Influence of shot peened surfaces on friction in hot forging

Dies in hot forging operations are subjected to a complex load combination consisting of thermal, mechanical and tribological loads. Therefore, hot forging dies exhibit short service life in comparison to other forming operations. An extension of tool life represents an effective way of increasing the efficiency of hot forging operations since the dies represent a large share of the process chain's production costs. Since there is no prior knowledge of ideal die surface properties, a shot peening surface treatment was applied in order to create enhanced tribological properties in comparison to machined die surfaces. Peening is a surface treatment procedure which enables a repeatable and accurate adjustment of topographical parameters. In this scientific contribution, the technique is investigated as a method to enhance tribological properties of hot forging die surfaces resulting in less friction and wear. Hot forging dies of AISI H11 were peened using different peening media creating die surfaces with varying topography. Tribological properties of the peened die surfaces were analysed in ring compression tests in dry and lubricated conditions using water-based graphite lubricant. The lowest friction was detected under lubricated conditions for peened surfaces with slightly increased roughness compared to the initial smooth, unpeened die surfaces.