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It has actually long been understood that the properties of some metals could be altered by heat treating. Grains in metals tend to grow larger as the metal is heated. A grain can grow larger by atoms moving from another grain that may ultimately disappear. Dislocations can not cross grain borders quickly, so the size of grains determines how quickly the dislocations can move. As expected, metals with little grains are stronger however they are less ductile. Figure 5 shows an example of the grain structure of metals. Satiating and Solidifying: There are many methods which metals can be heat treated. Annealing is a softening procedure in which metals are heated and then permitted to cool slowly. The majority of steels may be solidified by heating and quenching (cooling rapidly). This process was utilized quite early in the history of processing steel. In fact, it was thought that biological fluids made the best quenching liquids and urine was often used. In some ancient civilizations, the red hot sword blades were often plunged into the bodies of hapless detainees! Today metals are quenched in water or oil. In fact, quenching in seawater solutions is much faster, so the ancients were not completely GI Pipe wrong.Quenching lead to a metal that is extremely tough but likewise fragile. Gently heating up a hardened metal and enabling it to cool slowly will produce a metal that is still difficult but likewise less brittle. This procedure is known as tempering. (See Processing Metals Activity). It leads to many small Fe3C speeds up in the steel, which block dislocation movement which thus supply the strengthening.Cold Working: Since plastic contortion results from the motion of dislocations, metals can be strengthened by avoiding this motion. When a metal is bent or shaped, dislocations are produced and move. As the number of dislocations in the crystal increases, they will get twisted or pinned and will not have the ability to move. This will enhance the metal, making it more difficult to deform. This procedure is known as cold working. At greater temperatures the dislocations can reorganize, so little reinforcing occurs.You can try this with a paper clip. Unbend the paper clip and flex one of the straight areas back and forth a number of times. Imagine what is happening on the atomic level. Notification that it is more difficult to flex the metal at the same location. Dislocations have actually formed and become twisted, increasing the strength. The paper clip will eventually break at the bend. Cold working undoubtedly only works to a specific level! Too much contortion results in a tangle of dislocations that are not able to move, so the metal breaks instead.Heating removes the impacts of cold-working. When cold worked metals are warmed, recrystallization occurs. New grains form and grow to take in the cold worked portion. The new grains have less dislocations and the original residential or commercial properties are brought back.

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