High temperature tribological studies on hardfaced tool steels for press hardening of Al-Si coated boron steel

Abstract

Press hardening tools are expected to operate satisfactorily for thousands of forming cycles before being replaced. An approach to enhance the durability of tools in operation, and improve the process economy, is to refurbish them by depositing new material on the tool through welding, or hardfacing, when its surface is adversely damaged. The welding process changes the microstructure of the original tool steel and the deposited material also has a different microstructure, as well as different chemical composition to facilitate the welding process. Dissimilar friction behaviour between the original tool steel and the welded material can lead to unstable friction forces during forming. This work therefore focuses on understanding the tribological behaviour of different hardfacing materials, deposited through TIG welding, and their interaction with Al-Si coated boron steel at high temperatures. The tribological behaviour is evaluated using a hot-strip drawing tribometer capable of simulating the sliding conditions in the press hardening process. The effect of temperature on the microstructure and on the friction and wear behaviour of the weld material is also studied by conducting tests at room temperature and at 500 °C. In general, friction and the governing wear mechanisms are similar for the evaluated hardfacing materials. Friction stability was observed to be closely related to the temperature of the workpiece material. Temperature of the tool significantly affects the formation and structure of material transfer layers onto tool steels.