Post 172 – by Gautam Shah
Metals are deformed into useful shapes in their cold and hot states. Besides this, the type of metal used, determines the shape, function, and appearance of the formed object. Precious metals like the gold and silver, are comparatively softer to work with, whereas base metals such as the copper, tin, lead, and iron and their alloys like bronze, brass, and pewter may differ widely in their work characteristics.
Metal working processes affect both the core and surface portions of the object depending on the shape and size of the sub-parts of the object. Some of the heavy duty cold working processes may change the surface quality, even while not affecting the core section. For all Hot working processes the range of temperature and the cooling method and period affect the quality of material. Metals are susceptible to stresses of working processes depending on their constitution and pre-process grain structure.
Some of the processes though are cold working, there is a rise in temperature that affects the quality of the surface. In cold-rolling a specific surface, roughness is rolled into the strip at the tempering-mill to improve the deep-drawing operation, and to assure a good surface finish over the final product.
In hot conversion processes the reheating conditions, in-line scale removal, rolling temperature, and cooling rate, all determine the surface quality of the product. The surface quality post working processes affect the atmospheric corrosion, paint-ability, and many other subsequent operations. Sometimes the final pass in hot-rolling generates specific surface patterns, such as with the protrusions on reinforcing bars or checkers on floor plates, ribs.
The visually perceptive surface quality of a metal begins to emerge during the conversion processes like casting, forging or rolling. The working processes can be said to generally affect the constitutional aspect of the material.
Literally hundreds of metalworking processes have been developed some general and many other for specific purposes, but these can be divided into SIX broad groups: Casting, Rolling, Extrusion, Drawing, Forging, and Sheet-metal forming.
The first five processes subject a metal to large amounts of strain. However, if deformation occurs at a sufficiently high temperature, the metal will re-crystallize, that is, its deformed grains will be consumed by the growth of new, strain-free grains. For this reason, a metal is usually rolled, extruded, drawn, forged above its re-crystallization temperature. This is called hot working, and under these conditions there is virtually no limit to the compressive plastic strain to which the metal can be subjected. Other processes are performed below the re-crystallization temperature. These are called cold working. Cold working hardens metal and makes the part stronger. However, there is a definite limit to the strain that can be put into a cold part before it cracks.