Post 151 -by Gautam Shah
Metals in cold and hot state can be deformed into useful shapes. The shape, function, and appearance of metal objects are largely determined by the type of metal used. Precious metals like the gold and silver is 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 characteristics.
The surface quality of a metal begins to emerge during the conversion processes like casting, forging or rolling. Even when some of the processes are cold working, there is a rise in temperature that affects the quality of the surface. In hot conversion processes the reheating conditions, in-line scale removal, rolling temperature, and cooling rate, all determine the surface quality of the product. All these also 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. 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.
Literally hundreds of metalworking processes have been developed 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 a set 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.
Frequent heating and graduated cooling anneal the metal mass more ductile or softer. Similarly sudden cooling by quenching in oil or water, causes the surface to cool much faster then the inner or core mass. This makes a surface of metal objects harder. Annealed and surface hardened materials have not only different structural properties but also have special surface qualities, (e.g. colour, hardness, etc.).
Metal grains or crystals are far from perfect. Due to the imperfect structure, metals are capable of taking shock loads and reversal of stress, unlike non metal compounds which have very regular crystal structure. When metals, such as steel is used at high temperatures and under uninterrupted stress as in case of boilers, jet engines, power house turbines, hot discharge nozzles, they yield very slowly, stretch, and eventually fracture. Metal components, which under go stress reversals very frequently, fail due to fatigue. These are more pronounced in bridges, crankshafts, etc.
Metals are heated and cooled, without reaching to a melting stage, to provide hardening, strengthening, softening, improved formability, improved machinability, stress relief and improved dimensional stability. These thermo mechanical processes are known as annealing, normalizing, stress relief anneals, quench hardening, tempering, nitriding, martempering, austempering, carburizing, solution anneal, ageing, etc. All metals and alloys in common use are heat treated at some stage during processing. Iron alloys, however respond to heat treatment in a unique way because of the multitude of phase changes which can be induced.