Titanic Steel There is reason to believe that the steel used in the RMS Titanic was below the ductile– brittle transition temperature at the time the ship struck the iceberg. This added to the potential for brittle fracture of the hull rather than deformation upon impact. DID YOU KNOW? DID YOU KNOW? 11.9.2 Heat-Treated Steels Cannot Be Welded During welding, the parent metal in some of the heat-affected zone will exceed the critical (A 1 ) temperature, so the martensite and high strength will be eliminated. The design must accommodate the slightly lower strength of the steel in the weld area. If a part is to be both welded and heat-treated, then it must be welded first and then heat-treated. The Impact of Heat Treatment Heat treatment helps the environment by using less metal for a part and ensuring the part will last longer with less maintenance. These improved properties offset the cost to prevent environmental damage, provided that waste materials are treated properly. Any changes made to reduce the waste from the plant will improve on this balance. SUSTAINABLE METALLURGY SUSTAINABLE METALLURGY Did You Know? features provide high-interest facts related to the main topics. Sustainable Metallurgy features discuss advances in metal work and manufacturing that increase efficiency and lessen the impact on the environment. Chapter 4 Basic Structure of Metals 65 64 Section 2 Chemistry and Mechanics of Metals of the pan a mixture or a solution? The definition of mixture depends on the number of phases present in the total volume (in the pan), not the amount of one phase. At room temperature, the water could hold less than 1/2 cup of sugar in solution. When you heated it hot enough, you easily dissolved 2 cups of sugar in the same water. When the water cooled back to room temperature, it was then supersaturated. That is, the water had more sugar in it than it would “like” to hold. This is the same thing that happens with some nonferrous alloys (discussed in Chapter 15), as well as in some stainless steels (discussed in Chapter 13). The sugar takes a long time to grow crystals from the liquid. In most commercial alloys, the reaction occurs at a pace that can be controlled fairly easily. In similar cases involving metals, at the high temperature the alloy is a solution of the alloy elements. When stabilized at room temperature, the alloy is a mixture. Analysis How could you tell that the solution with sugar crystals at the bottom was a mixture and not a strange solution? How could you tell that the solution, with no sugar crystals visible at the bottom, was a solution? Suppose you did not touch a wet string end to some sugar crystals, but instead just put one end into the supersaturated solution of sugar in water. What do you suppose would happen? How could you do a test to see if you are right? of the water. What happens to the crystals as you stir? 4. When the sugar crystals are no longer visible, they have not evaporated. The mass of sugar remains in the pan, but it has dissolved into the water. You have made a solution of sugar in water. 5. Add a quarter of the cup of sugar, and stir again. If the water is warm, the sugar will all go into solution as before. 6. Add the rest of the 1 cup of sugar. This time, not all the sugar goes into solution. Some sugar crystals remain at the bottom of the pan even after you stir it gently for a minute or more. This is a mixture. You can see two phases. One phase is solid sugar, and the other phase is the liquid made up of water and dissolved sugar. 7. Put the pan on the stove and set it to a moderate heating rate, a bit less than halfway between simmer and full heat. Stir occasionally. As the mixture heats, the remaining sugar crystals will disappear. What phase or phases do you see in the pan now? What happened to the sugar crystals? 8. Add the second full cup of sugar. Stir occasionally as you continue to gently heat the pan. You may need to increase the burner heat setting slightly (not to full heat!). 9. As the sugar goes into solution, the boiling temperature will rise. Be extremely careful to avoid spilling hot solution, as it can burn you more than pure water. As the sugar goes into solution, the color of the liquid will shift from a translucent yellowish color to clear. The liquid will also become more viscous, as befits the syrup it is. 10. Bring the pan up to boiling, but do not let the solution boil over the sides. (A hot sugar-water solution will likely react with a glass stove top, pitting the surface.) Figure 4-23 shows the solution just as it begins to boil. Boiling for a short time will ensure that the sugar has completely dissolved into the water. 