Examination of frictional behavior in press hardening

Tribology is a process parameter with a significant influence on process stability, both in cold and hot forming. In both processes, the coefficient of friction between the tool and the sheet metal has a significant effect on the forming result, thus determining whether the part produced is sound or defective. In contrast to cold forming, however, there are additional wear mechanisms in play during press hardening, which place high demands on the tool surface. 

The key wear mechanisms in this case are signs of fatigue due to alternating thermal stress, signs of adhesion and abrasion (depending on the applied sheet metal coating), localized erosion due to the microstructural transformations caused by the high temperatures and diffusion processes on the tool surface. 

The service life of tools can be extended and maintenance intervals lengthened by the targeted selection of the tool material, tool coating and, where applicable, the lubricant. 

Various hard coatings and an uncoated tool steel were tested for suitability for use in press hardening tools by means of tempered strip drawing tests. During the tests, the press hardening material 22MnB5 was used, in both an x-tec®-coated and uncoated version. Following austenization in the system’s integrated furnace, the sheet metal strips were drawn over the coated drawing edges while applying surface pressure. The trials were evaluated with regard to tool wear, the surface condition of the sheet metal and the friction coefficient. 

The welding of the uncoated sheet metal to the tool can be avoided by the use of PVD coatings. The friction coefficients for the respective sheet metal materials remain almost unaffected by the tool coating.