Heat Extraction and Cooling Rates

Heat Extraction and Cooling Rates Two important factors influence cooling rates or the rates at which heat can be removed from a steel part. (continued below)

One is the ability of the heat to diffuse from the interior to the surface of the steel specimen, and the other is the ability of the quenching medium to remove heat from the surface of the part. The ability of a steel to transfer heat is characterized by its thermal diffusivity (units of area per unit time) or the ratio of its thermal conductivity to the volume specific heat. The thermal diffusivity of austenite transformation products increases with decreasing temperature. The slower cooling rates at positions removed from the surface of a bar permit more time for diffusion-controlled transformations, and it is this type of cooling behavior that results in the lower center hardness of bars, especially in the larger sizes.

09.11.08 Supporting Graphic

MS&T '08                 Heat Extraction and Cooling Rates Two important factors influence cooling rates or the rates at which heat can be removed from a steel part. (continued below) One is the ability of the heat to diffuse from the interior to the surface of the steel specimen, and the other is the ability of the quenching medium to remove heat from the surface of the part. The ability of a steel to transfer heat is characterized by its thermal diffusivity (units of area per unit time) or the ratio of its thermal conductivity to the volume specific heat. The thermal diffusivity of austenite transformation products increases with decreasing temperature. The slower cooling rates at positions removed from the surface of a bar permit more time for diffusion-controlled transformations, and it is this type of cooling behavior that results in the lower center hardness of bars, especially in the larger sizes. View associated graphic.   Cleaning Requirements for Heat Treatment - Developments and Applications October 29-30, 2008 Worcester Polytechnic Institute, Worcester, Mass. Visit http://www.asminternational.org/htcleaning08/ Click here for past One-Minute Mentors.

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