11. When all the sugar has gone into solution, take the pan off the heat and let it cool. 12. While the pan is cooling, dip about 1/2″ (12 mm) of one string into the liquid, then remove it. Mixtures, Solutions, and Phases—Sugar in Water Equipment • Small saucepan • Measuring cup (at least 1 cup volume) • Cooking stove • Three pieces of string, each about 10″ (25 cm) long • A pencil or a long, thin stick Materials • 2 cups granulated sugar (the most common kind) • 1 cup tap water The equipment and materials are shown in Figure 4-22. Procedure 1. Measure 1 cup of warm tap water into the measuring cup. The water should be room temperature or warm, not hot. Set the small saucepan on the counter (not stove), and pour the water into the small saucepan. 2. Dry the measuring cup, then measure out 1 cup of granulated sugar. 3. Add a small amount of sugar to the water, and stir. The sugar crystals can be seen at the bottom SELF-DISCOVERY John Falk Figure 4-22. Pictured is the equipment and materials you need for the trials of sugar in water. The person who did this trial used a small fry pan in place of a saucepan, and a laboratory heater instead of a cook stove. The digital thermometer on the right is not needed for this Self- Discovery, but was fun to use. John Falk Figure 4-23. The solution of water and sugar bubbles up slightly as it reaches the boiling temperature. The bubbles do not burst immediately, as with pure water, so reduce the heat as needed to keep the boiling solution inside the pan. This solution boiled at 219°F (104°C). The digital thermometer probe is on the right. John Falk Figure 4-24. A string is suspended in the sugar-water solution after the solution has cooled to room temperature. 13. While the string is still wet, touch it to some granulated sugar so some crystals stick to it. Let it dry. 14. Let the pan cool to room temperature. Then put the sugared tip of the string into the solution. Hang the string across the pencil or long stick so it is away from the edge of the pan, as shown in Figure 4-24. More than one string can be suspended in the solution. 15. Keep the pan covered to shield from airborne lint and dust, and let it sit for a week. During the week, check the end of the string in the liquid every day for crystal growth. The sugar crystal in Figure 4-25 developed in two days. Thoughts and Observations How many phases are present in the pan at the end of step 15? What are they made of? Are the contents John Falk Figure 4-25. A large crystal of sugar has grown on the string in just two days. Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. Chapter 5 Physical and Chemical Properties of Metals 89 88 Section 2 Chemistry and Mechanics of Metals 16. True or False? A single metal part is not sufficient in most applications, so other metal parts have to be attached to it. 17. True or False? Alloys with long freezing ranges are typically easier to weld. 18. During the _____ process, a filler metal is melted but the parent metals are not. A. welding B. brazing C. friction welding D. diffusion bonding 19. _____ is a metal’s ability to remain in the metallic condition. A. Sacrificial corrosion B. Surface corrosion C. Corrosion resistance D. Intergranular corrosion 20. True or False? Metals with higher corrosion potential are more electronegative than metals with less tendency to corrode. 21. _____ is the intentional joining of two metals in order to protect one from corrosion at the expense of the other. A. Surface corrosion B. Sacrificial corrosion C. Intergranular corrosion D. Corrosion resistance Apply and Analyze 1. Explain how the process of hot-working improves the properties of the resulting material. 2. Name and describe three inhibitors to dislocation. 3. Why does inhibiting the motion of dislocations increase the strength of a metal? 4. How can cold-worked metal be restored to a dislocation-free condition? 5. Why is it important to take thermal expansion into account when machining a part? 6. What can happen if a metal is worked in the freezing range? 7. List the three elements that exhibit ferromagnetism at room temperature. 8. Why is soldering the preferred joining method for electronic circuit boards? 9. Why is it important to protect steel from corrosion? Critical Thinking 1. How are chemistry and metallurgy related to each other? 5. When iron transforms to the ferrite phase, the iron carbide in the changed metal is called _____. A. cementite B. pearlite C. hot strip D. austenite 6. In the surface treatment known as _____, a coil of hot-rolled steel is dipped into an acid bath to remove the oxide scale and surface impurities formed during rolling and cooling. A. pinning B. edge dislocation C. cold-working D. pickling 7. True or False? Metal strength decreases during cold work. 8. True or False? Displacement by one row of atoms at a time happens in all metals. 9. Which of the following is used as a major mode of strengthening metal? A. Ferromagnetism B. Dislocation tangles C. Grain boundaries D. Bending 10. True or False? Recrystallized metal is just as formable as the original hot-worked metal. 