199 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 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 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.