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Quenching, or rapid and controlled cooling, has been used for centuries to harden steel and increase the toughness of metal alloys. Quenching poses a difficult heat transfer problem, as it deals with high-temperature materials being cooled in a controlled way, in mediums such as air, water, oil, liquid nitrogen, or some other special quenching fluid. The material properties and the fluid properties change fast, and during the quenching process, heat transfer between the material body and the medium can go through several regimes. The medium surrounding the hot body can have film boiling, transition from film boiling to nucleate boiling, nucleate boiling, and natural convection as quenching time progresses.
In this chapter, the quenching of a small spherical ball made out of steel is analyzed in a forced convection air flow. The conduction heat transfer resistance within the ball is assumed to be much less than the heat transfer resistance between the surface of the ball and the quenching medium—the Biot number that is defined in Eq. (16-2) is less than 0.1. This assumption allows the use of the following energy balance in the present unsteady-state heat transfer application (see Reference by J. P. Holman ).