210 Section 3 Ferrous Metallurgy Steel Terminology A great many terms are used to describe different alloys of iron and carbon, because s mall differences in carbon content have a dramatic effect on the microstructures. Alloys with less carbon than the eutectic composition of 0.77% carbon are called hypoeutectoid—“hypo” meaning “less than.” Alloys with more than 0.77% carbon are called hypereutectoid— “hyper” meaning “more than.” Hypereutectoid alloys produce layers of cementite around grains upon cooling, in addition to pearlite. Hypoeutectoid alloys produce only pearlite. Common usage defines three categories of carbon steel: low carbon, medium carbon, and high carbon. These terms relate directly to the phase diagram, as illustrated in Figure 10-15. The categories are useful in practice because each category responds differently to cold work and heat treatments. DID YOU KNOW? DID YOU KNOW? Goodheart-Willcox Publisher Figure 10-15. Hypoeutectoid (less than 0.77% carbon) and hypereutectoid (greater than 0.77% carbon) descriptions are based on the phase diagram and produce significantly different microstructures. The low-, medium-, and high-carbon designations are related to differences in processing and performance. 800 Medium carbon Low carbon High carbon Hypoeutectoid Hypereutectoid 1000 0 0.2 0.5 1 2 Fe Austenite Ferrite plus austenite 1200 1400 1800 1600 2000 Temperature, ° F Ferrite and cementite 0.77% Percent carbon Austenite and cementite 1340°F Rod mills use long rods to grind ore as they tumble within the turning mill, Figure 10-16. The grinding rods within the mill make use of the wear resistance provided by the hypereutectoid carbon in G10950 steel. Minerals, such as ore to be crushed, are placed into the cylindrical chamber with the long steel rods. As the chamber is rotated, the rods fall onto the rock-size minerals, crushing and reducing the minerals to grit or powder. Some highly abrasive applications specify alloys with even more carbon. Copyright Goodheart-Willcox Co., Inc.