Heat treating, tempering and annealing metals are the most common ways to treat such material. These treatments are used to increase or decrease the brittleness, soften or harden or weaken or strengthen metals.
In order to achieve the desired effect, it is important to understand the difference between all of these techniques. For example, if the desired end material is high-tensile strength steel low brittleness, it may be necessary to use a variety of treatments.
Heat Treating
Actually, tempering and annealing metals are types of heat treating. Heat treating alters the chemical and physical properties of metal by using the controlled application of heat. This method can also be used on other materials as well, such as glass.
The microstructures, which is called a phase, of metal are composed of little crystals known as grains. Heat treatment causes these crystals to shrink or grow. The desired characteristics of the material is achieved by heat treating to different grain size and concentration. For instance, the concentration and presence of carbon characterized the various phases of carbon steels. A phase diagram is used by an engineer to obtain the anticipated results.
Tempering
When metal is heated below critical temperature, this is known as tempering. The process is used to increase the toughness and ductility of the material. However, this process typically will reduce the strength of the metal, which is acceptable because the end result will be stronger. In addition to increasing toughness and ductility, tempering also increases workability and decreases the chances of cracking.
The steel is first austenized and then quenched in oil, air, water or other fluid quickly to lower the temperature rapidly. This causes the formation of martensite, which is brittle but strong. By cooling the medical quickly, the molecules diffuse from the high temperature formation and stay at the lower temperature.
Finally, the metal is tempered between 302° to 500°F or 698° to 1202°F. However the 698° to 1202°F can lead to brittleness. The result of tempering is the transformation from martensite to pearlite or bainite.
Annealing
When annealing metals, it is heated to a specific high temperature and held for a certain length of time. The amount of time could be anywhere from several hours to several days. After heating, the metal is allowed to cool slowly.
Special furnaces are required in order for the annealing process to occur properly. The reason for this is because such furnaces are able to be controlled in a way that ensures the anticipated changes will take place.
There are several reasons for annealing metals. It alleviates internal stress and increase ductility, which limits brittleness. In addition, it increases the homogeneity and toughness as well.
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