Thermal contact resistance estimation and metallurgical transformation identification during the hot stamping

This paper is devoted to the thermal aspects of hot stamping. An original way to estimate the thermal contact resistances between the tools and the blank, the metallurgical model parameters and the latent heat that describe the austenite to martensite transformation during a hot stamping process is presented. The measurements are made thanks to two thermocouples in the die, two in the punch and one the half-thickness of the blank. All thermocouples are aligned according to the heat flow direction, reducing the heat problem to one dimension. With the measured temperatures in the tools, the heat fluxes that cross the tools-blank interfaces are estimated solving two Inverse Heat Conduction Problems (IHCP). Using these estimated heat fluxes as boundary conditions in the blank and the measured temperature in the blank, a third IHCP resolution allows the estimation of a source term in the heat equation that describes the exothermic nature of the metallurgical transformation. This heat release can be also determined by direct calculus. The validity of the assumptions used by these techniques is discussed. Finally, the evolution of the heat release can be linked to the metallurgical transformation kinetic as well as the latent heat of the transformation allowing the estimation of Koistinen-Marburger parameters. These data will be used to perform accurate hot stamping FE simulations. © 2013 Elsevier B.V. All rights reserved.