Heat Treating - 5: Quench Tempering
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.

Quench Tempering is the process for making material harder. This method has been known for hundreds of years but was only perfected in the last century. The metal* is heated to a specific temperature and rapidly cooled (quenched) in a bath of water, brine, oil, or air to increase its hardness

One drawback of using this method by itself is that the metal* becomes brittle. This treatment is therefore typically followed by a tempering process which is a heating process at another lower specific temperature to stress relieve the material and minimize the brittleness problem. The temperature chosen for the tempering process directly impacts the hardness of the work piece . The higher the temperature in the tempering process, the lower the hardness.

The benefits of the quench tempering process is the restructuring of the crystalline nature of the metal*. Metals have a specific structure at the molecular level and the material used most in the machine shop, the steels, have a structure that repeats itself throughout the material. Iron alloy structure is different depending on the temperature they are heated to. Their structure is Body Centered Cubic (left) when the steel is allowed to cool at a slow pace . Their structure is Face Centered Cubic (below right) when the temperature of the iron alloy is more than approximately 1400 degrees Fahrenheit . So heat-treating consist of freezing the crystalline structure obtained at high temperature by cooling the iron alloy very fast. The tempering process permits a better re-diffusion of the carbon atoms in the material and imparts better ductility to the workpiece (although it reduces the hardness of the material).

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label: heat treating, annealing, normalizing, stress relieving,. qunench, quench temper, tool steels, hardness, Brinell, rockwell, alloyed steel, carbon contenet