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Heat Treatment Heat treatment can make a metal harder, stronger, and more resistant to impact. This process consists, in general, of a series of operations involving controlled heating and cooling of metals in a solid state. The purpose is to change the metal's property or combination of properties so that the metal is more useful, serviceable, or safe for a particular application or design. Heat treatment can also make a metal softer and more ductile. No one heat-treating operation can produce all of these characteristics. Some properties are often improved at the expense of others. In being hardened, for example, a metal can become brittle. The various heat-treating processes are similar in that they all involve heating and cooling the metals. However, they differ in three important ways. The first two ways are the temperatures to which the metal is heated and the rate at which it is cooled. The third difference is the finished metal. The most common forms of heat treatment for ferrous metals are hardening, tempering, annealing, normalizing, case hardening, and hot-working. The paragraphs that follow discuss each of these treatments. Most nonferrous metals can be annealed and many of them can be hardened by heat treatment. However, only one nonferrous metal, titanium, can be case-hardened, and none can be normalized or tempered. Hardening. For most steels the hardening treatment consists of heating the steel to the correct temperature and then cooling it rapidly by plunging the hot steel into oil or brine. Although most steels must be cooled rapidly for hardening, a few can be cooled from the hardening (specified) temperature by air. Hardening increases the durability and strength of the steel, but it makes it less ductile. Many nonferrous metals can also be hardened and strengthened by the same method. Tempering. After the hardening treatment, steel is often harder than necessary and it is too brittle for most practical uses. In addition, rapid cooling from the hardening temperature causes internal stresses that can cause flaws in the metal. To relieve the internal stresses and reduce brittleness, steel is tempered after hardening. Tempering consists of heating the steel to a temperature below that at which it was hardened, holding the metal at that temperature for a predetermined time, and then cooling it, usually in still air. The resultant strength, hardness, and ductility depend upon the temperature to which the steel is heated during the tempering process. Annealing. In general, annealing is the opposite of hardening. Metals are annealed to relieve internal stresses, soften them, make them more ductile, and refine their grain structure. Annealing consists of heating the metal to the proper temperature, holding it at that temperature for the required time, and then cooling the metal to room temperature. The big difference between hardening and annealing is the cooling rate. To produce maximum softness in steel, the metal must be cooled very slowly. This can be done by burying the hot metal in sand, ashes, or some other substance that does not conduct heat readily. Another method is to shut off the heat and allow the furnace and metal to cool together. The first method is called packing and the latter is called furnace cooling. Normalizing. Only ferrous metals can be normalized. In the process, the metal is heated to the required temperature, held at that temperature until it is uniformly heated, and then removed to cool in still air. Steel parts are normalized to relieve the internal stress set up by machining, forging, bending, or welding. Case Hardening. When a low-carbon steel is used where a hardened surface is desirable, the character of its surface can be altered to form a very hard case. The hard surface is obtained by adding carbon to the steel. This is done by keeping the steel at a sufficiently high temperature and in contact with a suitable material containing carbon. The steel is heated to 1,700° to 1,800°F for several hours. The prolonged heating at a high temperature develops a coarse grain in the core. To refine the structure, the metal must be reheated slightly above the critical temperature of the core and then quenched. It must then be reheated slightly above the critical temperature of the case, and again quenched. The double heat treatment produces a hardened case with a fine structure and a ductile core with a full measure of toughness. Hot Working. Almost all steel is hot-worked from the ingot into some form from which it is either hot- or cold-worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the core is still molten. The ingot is then placed in a soaking pit to retard heat loss so the core solidifies gradually. After soaking, the temperature is equalized throughout the ingot. Then, to make it easier to handle, the ingot is reduced to intermediate size by rolling. The rolled shape is called a bloom when its sectional dimensions are 6 x 6 inches or larger, and it is approximately square. The section is called a billet when it is almost square and its sectional dimensions are less than 6 x 6 inches. Rectangular sections that have widths greater than twice their thicknesses are called slabs. The slab is the intermediate shape from which sheets are rolled.
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