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Text C: «METALWORKING AND METAL PROPETIES»

An importantfeature of hot working is that itpro­vides theimprovement of mechanicalproperties of met­als . Hot-working (hot-rolling or hot-forging)eliminates porosity, directionality, andsegregation that are usu­ally present in metals. Hot-worked products have better ductility and toughness than the unworkedcasting. Dur­ing the forging of a bar, the grains of the metal become greatlyelongated in the direction of flow. As a result, the toughness of the metal is greatly improved in this direction andweakened in directionstransverse to the flow. Good forging makes the flow lines in thefinishedpart oriented so as to lie in the direction of maximum stress when the part is placed in service.

The ability of a metal to resist thinning and fracture during cold-working operations plays an important role in alloy selection. In operations that involve stretching, the best alloys are those which grow stronger with strain (are strain hardening) — for example, the copper-zinc alloy, brass, used for cartridges and the aluminum-mag­nesium alloys in beverage cans, which exhibit greater strain hardening.

Fracture of the workpiece during forming can result frominner flaws in the metal. These flaws often con­sist of nonmetallicinclusions such as oxides or sulfides that aretrapped in the metal duringrefining. Such in­clusions can beavoided by proper manufacturing pro­cedures.

The ability of different metals toundergo strain var­ies. The change of the shape after one forming operation is often limited by thetensile ductility of the metal. Met­als such as copper and aluminum are more ductile in such operations than other metals.

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