Energy savings by implementing a two-stage heating strategy for coated patch blanks using inductive preheating for press hardening

Press hardening is an energy-intensive process in which the furnace for austenitizing the blanks is the largest energy consumer. Furnace systems with a length of up to 45 m are used to produce large components, such as press hardened door rings. The blanks used to produce large components usually have sheet thickness variations in the form of tailor welded blanks and patchwork blanks, whereby the area with the largest sheet thickness is relevant for the minimum furnace heating time. In this study a two-stage heating strategy was developed for patchwork blanks with inductive preheating in the partial area of the blank with the largest sheet thickness (3 mm). Patchwork blanks of 22MnB5 steel with aluminum‑silicon and zinc coating were used in this research. After heat treatment, the aluminum‑silicon coated samples show a very inhomogeneous coating thickness, while the zinc coating shows a high degree of homogeneity. The application of inductive preheating resulted in energy savings of up to 27 %, achieved with completely homogeneous heating. This development significantly reduced the impact of sheet thickness variations on the furnace process window, improving process stability and reducing scrap.