212 Section 3 Ferrous Metallurgy workpiece, when thrust into a salt bath, will reach the bath temperature in under one second. If quenched from 1500°F (820°C) to 1300°F (700°C) and then soaked for an hour before cooling in air, the G10800 workpiece will have a coarse pearlite structure. The same G10800 steel workpiece quenched from 1500°F (820°C) into a salt bath at a lower temperature of 1000°F (540°C), then soaked, will form fine pearlite, as shown in Figure 10-17. Railroad rails made of G10800 steel are much larger than a small sample. After hot-rolling, water must be splashed on the head side of the rail to cool it to 1000°F (540°C) more quickly than an air cool and produce the finest pearlite possible in production. Note that the rail is not dropped into a water tank, which would cool it rapidly, but rather is only splashed with water. The head of a railroad rail is the area where engine and car wheels roll on the rail. The fine pearlite produced by quickly cooling the head translates to a very wear-resistant contact area. Mechanism and Effects of Pearlite Pearlite colonies form by growing the ferrite and cementite platelets together, as illustrated in Figure 10-18. The sample cooled and held at 1300°F (700°C) for an hour develops wide bands of ferrite and cementite, because the carbon can diffuse further at the high temperature as the austenite transforms to ferrite and cementite. By contrast, a sample cooled quickly to 1000°F (540°C) will have much narrower bands. The carbon simply cannot move as far at the lower temperature before platelets grow into the austenite. Austenite transforming into ferrite and cementite is called a diffusion-controlled reaction, because the carbon in the austenite must diffuse and migrate to form the cementite. The thickness of the cementite layers depends on the rate of that diffusion. Figure 10-19 shows that fine pearlite has higher strength than coarse pearlite. As Chapter 6 made clear, higher strength also means greater hardness, which in turn means greater wear resistance. As mentioned, you can change the lamellar spacing of pearlite by cooling the steel to 1000°F (540°C) quickly and then cooling more slowly. This cooling path increases the hardness and tensile strength, yet retains much of the impact strength of the alloy. Goodheart-Willcox Publisher Figure 10-19. The yield strength and tensile strength of UNS G10800 steel for railroad rails increases as the lamellar spacing decreases. Elongation decreases only slightly. A G10800 steel with a pearlite lamellar spacing of 0.20 μm will hav e a yield strength of about 72 ksi (500 MPa). 0 0.10 0.20 0.30 Grade 900 0.40 200 400 600 0 10 20 30 EI 800 1000 1200 MPa 0 29 58 87 116 145 174 ksi Yield and tensile strength Elongation (%) Spacing of pearlite lamellae (μm) TS YS Buehler Ltd. Figure 10-17. If steel is cooled quickly to 1000°F (650°C), the pearlite lamellar spacing is very small. The ferrite and cementite phases present in this sample are the same as in Figure 10-12, but the microstructure here is a fine pearlite. This narrower spacing is due to the lower soak temperature. Growth direction Cementite Fe3C Lamellar spacing Ferrite Ferrite Austenite Carbon diffusion Goodheart-Willcox Publisher Figure 10-18. The transformation from austenite to pearlite occurs as the ferrite and cementite platelets grow side by side, from left to right in the figure. The carbon in the austenite moves away from the growing ferrite and into the cementite platelets. Copyright Goodheart-Willcox Co., Inc.