Tailoring by Direct Contact Heating During Hot Forming/Die Quenching

The well-known drawbacks of furnace-based technologies used for hot-stamping ultrahigh strength steel (UHSS) parts motivate development of alternative approaches for austenitizing blanks, among them direct contact heating. While the objective of most hot-stamping operations is to produce fully martensitic parts, there is growing interest in developing tailoring techniques for making parts having inhomogeneous mechanical properties for improved crash performance. This work shows how direct contact heating can produce parts having highly controlled microstructures by virtue of the different thermal effusivities of steel and ceramic regions on the heating die, even though the contact surface remains nearly isothermal. The process is designed with the aid of a finite element model that incorporates an austenitization submodel for 22MnB5, a common ultrahigh strength steel. Microhardness measurements and metallographic analysis on heated and quenched blanks are consistent with modeled austenite phase fractions, and highlight the potential of this technique for producing strong and lightweight automotive parts.