Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 211 10.4 Effect of Cooling Rate on Structure and Properties So far, only the cooling of steel at a moderate rate has bee n considered. The phase diagrams in this chapter apply only under that condition. Even differences in that moderate cooling rate can change the pearlite lamellar spacing, and thus strength and other properties. But how fast is “moderate”? Could you splash some water onto steel workpieces to cool them faster and speed up production, and yet still maintain a pearlitic microstructure with desirable properties? 10.4.1 Slow to Moderate Cooling—Pearlite The processes of quenching and soaking are important aspects in controlling t he rate of cooling of a metal workpiece, and thus producing desired properties. Quenching is the act of thrusting a sample into a salt bath, water, or other liquid to change the sample temperature rapidly to the bath temperature. A salt bath is a container holding melted salt that can be heated to a wide range of temperatures. Soaking a piece of metal means holding the workpiece at a constant temperature until desired structural changes take place. Suppose you cool a small sample of UNS G10800 steel from 1500°F (820°C) by quenching it into a salt bath and then soaking it. This small Goodheart-Willcox Publisher Figure 10-16. In this rod mill, fist-sized pieces of metal ores or other minerals are fed into the cylindrical chamber from the left. As the chamber rotates, the grinding rods are lifted up and dropped onto the minerals. At the far end, the exiting minerals have been reduced to a fine grit. Grinding rods Copyright Goodheart-Willcox Co., Inc.