This medium varies in their heat abstractive powers. If one cools a piece of steel in a furnace, the rate of cooling is relatively slow. If the steel is cooled in air, the rate of cooling will be more rapid. If the steel is quenched in oil, the rate of cooling is still faster and when quenched in water; the rate is very much greater. With very rapid cooling of the material, the resulting material is known as martensite (a hard brittle crystalline structure).
The hardness of the quenched material depends on the amount of iron carbide, essentially carbon present in the alloys. The hardness of the material is directly proportional with its carbon content. For eample, the steels Brinell hardness of 500 contains 0. 35 percent carbon. When the material contains 1. 5 percent carbon, it attains its maimum strength. It should be remembered that the ductility of the material increases upon increase of carbon content, hence, it is vital for machine parts and other similar material not to contain to high carbon content.
In this way, the materials will maintain its good ductility. After a material has been quenched for the purpose of hardening, it will be in a state of severe strain because of the deformation that has taken place in the structure. In most cases, the material will be too hard and ductile for practical use. In this case, it is therefore necessary to quench the material above the desired hardness. To relieve internal stresses, and in many cases to decrease the hardness of the material and to improve its toughness, it should be tempered.
Tempering of the steel is the process in which the material is heated to any temperature below the lower critical temperature, followed by any form of cooling. Once a material was tempered, the properties will certainly change. The tensile strength, yield point and Brinell hardness decrease, while elongation and reduction of area increases with increasing tempering temperature. If a material is tempered for along time in a temperature just below the lower critical point, the iron carbide will completely agglomerate to form spheres. This process is known as spheroidizing.
It is often to soften high- carbon tool steel preliminary to machining. When it is desired that a material should be as soft as possible, it may be heated to a temperature above the upper critical temperature and held there for the proper length of time to insure homogeneous austenite, and then allowed to cool very slowly, as in the furnace. In this treatment, the internal stresses brought about by severe cold working or other causes are completely relieved. If heating is continued above the upper critical temperature, the grain size increases with increase in temperature.
If after heating above the upper critical temperature, the material is allowed to cool slowly to room temperature, it will be found to have a fine grain structure and to be tough. This process of heating to above the upper critical temperature, followed by a very slow cooling is called annealing. In some cases, grain refinement without maimum softness is desired. In this case, the material may be heated to any temperature above the upper critical temperature, followed by cooling in still air.
This treatment is referred to as normalizing. In order to have an effective material with its properties matching its practical use, proper heat treatment should be done to the material. This will help manufacturers of many materials (mostly steel and cast iron) to improve the optimal capability of their products when it comes to its use.
References Material Properties. 2002. Retrieved March 19, 2009 @ http://www. aws. org Materials: Heat Treatment. 2004. Retrieved March 19, 2009 @ http://www. ndt. org