Local Patchworking of Sheet Properties Combining Carburization and Hot Stamping

Hot stamping is now commonplace in the automotive industry. The continuing need by automotive manufacturers to reduce weight while increasing crashworthiness has driven the industry to seek new hot stamping solutions. Currently tailored hot stamping is being introduced to hot stamp panels for automotive body structures. A tailored hot stamped part has at least two regions of different material properties to satisfy engineering design targets. For example, the B-pillar can be separated into the incredibly strong upper region to reduce intrusion, while the lower region is softer and tailored for energy absorption. We can then consider the tailored hot stamped part as a patchwork of hard and soft regions. Patchwork in this context means there is a relational organization (topology) to the network of hard and soft regions. The next generation of tailored hot stamping sheets may see extremely small regions of the blank have tailored properties.We have investigated local carburization of a low-carbon blank sheet to locally tailor the properties of a hot stamped component. After hot stamping and in-die quenching, regions with a high carbon concentration (and high hardenability) transform to martensite while regions with low carbon concentration (and low hardenability) transform to ferrite, pearlite, or bainite. The location of carburizing can be controlled by using a commercially available stop-off coating (or mask) for carburizing. When this coating is used, the gradient in carbon concentration across the boundary between the masked and non-masked (that is, carburized and non-carburized) region of the sheet blank is on the order of the sheet thickness. The results of this process are reported in this paper.