cooling it slowly, produces a coarser pearlite than holding it at 1000°F (540°C). The austenite-to-cementite-and-ferrite (pearlite) reaction is diffusion controlled. At the higher hold temperature, carbon diffuses faster and further, allowing for wider cementite layer spacing. • Steel cooled rapidly, or quenched, to near room temperature develops a martensite microstructure, consisting of a phase with a body-centered tetragonal (bct) unit cell. This phase will decompose in an hour into ferrite and extremely fine cementite particles upon heating above 300°F (150°C). CHAPTER REVIEW Copyright Goodheart-Willcox Co., Inc.