328 Section 4 Nonferrous Metallurgy Annealing Troubleshooting A degraded oven heating element or seizing of a belt drive in a continuous-feed oven can change the annealing temperature or time, producing changes in the microstructure and product performance. PRACTICAL METALLURGY PRACTICAL METALLURGY Another process consideration is that two different procedures for achieving a certain final size may produce different microstructures, and hence different properties. For example, consider two alternative procedures for reducing a metal strip from 0.200″ (5.00 mm) to 0.020″ (0.50 mm), illustrated in Figure 15-8. Alert technicians can assure that correct routing instructions are followed, and that questionable procedures are checked with an engineer before committing the coil of metal. 15.3 Strengthening through Alloying Nonfe rrous metals can be strengthened by cold work, and some alloy additions increase the effects of cold work. Certain alloy additions form large precipitates that increase yield strength even at hot-working temperatures, and some nonferrous alloys can be heat-treated to dramatically increase strength and hardness. Initial: 0.200” (5.00 mm) Reduce: 0.140” (3.56 mm) Reduce: 0.040” (1.00 mm) Cold roll: 90% Final: 0.020” (0.50 mm) Finish: annealed, 0.020” (0.50 mm) Anneal A Initial: 0.200” (5.00 mm) Reduce: 0.140” (3.56 mm) Reduce: 0.040” (1.00 mm) Final: 0.020” (0.50 mm) Finish: annealed, 0.020” (0.50 mm) Anneal Anneal Cold roll: 67% B Goodheart-Willcox Publisher Figure 15-8. One procedure for producing 0.020″ (0.50 mm) annealed strip from 0.200″ (5.00 mm) is to roll it to final gage in two passes, then anneal it. Procedure A illustrates a two-stand rolling mill, followed by a recrystallization anneal in a box oven with a vertical sliding door. Another procedure (B) is to do a partial reduction in a single-stand mill, anneal the strip, and then make the final reduction to the finish gage of 0.020″ (0.50 mm), with another recrystallization anneal. The final strip dimensions are the same. The percent of cold work before annealing will control the density of dislocation tangles. The density of dislocation tangles will impact the recrystallized grain size and the minimum recrystallization temperature. In general, a smaller final reduction will produce larger recrystallized grains. Thus, the two procedures may produce different microstructures, even though the two finished strips appear identical to the eye. Copyright Goodheart-Willcox Co., Inc.