Metals work hardening

It is thought that the increased hardness (Tab. 6.11) is due in part as a result of the reduced pore density and the increase in dislocation density caused by work hardening. It is well known that raising the temperature of a metal work hardens that metal and it is thought that the implosion of cavitation bubbles dose to the electrode raises the microscopic temperature. In addition collapse will produce surface impacts by solvent also generating work hardening. 

Fabrication Expanding the tube into the tube sheet reduces the tube wall thickness and work-hardens the metal. The induced stresses can lead to stress corrosion. Differential expansion between tubes and shell in fixed-tube-sheet exchangers can develop stresses, which lead to stress corrosion. 

More uniform results may be expected if a substantial layer of metal is removed from the specimens to ehminate variations in condition of the original metaUic surface. This can be done by chemical treatment (pickling), electrolytic removal, or grinding with a coarse abrasive paper or cloth, such as No. 50, using care not to work-harden the surface. At least 2.5 X 10 mm (0.0001 in) or 1.5 to 2.3 mg/cm (10 to 15 mg/iu") should be removed. If clad alloy specimens are to be used, specif attention must be given to ensure that excessive metal is not removed. After final preparation of the specimen surface, the speci-... 

Pure metals are very soft indeed, and have a high ductility. This is what, for centuries, has made them so attractive at first for jewellery and weapons, and then for other implements and structures they can be worked to the shape that you want them in furthermore, their ability to work-harden means that, after you have finished, the... 

The result is work-hardening the steeply rising stress-strain curve after yield, shown in Chapter 8. All metals and ceramics work-harden. It can be a nuisance if you want to roll thin sheet, work-hardening quickly raises the yield strength so much that you have to stop and anneal the metal (heat it up to remove the accumulated dislocations) before you can go on. But it is also useful it is a potent strengthening method, which can be added to the other methods to produce strong materials. 

Niobium is always found in nature associated with tantalum and it closely resembles tantalum in its chemical and mechanical properties. It is a soft ductile metal which, like tantalum, work hardens more slowly than most metals. It will in fact absorb over 90% cold work before annealing becomes necessary, and it is easily formed at room temperature. In addition, welds of high quality can be produced in the metal. In appearance the metal is somewhat similar to stainless steel it has a density slightly higher than stainless steel and a thermal conductivity similar to 1% carbon steel. 

Platinum work-hardens at approximately the same rate as palladium or copper, a 25% cold reduction in thickness raising the hardness from about 40 HV in the fully annealed state to 75 HV. Cast platinum is usually slightly harder than the same grade of metal in the wrought and annealed state. 

The rolling process is used to transform cast aluminum ingot into any one of a number of intermediate or final products. Pressure exerted by the rollers as aluminum is passed between them flattens the metal and may cause work hardening. 

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