326 Section 4 Nonferrous Metallurgy 15.2 Developing Desirable Properties by Working Metal Nonferrous alloys are processed to achieve microstructures with desirable properties. During fabrication, ductility and formability are often required. Once a product is in use, high strength is often required more than ductility. A part may need to be processed so that, when finished, it has electrical conductivity, toughness, low density, or a polished finish as its key property. 15.2.1 Improving Metal Pro perties by Hot Work When bulk deformation is done while the workpiece is hot enough to recrystallize, we call it hot-working. The temperature for hot-working is determined by the behavior of the metal. Nickel (Ni) can be hot-worked at 1251°F (677°C). Tin (Sn) is hot-worked at room temperature. When metal is hot-worked at between 60% and 75% of the melting-point temperature, the dislocation tangles disappear as fast as they are created in the alloy. This is called dynamic recrystallization, illustrated in Figure 15-7. Hot-working ingots of all metals disrupts large grains and large precipitate particles of the cast microstructure. Hot-worked microstructures are more uniform, with finer grains and, consequently, higher strength. As a result, hot-worked metal tends to be tougher and more ductile than cast ingots. Since hot-worked metal recrystallizes before it cools, the ductility is higher and the yield strength is at a minimum. Recrystallization Temperatures for Pure Metals Name Symbol Melting point Recrystallization temperature (55% of melting point, K) °F °C °F °C Tin Sn 449 232 40 5 Lead Pb 621 327 135 57 Zinc Zn 787 420 226 108 Aluminum Al 1221 660 465 240 Magnesium Mg 1202 650 454 235 Silver Ag 1763 962 763 406 Copper Cu 1985 1085 885 474 Iron Fe 2800 1538 1333 723 Nickel Ni 2651 1455 1251 677 Molybdenum Mo 4753 2623 2407 1320 Tungsten W 6191 3422 3198 1759 Goodheart-Willcox Publisher Figure 15-6. The recrystallization temperatures for select pure metals are listed here. The recrystallization temperature and degree of recrystallization depend on the amount of impurities and precipitates, prior cold work, time at soak temperature, and speed of heating to soak temperature. Copyright Goodheart-Willcox Co., Inc.