332 Section 4 Nonferrous Metallurgy and alloy elements. If the furnace time at the final step is continued further, the second-phase particles grow too large, the stresses surrounding each particle relax, and the strength and hardness drop. This is called overaging. Inside the metal, the pre-precipitates grow into particles. By the time these particles become visible at 1000X magnification, they have lost the coherent interface match, which results in the drop in strength and hardness of the alloy. Figure 15-12 shows the effect of aging on the hardness of an aluminum-copper (Al-Cu) alloy. In some metals, the overaged alloy resists corrosion better than when aged to peak strength, so overaging may be desirable. UNS S17700 (17-7PH) Stainless Steel As discussed in Chapter 13, UNS S17700 (17-7PH) stainless steel achieves strength by precipitation hardening, just as the UNS C17200 alloy does. One percent aluminum in the stainless steel strengthens the alloy with an aluminum-iron precipitate when it is heat-treated in the cycle shown in Figure 15-11. The high-temperature solutionizing step is 1800°F (980°C), the quench is in oil, and the artificial aging is done at 1200°F to 1400°F (650°C to 760°C) for one hour. In the quenched condition, the metal has all the formability and machinability of an austenitic stainless steel, and after aging it has substantially higher strength. DID YOU KNOW? DID YOU KNOW? 15.4 Reactions of Nonferrous Metals during Refining and Use Just like ferrous metals, nonferrous metals must first be separated from their ores and refined before they can be used. This is accomplished using certain chemical or electrical reactions. The reactions of certain nonferrous metals, such as their rate of oxidation, can be put to good use. 15.4.1 Extracting and Refining Nonferrous Metals All metals are obtained from the earth by mining ores or by extraction from mineral-rich water called brine. Refining metal from the ores is done using a variety of chemical and electrolytic methods. In oxide form, some ores, such as iron oxide, can be reduced to metal using carbon, producing metal and carbon dioxide (CO2). Some ores are ground to a fine powder, then dissolved in a chemical solution and refined electrolytically, which uses an electric current to reduce the metal from the oxide to metal. Some metal ores, found as sulfide compounds, are first roasted in air to convert them to metal oxides plus sulfur dioxide (SO2). The sulfur dioxide by-product is converted into sulfuric acid (H2SO4) and sold, or it escapes into the air. 0.001 100 90 80 70 60 0.01 0.1 10 Time, days Hardness, Vickers pyramid 100 1000 10,000 1 As-quenched hardness Goodheart-Willcox Publisher Figure 15-12. Shown here is the hardness of an Al-4% Cu alloy solutionized, quenched, and then aged at 374°F (190°C). A part made of th is alloy would reach maximum hardness when artificially aged for one day. Aging for over one day causes “decoherence” to the precipitates, and strength drops. Note the logarithmic time scale on the x-axis. Copyright Goodheart-Willcox Co., Inc.