Chapter 15 Introduction to Nonferrous Metals 327 15.2.2 Improving Metal Propert ies by Cold Work When the workpiece is deformed and the dislocation tangles remain, we call it cold work. Cold work occurs at temperatures below one-half the melting point on the kelvin temperature scale. The dislocation tangles developing during cold work increase strength and reduce ductility. 15.2.3 Improving Metal Structur e by “Warm Work” During bulk deformation at temperatures between hot work and cold work, the dislocation tangles develop, and recrystallization removes only the most severely tangled portions. The most severely stressed atoms move and relax severe stresses, while the rest of the tangles remain. Work at this temperature is sometimes referred to as warm work, and the change in the microstructure is called recovery. Eliminating the most severely stressed tangles allows some dislocation motion. Warm-worked metal has some ductility and somewhat higher strength than the recrystallized metal. 15.2.4 Restoring Ductility by Anne aling Annealing metals is a thermal process. Recovery and recrystallization occurs when the temperature is high enough to allow the atoms to move slightly, and the changes happen faster at higher temperatures. Restoring ductility after the metal has been cold-worked is accomplished by annealing the metal at 55% to 65% of the melting temperature. The metal recrystallizes in a few minutes at temperature, to form new, strain-free, “clean” grains. 15.2.5 Different Process Sequences Ma y Produce Different Microstructures Heating slowly to the soak temperature tends to produce larger grains, while heating quickly produces finer grain size, higher yield strength, and better ductility. In large batch ovens, large coils of metal may require hours to reach the oven set-point temperature, which usually produces a larger grain size than well-spaced sheets of metal in a small oven that reaches the set point in four minutes. Therefore, production routing sheets need to state which oven to use, as well as the annealing temperature. Original grains Deformed grains Top roll Bottom roll New grains growing Recrystalized new grains Goodheart-Willcox Publisher Figure 15-7. During hot-rolling and hot forging, metal is reduced in thickness, or worked, and the resulting dislocation tangles are immediately removed by dynamic recrystallization. Copyright Goodheart-Willcox Co., Inc.