Investigation of the factors influencing the interfacial heat transfer coefficient in hot stamping

Abstract

In hot stamping of advanced high strength steel (AHSS) parts, the interfacial heat transfer coefficient (IHTC) between part and die has a direct impact on the temperature distribution in the part and consequently affects the microstructure and mechanical properties of the part. In this paper, a cylindrical-die experimental model was designed, experiments on boron steel 22MnB5 were conducted, the Beck's non-linear estimation method was used to calculate the IHTC, and the factors that influence IHTC were investigated. First, the influence of surface topography was studied. Both contact pressure and blank surface roughness impact the topography at the interface between die and blank. The interfacial topography determines the actual contacting area, which impacts the interfacial heat flow resistance and consequently the value of IHTC. The IHTC correlates highly with contact pressure by a positive power function. It also increases significantly with decreasing surface roughness but slows down when the roughness is less than 1μm and saturates at high contact pressure. Second, the influence of material thermo-physical properties was analyzed. The IHTC between 22MnB5 blank and #45-tool-steel die is approximately twice as much as that of H13-tool-steel die. The martensitic transformation yields a positive gain on IHTC by approximately 30%. The combination of phase transformation induced thermal conductivity increasing and latent heat releasing results in a sharp increase of the IHTC near the Ms point. Third, the influence of anti-oxidation surface coating was evaluated. Although the oxidation preventive oil coating can reduce oxidation of the blank to a certain degree, it weakens the heat transfer performance. The IHTC decreases more with a thicker coating.