Heat treatment is a term applied to a variety of procedures for changing the characteristics of metal by heating and cooling. By proper heat treatment, it is possible to obtain certain characteristics in metal such as hardness, tensile strength (ability to resist stretching), and ductility. Heat treatment can be a simple process requiring few tools. In industry, it is a highly scientific and complicated-procedure requiring much equipment.
        Many of the projects or products made in the machine shop have little or no value until they are heat-treated. This article includes only the most elementary information about the heat treatment of steel. Heat treatment can also be done on many of the nonferrous metals such as aluminium, copper and brass. The procedures are different, however, and will not be considered here.
       They procedures of  of heat treatment  of steel include hardening, tempering, annealing, and case hardening.

Hardening

      Hardening is a process of heating and cooling steel to increase its hardness and tensile strength, to reduce its ductility, to obtain a fine grain structure. The procedure includes heating the metal above its critical point or temperature, followed by rapid cooling. As steel is heated, a physical and chemical change takes place between the iron and carbon. The critical point, or critical temperature, is the point at which the steel has the most desirable characteristics. When steel reaches this temperature--somewhere between 1400 and 1600 F--the change is ideal to make for a hard strong material if it is cooled quickly. If the metal cools slowly, it changes back to its original state. By plunging the hot metal into water, oil, or brine (quenching), the desirable characteristics are retained. The metal is very hard and strong and less ductile than before.
Heating is  done in a furnace fired by gas, oil, or electricity. A device called a pyrometer is attached to the furnace. This accurately registers the exact temperature in the furnace (FU.8-1). The temperature of the metal can also be determined by observing its You can make use of the colors when heat-treating simple metal parts and tool. Colors are not very accurate, however. Even the expert heat-treater will be off as much as 20 F from the true temperature.

The hardening procedure is:
1.Light the furnace, and allow it to come to the right temperature.

2.Place the metal in the furnace, and heat it to the critical temperature. For carbon tool steel, allow about 20 to 30minutes per inch of thickness for coming up to heat. Allow about 10to 15minutes per inch of thickness for soaking at hardening temperature.

3.Sellect the correct cooling solution. Some steels can be cooled in water, and others must be cooled in oil or brine. Water is the most widely used material for quenching carbon steels because it is inexpensive and effective. Brine is usually made by adding about 9 percent of common salt to the water. Brine helps to produce a more uniform hardness. The brine cooles the parts all over more quickly. Oil is used for a some what slower speed of quenching. Most oils used for quenching are mineral oils.

4.Remove the hot metal with tongs and plunge it into the cooling solution. Agitate so that the metal cools quickly and evenly. If it is a thin piece (like a knife or a blade), cut the cooling solution with the object so it won't warp. If one side cools faster than the other  there will be some warping.

5.A properly hardened piece of steel will be hard and brittle and have high tensile strength. It will also have internal strain. IF left in this state, these internal strains could caouse the metal to crack.

Tempering

Tempering is a process of reducing the degree of hardness and strength and increasing the toughness. It removes the brittleness from a hardened piece. It is a process that follows the hardening procedure and makes the metal as hard and tough as possible. Tempering is done by quenching or by cooling in air. As the metal is heated for temperingk it changes in color. These colors are called temper colors. You can watch these colors to know when the correct heat is reached. A more accurate method, of course, is to watch the pyrometer. Many parts and projects are completely tempered. Others are tempered in one section, and the rest remains in the hardened state.

The tempering procedure is:
1. To temper the entire piece, place it in the furnace. Reheat to the correct temperature to produce the hardness and toughness you want. Remove the metal and cool it quickly.

2.To temper small  cutting tools:
a. Harden the entire tool. Clean off the scale with abrasive cloth.
b. Heat a scrap piece of metal red hot.
c. Place the tool on the metal with the point extending beyond the hot piece of metal.
d. Watch the temper colors. When the correct color reaches the point of the tool quench it.

Annealing

annealing is the process of softening steel relieve internal strain. This makes the steel easier to machine. The metal is heated above the critical temperature and cooled slowly. The most common method is to place the steel in the furnace and heat it thoroughly. Then turn off the furnace, allowing the metal to cool slowly. Another method is to pack the metal in clay, heat it to the critical temperature, remove it from the furnace, and allow it to cool slowly.

Case hardening

Case hardening is a process of hardening the outer surface or case of ferrous metal. By adding a small amount of carbon to the case of the low-carbon steel, It can be heat-treated to make the case hard. At the same time the centre. or core.

There are many methods of case hardening. In industry, molten cyanide is used (this is called cyaniding). Another industrial method is carburizing. This is a case-hardening procedure in which carbon is added   to steel from the surface inward by one of the following methods: pack method, gas method of liquid-salt method.

This process can be done on such items as hammer heads, piston pins, and other items that must stand a good deal of shock and wear.  It can never be used on anything that must be sharpened by grinding.