Analysis of the friction and wear behaviour of hot work tool steel for forging

A significant part of the energy in forging is used to break the interfacial junctions due to friction between the tool and the workpiece. The life of hot-forging tools is usually limited by complex interactive mechanisms under cyclic loading such as abrasive, adhesive and scaling wear, thermal and mechanical fatigue, and plastic deformation. This contribution deals with the wear mechanisms of the tempered martensitic X38CrMoV5 steel (AISI H11) under high-temperature and dry-sliding wear. The investigations are carried out with high-temperature pin-on-disc tests. The pin is cut from bars of X38CrMoV5 steel treated at 42 and 47 HRC. The disc is made of common steel (AISI 1018, XC18). Temperature of the disc ranges from 20 to 950 °C. Before the test starts, the disc is first pre-heated for 1 h. The experiments are performed under constant load and velocity. The friction coefficient decreases quasi-linearly with the rising disc temperature up to 800 °C. Over this temperature, it decreases drastically for the 42 HRC steel but remains linear for the 47 HRC steel. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) investigations have revealed that wear is essentially due to abrasion, plastic deformation and fatigue. Set of cracks due to contact rolling fatigue is observed on the pin and the disc. Those cracks are located on the transferred scale on the pin and on the oxide scale of the disc wear track. The cross-section observations of the pin have revealed a plastically deformed zone beneath the surface. In this sub-surface layer, the tempered martensitic microstructure seems to be more aligned due to friction and the plastic deformation.