Metallurgy Fundamentals, 6th Edition Page 205 (223 of 528)

Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 205 2200°F (1200°C) for hot-working, or near room temperature for cold-working. Thus, the portion of the phase diagram of greatest interest is the corner shown in Figure 10-7. 10.2.2 A1, A3, and Acm Transformation Boundaries Three transformation lines, the A1, A3, and Acm lines, are marked on Figure 10-8. The most important of these is the A1 transformation. Above 1341°F (727°C), much of the steel is austenite, or face-centered cubic crystal structure. As the metal drops below 1341°F (727°C), the austenite transforms into ferrite and cementite. The A1 line marks this transition. At temperatures and compositions above the A3 line, all of the steel is austenite. Below the A3 line, some is austenite, while some transforms into ferrite, the body- centered cubic crystal structure. At temperatures and compositions above the Acm curve, the metal is entirely austenite. Below and to the right of the Acm line, the iron metal contains particles of cementite, Fe3C. Delta iron and liquid Delta and austenite Austenite Ferrite Ferrite and cementite Austenite and cementite Steel Cast iron Austenite and liquid Austenite and liquid Carbon in liquid iron Cementite and liquid 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 Delta Percent carbon 0 Fe Fe3C 1 2 3 4 5 6 6.67% Temperature, ° F 2.14% Ferrite plus austenite Goodheart-Willcox Publisher Figure 10-6. Iron-carbon alloys between 2.14% and 6.67% carbon solidify with large cementite particles. This is the domain of cast iron. Copyright Goodheart-Willcox Co., Inc.

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Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 205 2200°F (1200°C) for hot-working, or near room temperature for cold-working. Thus, the portion of the phase diagram of greatest interest is the corner shown in Figure 10-7. 10.2.2 A1, A3, and Acm Transformation Boundaries Three transformation lines, the A1, A3, and Acm lines, are marked on Figure 10-8. The most important of these is the A1 transformation. Above 1341°F (727°C), much of the steel is austenite, or face-centered cubic crystal structure. As the metal drops below 1341°F (727°C), the austenite transforms into ferrite and cementite. The A1 line marks this transition. At temperatures and compositions above the A3 line, all of the steel is austenite. Below the A3 line, some is austenite, while some transforms into ferrite, the body- centered cubic crystal structure. At temperatures and compositions above the Acm curve, the metal is entirely austenite. Below and to the right of the Acm line, the iron metal contains particles of cementite, Fe3C. Delta iron and liquid Delta and austenite Austenite Ferrite Ferrite and cementite Austenite and cementite Steel Cast iron Austenite and liquid Austenite and liquid Carbon in liquid iron Cementite and liquid 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 Delta Percent carbon 0 Fe Fe3C 1 2 3 4 5 6 6.67% Temperature, ° F 2.14% Ferrite plus austenite Goodheart-Willcox Publisher Figure 10-6. Iron-carbon alloys between 2.14% and 6.67% carbon solidify with large cementite particles. This is the domain of cast iron. Copyright Goodheart-Willcox Co., Inc.

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204 Section 3 Ferrous Metallurgy 10.2.1 Phase Diagram Regions Important to Processing Any liquid iron-carbon alloy with 2.14% to 6.67% carbon will form cast iron upon cooling to room temperature, made of ferrite and large cementite particles. A liquid iron-carbon alloy with 0.022% to 2.14% carbon will solidify to steel, made of ferrite and fine cementite. The division between steel and cast iron composition is indicated by the vertical line at 2.14% carbon in Figure 10-6. Most steel working materials use compositions determined by the designer and specified on the purchase order, so the focus of this chapter is mostly on the effects of temperature changes on a single composition. Most steel alloys contain less than 1.0% carbon. Processing is usually done between 1800°F (980°C) and Delta iron and liquid Delta and austenite Austenite Ferrite plus austenite Ferrite Ferrite and cementite Austenite and cementite Austenite and liquid Liquid iron Cementite and liquid Cementite 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 Delta Percent carbon 0 1 2 3 4 5 6 Temperature, ° F Goodheart-Willcox Publisher Figure 10-5. This iron-cementite phase diagram indicates the phases of iron and cementite present across a wide range of temperatures and compositions. Regions with only one phase, known as phase domains, are: liquid iron with dissolved carbon, delta iron (bcc iron with some carbon), austenite (fcc iron with dissolved carbon), cementite (Fe3C), and ferrite (bcc iron with almost no carbon). Regions of mixtures of two phases are: delta plus liquid, liquid plus austenite, liquid plus cementite, delta plus austenite, austenite plus ferrite, austenite plus cementite, and ferrite plus cementite. Copyright Goodheart-Willcox Co., Inc.

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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.

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Page 2Section 1 Introduction to Metallurgy click to expand contents

Page 42Section 2 Chemistry and Mechanics of Metals click to expand contents

Page 120Section 3 Ferrous Metallurgy click to expand contents

Page 318Section 4 Nonferrous Metallurgy click to expand contents

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