Experimental and Computational Study of Heat Transfer During Quenching of Metallic Probes

Heat transfer from hot bodies such as steel, aluminum and other metals is vitally important for a wide range of industries such as chemical, nuclear and manufacturing (including steel hardening) industries. Hardening of steels (so-called martensiticor bainitic-hardening) requires preheating (austenitizing) of the part to temperatures in the range of 750-1100 °C, from which the steel is quenched (i.e., rapidly cooled) in a defined way to obtain the desired mechanical properties such as hardness and yield strength. Most liquid quenchants used for this process exhibit boiling temperatures between 100 and 300 °C at atmospheric pressure. When parts are quenched in these fluids, wetting of the surface is usually time dependant, which influences the cooling process and the achievable hardness (Liscic et al., 2003). Heat transfer research related to cooling has been the source of fundamental studies since the early work by Fourier (Fourier, 1820). These early studies were typically performed by hot-wire anemometry (King, 1914; Russell, 1910). One of the first to report the results of fundamental heat transfer studies for the quenching of metals such as steel using cooling curve analysis (time vs. temperature curves) was Benedicks who utilized 4-12 mm diameter x 15-50 mm cylindrical carbon steel probes in his now-classic work (Benedicks, 1908). The advantage of using probes larger in diameter than thin platinum wire used for hot-wire anemometry tests is that it is possible to more easily measure thermal gradients through the cross-section upon cooling and to view surface cooling mechanisms. Benedicks work involved cooling hot steel (1000 oC) in water at 4.5 – 16 oC and in addition to cooling time from 700 oC – 100 oC, effects of the ratio of mass/surface area on cooling time were evaluated. In 1920, Pilling and Lynch measured the temperature at the center of 6.4 mm dia x 50 mm cylindrical carbon steel probes cooled (quenched) from 830 oC into various vaporizable liquids (Pilling & Lynch, 1920). From this work, they identified three characteristic cooling mechanisms, so-called: A, B and C-stage cooling which are currently designated as film boiling, nucleate boiling and convective cooling, based on the cooling time-temperature and