Thermal contact resistance estimation at the blank/tool interface: experimental approach to simulate the blank cooling during the hot stamping process

In the present paper, an experimental device designed and developed to estimate the thermal contact resistance (RC) at the blank/tool interface is presented along with the simulation chain used to validate the experimental results. The designed stamping tool is composed of a die and a punch made in Z160CDV12 steel and presenting an omega shape. Sample and tools are thermally instrumented with thermocouples type K sheathed with silky glass, forming heat flux-meters in the most interesting locations in the tools. Hot stamping tests are carried out under different contact pressure values covering the range from 5 to 30 MPa. Tested blank material is a C-Mn steel, named Usibor 1500P. Temperatures measurement in the tool and the blank allow the estimation of Rc evolution for every contact pressure. Experiments were carried out in the same conditions of the industrial process. The recorded blank temperature shows systematically a slope change around 400°C; it is linked to the microstructure transformation from austenite to martensite. This metallurgical transformation is also observed on the RC evolution curve. Linked to the microstructure transformation, this singularity tends to vanish with increasing pressure. That is due to the increasing of the cooling velocity with the stamping pressure increasing. Results have been established as correlation of type: RC = RC(P) to be used for numerical simulation through Pam-Stamp and Abaqus. Experimental and numerical parameters have been compared and the small temperature difference shows the good quality of results.