Guided Tour 199 198 Chapter Opening Image. Here, several tons of steel will soon be quenched into agitated water. The method and medium used to quench or cool hot steel, together with the speed of cooling, have dramatic effects on what microstructures develop, which determines the final properties of the metal. Steelwind Industries, Inc., Oak Creek, WI CHAPTER 10 Phase Diagrams: The Road Map to Phases and Structures CHAPTER OUTLINE 10.1 Solutions and Mixtures You Know—Water and Antifreeze 10.1.1 Water-Antifreeze Solution 10.1.2 Composition and Temperature Determine Phases Present 10.2 The Iron-Carbon Phase Diagram 10.2.1 Phase Diagram Regions Important to Processing 10.2.2 A 1 , A 3 , and A cm Transformation Boundaries 10.3 Transformations of Different Steel Compositions 10.3.1 UNS G10200 Steel 10.3.2 UNS G10800 Steel 10.3.3 UNS G10950 Steel 10.4 Effect of Cooling Rate on Structure and Properties 10.4.1 Slow to Moderate Cooling—Pearlite 10.4.2 More Rapid Cooling 10.5 Additional Thermal Processing to Improve Properties 10.5.1 Tempering Martensite 10.5.2 Spheroidizing Pearlite LEARNING OBJECTIVES After studying this chapter, you will be able to: • Understand how the Fe-C phase diagram describes the phases present in iron-carbon alloys. • Understand how different cooling paths in steel produce different microstructures. • Define by examples the difference between “phase” and “microstructure.” • Explain why UNS G10200 (Fe with 0.2% C), G10800 (Fe with 0.8% C), and G10950 (Fe with 0.95% C) steels develop different microstructures with the same moderate cooling rate. • Understand the difference between moderate cooling and rapid cooling in terms of the isothermal temperature transformation diagram. • Discuss the different microstructures developed in carbon steel by slow cooling and very fast cooling to room temperature. • Describe the major properties resulting from the microstructures developed by moderate, interrupted, and rapid cooling of UNS G10800 steel from 1500°F (816°C) to room temperature. • Understand why tempering improves the toughness of martensitic steel. • Understand how a spheroidizing anneal changes pearlite microstructure, and why this microstructure is easily formable. TECHNICAL TERMS bainite deep-drawing eutectoid point hypereutectoid alloy hypoeutectoid alloy isothermal transformation (IT) diagram martensite pearlite nose phase boundaries phase diagram phase domain quenching retained austenite salt bath soaking spheroidizing tempering Introduction Up to this point, the discussion of the production of steel has focused on what is called plain carbon steel. There is little need for thermal processing beyond annealing to make the goods you have read about. Slabs and billets are worked into semifinished goods such as strip, plate, or bars by hot-working and annealing. Metal for finished goods, such as washing machines and toasters, may also be annealed during manufacture. But heat treatment is important to create desired properties in certain products. To understand the heat treatment of steel, we must understand how iron changes with different carbon compositions and at different temperatures. The goal with this chapter, Figure 10-1, is for you to learn to decipher the graph of alloy composition, temperature, and phase that is called a phase diagram. With the information in a phase diagram, you can predict what is happening to a piece of metal as it is being processed. You will understand what steel microstructures form at different temperatures and alloy compositions. Arc Welding Arc welding is the most common method of joining metals. In shielded metal arc welding (SMAW), illustrated in Figure 9-22, the weld area is shielded by a flux, and a filler metal is added. Another common arc welding method, called gas metal arc welding (GMAW), uses an inert or chemically reducing cover gas in place of flux. There is no slag to chip off when a GMAW weld is complete, which is a distinct advantage. The setup used in GMAW is shown in Figure 9-23. When the arc is struck from a tungsten tip that does not melt and a cover gas is used, the process is called gas tungsten arc welding (GTAW). GTAW gives the welder great flexibility for arc positioning and filler feeding, but superior operator control is needed. The welder needs excellent training and experience for the more difficult procedure. Safe Arc Welding When arc welding, the welder must wear protective clothing and equipment. Clothing of synthetic material should be avoided in favor of cotton. Long-sleeved shirts and trousers are expected. Leather chaps and forearm protection are often used. The arc produces intense light, including UV radiation that sunburns exposed skin and permanently damages eyes quickly. Thus, opaque face shields and dark-tinted glass view ports are an absolute requirement. Even people 11 yards (10 m) away should not look directly at an arc. Of course, stray water on the workpiece or underfoot is always to be avoided. Yellow transparent plastic curtains should surround the welding area to filter out UV radiation from those not directly involved. SAFETY NOTE SAFETY NOTE Flux covering Electrode wire Welding arc Weld pool Slag Weld metal Joint Base metal Workpiece lead Base metal Joint Covered electrode Electrode clamp handle Electrode lead AC or DC power source Input power Electrode holder Goodheart-Willcox Publisher Figure 9-22. A shielded metal arc weld uses an electric arc from a filler alloy rod, or electrode, to the workpiece. The melted filler and arc melt the parent, disrupting the surface oxide. Flux melted from the filler rod protects the liquid metal by forming a liquid of silicon dioxide and other compounds over it. C i ht G dh t Will C I Tougher Rails Increasing the tensile strength by 29 ksi (200 MPa) effectively doubles the wear resistance of railroad rails, which doubles their service life. Railroad rails to carry modern larger freight cars are specified as “Grade 900A.” This is a UNS G10800 alloy fabricated with fine pearlite on the wear surface. Rails made in the late 19th century had less pearlite because the carbon content was below 0.80%, and the lamellar spacing was larger because the hot-rolled rails were cooled in air. One can imagine the care necessary to direct a water spray onto the head of the rail just long enough to bring the head down to 1000°F (540°C) but not cool it further, nor cool the web or foot of the rail so quickly. Slower cooling increases the elongation and toughness, which is desirable in the web and foot of the rail. The technicians and operators making the rail need to monitor the positioning of the spray heads and the water flow to assure that both hardness in the rail head and toughness in the web and foot are maintained. PRACTICAL METALLURGY PRACTICAL METALLURGY Learning Objectives clearly identify the knowledge and skills to be obtained when the chapter is completed. Objectives are tied to the chapter outline, to the text, and to our end- of-chapter materials. Technical Terms list the important terms to be learned in the chapter. Introduction provides an overview and preview of the chapter content. The instructional design includes student-focused learning tools to help students succeed. This visual guide highlights the features designed for the textbook. Illustrations have been designed to clearly and simply communicate the specific topic. Illustrations have been updated for this edition. Photographic images have been updated to show the latest equipment and practices. Safety Note features point out safety-related issues to help you avoid potentially dangerous materials and practices. Practical Metallurgy features demonstrate how metallurgical knowledge applies to everyday jobs and situations. Chapter Outline provides a preview of the chapter topics and serves as a review tool.