208 Section 3 Ferrous Metallurgy 10.3.2 UNS G10800 Steel When UNS G10800 steel (iron with 0.8% carbon) is heated to point 4 on Figu re 10-11, it forms a single phase, austenite, with all the carbon in solution. As this alloy is cooled from point 4 to point 5, it completely transforms to ferrite and cementite in a pearlite microstructure. In three dimensions, you would see that the cementite forms layers of platelets. Cutting across these platelets for the metallurgical sample makes them appear as bands of ferrite and cementite, as shown in Figure 10-12. Applications of G10800 Steel A major use of G10800 steel is for railroad rails. Rails must withstan d wear, especially on tight turns, and must exhibit a high degree of toughness so they will not chip or crack. An alloy designated “Grade 900” in the railroad industry, with 0.8% carbon, is completely pearlite. Grade 900 resists rolling wear better than an older grade made with G10500 (0.5% carbon), which had a partly ferrite and partly pearlite microstructure. G10800 steel is often welded, but care must be taken to not cool the weldment in this high-carbon alloy too fast. Hence, electric arc welding is preferred for Grade 900 railroad rails, where the post-weld cooling can be controlled more easily than with a thermite weld process. Another major application of pearlite steel is for structural cables for bridges. The combination of strength and good ductility are very desirable where brittle failure absolutely must be avoided. Light: ferrite (bcc Fe) 100 μm Dark: cementite (Fe3C) ASM International Figure 10-12. When UNS G10800 steel is heated to 1500°F (820°C), then cooled in air to somewhat below 1341°F (727°C), it develops the banded structure called pearlite. The distance between layers is called the lamellar spacing. 800 1000 0 2 Fe 0.8%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 4 5 1340°F Goodheart-Willcox Publisher Figure 10-11. A UNS G10800 steel transforms from 100% austenite, point 4, to a mixture of ferrite and cementite, point 5, when it cools at a moderate rate. The microstructure of this mixture is 100% pearlite. Copyright Goodheart-Willcox Co., Inc.