11. The _____ of a metal depends on the atomic mass of the atom and the atomic spacing. A. density B. thermal energy C. joinability D. melting point 12. The heat required to raise the temperature of a unit mass of a material by one degree is called _____. A. thermal expansion B. thermal energy C. specific heat capacity D. thermal conductivity 13. True or False? When heated even slightly, a metal increases in length. 14. True or False? The units do not need to be consistent when calculating machining dimensions. 15. The physical property at the atomic level that causes particular materials to form or be attracted to magnets is known as _____. A. magnetic susceptibility B. electromagnetism C. Curie temperature D. ferromagnetism ferromagnetism. The ability to be magnetized is called magnetic susceptibility. • Joinability is the ease or difficulty of attaching two pieces of metal together. Common metallurgical joining processes include welding, brazing, and soldering. • Many metal parts must be machined into the final shape or to make a smooth surface. A machinability rating describes the ease or difficulty of cutting a certain workpiece alloy compared to a standard alloy. • The chemical properties of metals, such as corrosion and oxidation, are important considerations when working with metals. • Most metals tend to corrode and return to their natural oxidized state when exposed to typical moist air or immersed in water. • A battery that produces electrical power results when a highly electronegative metal is placed in a circuit with suitable electrolytes. Review Questions Answer the following questions using the information provided in this chapter. Know and Understand 1. Strands of cast steel cut into lengths that a mill can handle are called _____. A. dendrites B. grains C. austenites D. billets 2. True or False? A slower cooling rate produces smaller metal dendrite grains. 3. The process of _____ occurs any time a metal becomes hot enough for atoms from the disturbed region to realign into a grain with a definite crystal structure. A. bonding B. recrystallization C. corrosion D. hot-working 4. True or False? The forces that bond atoms across grains are less than the forces within the individual grains. Summary • Liquid metal has a shapeless structure with no repeating pattern. As it cools, it solidifies into a crystalline structure with the repeating pattern determined by the metal and temperature. • In the US today, most steel is first poured in continuous casting machines. The molds used are water-cooled so the metal solidifies quickly and does not melt the mold. • When large volumes of metal solidify, single crystal dendrites form. The dimensions depend greatly on the cooling rate in the mold. • A cast billet of steel has to be reduced in size for most applications. Through hot-working, it can be reduced by rolling, extrusion, or forging. • At hot-working temperatures, disrupted atoms re-form quickly into an aligned crystal structure. All metals recrystallize during this process if the temperature is high enough. • When steel cools below 1341°F (727°C), it undergoes a phase change, from austenite to ferrite. • Forging a part or rolling a strip thinner while the metal is near room temperature is known as cold work. • As steel cools and becomes ferrite, the newly formed grains are not perfect arrays. Some planes of atoms extend only partway through the grain, an imperfection in the crystal lattice called an edge dislocation. • No matter how it is developed, smaller grain size increases metal strength. Metal with large grains may exhibit less strength and ductility. • Physical properties of metals are those that can be measured without applying a force, such as density and melting point. • Thermal properties of metals are the ways in which they react to and conduct heat. They are important to consider when working with and producing parts with metals. • A key property of metals is their relatively high thermal conductivity compared to nonmetals. Metals are much better at conducting heat than most nonmetals. • Some metals, like silver, copper, and aluminum, are great conductors of electricity. • The physical property that causes specific materials to form or be attracted to magnets is called CHAPTER REVIEW Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. Self-Discovery activities allow students to perform hands-on activities at home or in class using easily obtained items to reinforce key concepts. Summary feature provides an additional review tool for you and reinforces key learning objectives. Know and Understand questions allow you to demonstrate knowledge, identification, and comprehension of chapter material. Apply and Analyze questions extend your learning and help you analyze and apply knowledge. Critical Thinking questions develop higher- order thinking, as well as problem solving, personal, and workplace skills.