206 Section 3 Ferrous Metallurgy The composition and temperature where all three lines touch, at 1341°F (727°C) and 0.77% carbon, is called the eutectoid point. In production, the A1 temperature is often referred to as the austenitizing temperature, when heated steel completely transforms from ferrite and cementite into austenite. Consider, for example, a piece of UNS G10200 steel (AISI 1020, 0.2% carbon) for an auto engine part. The piece is heated to 2200°F (1200°C) for forging, as indicated by the red dot (point 1) on the phase diagram in Figure 10-8. Between 2200°F (1200°C) and 1550°F (840°C), the alloy of iron (Fe) with 0.2% carbon (C) is single-phase austenite. When the part cools below 1550°F (840°C), it crosses the A3 phase boundary line and enters a two-phase region. Here it transforms into ferrite plus austenite, with increasing amounts of ferrite as the temperature drops to the green dot, point 2. Upon cooling further, the metal crosses the A1 phase boundary and enters another two-phase region, indicated by the blue dot, point 3. Here, all the austenite that existed above the A1 boundary has transformed into ferrite plus cementite. Since the different phases have different crystal structures, ductility, and bulk forming characteristics, clearly both temperature and composition influence how well this alloy forms. 10.3 Transformations of Different Steel Compositions The phase diagram tells what phases are present at each composition and temperature. It does not tell you what forms, or microstructures, will exist when the metal cools to room temperature. To understand the microstructures developed, 800 1000 1 2 Austenite Ferrite plus austenite Ferrite Austenite and liquid 1200 1400 1600 1800 2000 2200 2400 Temperature, ° F Austenite and cementite Ferrite and cementite Percent carbon Goodheart-Willcox Publisher Figure 10-7. Most iron-carbon alloys contain between 0.08% and 0.80% carbon. The phase transformations manipulated during production occur between 1300°F and 1700°F (700°C and 930°C). Hot forging and extrusion are done between 2000°F and 2400°F (1090°C and 1320°C). 800 1000 0 2 Fe 0.2%C Austenite A3 Acm 1 2 3 A1 Ferrite plus austenite Ferrite 1200 1400 1340°F 1800 2000 Temperature, ° F Ferrite and cementite 0.77%1 Percent carbon Goodheart-Willcox Publisher Figure 10-8. A sample of UNS G10200 steel is single-phase austenite at the red dot (point 1), then two-phase ferrite plus austenite at the green dot (point 2), and then two-phase ferrite plus cementite at the blue dot (point 3). Copyright Goodheart-Willcox Co., Inc.