Heat Transfer Modeling of Roller Hearth and Muffle Furnace
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In hot forming die quenching, furnaces are used to austenitize ultra high strength steel blanks. In the case of coated steels, like Usibor® 1500 P, furnace heating also transforms a protective Al-Si layer into a permanent Al-Si-Fe intermetallic coating. Modeling this process requires knowledge of the thermophysical properties of the material, specifically, radiative properties and how the sensible energy and latent heat of austenitization change with blank temperature. While the sensible energy is known, there is considerable uncertainty regarding the radiative properties and the latent heat of austenitization. In this work the effective specific heat of Usibor® 1500 P is inferred through inverse analysis of temperature data collected on coupons heated in a muffle furnace. This technique is first used to validate the heat transfer model, and then used to reveal the distribution of latent heat of austenitization at higher temperatures. The characterization of the radiative properties is carried out on Gleeble-heated coupons using a near-infrared spectrometer and a Fourier transform infrared reflectometer. Obtained thermophysical properties are employed in developing a heat transfer model for the patched blanks to gain insight into the non-uniform heating of patched blanks. The thermocouple measurements carried out in muffle and roller hearth furnaces are used to validate the modeled temperatures. Various strategies to optimize the heating process for patched blanks are proposed and evaluated, including the use of a high emissivity coating to compensate for the increased thermal mass of the patch.