Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 207 consider three alloys as they complete a thermal cycle. Each alloy has been cast and hot-rolled to 3/8″ (9.5 mm) rod, and 6″ (150 mm) lengths have been placed in a furnace at 1800°F (980°C) for one half hour. What transformations will occur in each piece during cooling? The three cooling paths for the different steels are shown in Figure 10-9. 10.3.1 UNS G10200 Steel When a UNS G10200 (AISI 1020, 0.2% carbon) sample is cooled from 1 800°F (980°C), it passes below the A3 temperature at approximately 1500°F (820°C), as shown in Figure 10-9. Some metal transforms from austenite to ferrite. Ferrite (bcc) cannot hold 0.2% carbon in solution, so carbon is rejected from the new ferrite, simultaneously increasing the carbon concentration in the remaining austenite (fcc). More ferrite forms as the steel continues to cool, further increasing the amount of carbon in the austenite. At 1341°F (727°C), the A1 temperature, all the remaining austenite transforms to ferrite and cementite, usually in a pearlite structure. The result is a microstructure with a mixture of ferrite and islands of pearlite, Figure 10-10. This steel is called a hypoeutectoid alloy, because the composition includes less carbon than the eutectoid, which contains 0.77% carbon. Applications of G10200 Steel When cooled at a moderate rate, alloy G10200 has good ductility and w eldability, plus reasonable machinability. However, strength is not outstanding, and abrasive wear resistance is low unless the surface is specially hardened. This steel is used for axles, shafts, light-duty gears, and spindles. It is often used for general-purpose cold-headed bolts. When G10200 is welded, the weld pool goes through the transformations indicated in the phase diagram as it solidifies and cools. The closely related alloy, UNS G10100, has 0.1% carbon, so the microstructure has more ferrite and fewer pearlite islands. Alloy G10100 is used for auto body hang-on parts such as fenders, deck lids, and roof lids. It is also a general-application steel, used for steel chairs, hide- a-bed frames, white goods, playground equipment, and many other products. Goodheart-Willcox Publisher Figure 10-9. Three typical steel alloys undergo transformations on cooling from 1800°F (980°C) to room temperature, following the paths indicated by the vertical lines in this figure. 800 1000 0 2 Fe 0.2%C 0.8%C 1.2%C Austenite A3 Acm A1 Ferrite plus austenite Ferrite 1200 1400 1800 2000 Temperature, ° F Ferrite and cementite 0.77%1 Percent carbon Austenite and cementite 1340°F Light areas: ferrite 50 μm Pearlite islands ASM International Figure 10-10. The UNS G10200 alloy transforms with large areas of ferrite (white regions in micrograph) and islands of pearlite (dark regions in micrograph) scattered throughout the metal. The layers in the pearlite are so fine that they cannot be seen clearly at this magnification. Copyright Goodheart-Willcox Co., Inc.