Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 209 10.3.3 UNS G10950 Steel UNS G10950, an iron-carbon alloy with 0.95% carbon, is completely austenite a t point 6 in Figure 10-13. When the alloy cools to point 7, below the Acm line, some cementite particles form in the austenite along grain boundaries. When the metal cools to point 8 in Figure 10-13, all of the remaining austenite transforms into ferrite and cementite, as shown in Figure 10-14. This alloy is called a hypereutectoid alloy, because the percent carbon composition is greater than the eutectoid composition of 0.77% carbon. Control of the Cementite Structure The cementite along the grain boundaries forms through a diffusion-co ntrolled reaction, so the size of the layer is strongly affected by the rate of cooling. Slow cooling produces large grains, with a wide cementite layer around each one. Quicker cooling produces many small grains, with a thinner layer around each one. The coarser cementite from slow cooling reduces the elongation and ductility significantly, compared to the more quickly cooled samples. The fine cementite also increases the hardness and wear resistance substantially. Applications of G10950 Steel Skid plates on snowplows and earthmoving equipment need the abrasion resistance of ver y high-carbon steels such as UNS G10950. Goodheart-Willcox Publisher Figure 10-13. When a UNS G10950 steel is cooled from 1800°F (980°C), it transforms first into cementite and austenite, then into cementite and ferrite phases. 800 1000 0 1 2 Fe Austenite Acm A1 Ferrite plus austenite Ferrite 1200 1400 1800 2000 Temperature, ° F Ferrite and cementite 0.95%C Percent carbon Austenite and cementite 6 7 8 1340°F ASM International Figure 10-14. This is a micrograph of a 1.12% carbon alloy cooled at a moderate rate from 1800°F (980°C). Cementite has precipitated along the austenite grain boundaries. Upon cooling to room temperature, the austenite transformed to cementite bands around the austenite grains, and these grains formed ferrite and cementite in the form of pearlite. 20 μm Cementite layer Pearlite colonies Pearlite colonies Pearlite colonies Copyright Goodheart-Willcox Co., Inc.