Development of a method of increasing the wear resistance of forging dies in the aspect of tool material, thermo-chemical treatment and PVD coatings applied in a selected hot forging process

The article discusses the wear of forging tools intended for hot forging processes. It presents the results of an experimental research of a selection of methods to improve the durability of forging tools in an industrial die forging process. Improved durability was obtained by the application of thermo-chemical treatment in the form of nitriding and the use of hybrid layers composed of a nitrided layer combined with PVD coatings deposited on the surface. In addition, various nitriding processes were applied and the influence of the phase structure of the nitrided layer was examined. The use of a new coating in this type of application based on non-stoichiometric chromium nitride (CrNx) with the thickness of 12 μm was analyzed. This type of coating was obtained with the use of an innovative technology combining magnetron sputtering and arc evaporation. The obtained effect of increased durability was verified by means of a comprehensive tool analysis after operational tests, compared with the obtained durability results for the traditionally used tools made of Unimax steel. The research included, among others, determination of the amount of wear by 3D scanning, analysis of the changes in the surface layer of the tools in selected areas using a scanning electron microscope (SEM), observation of the microstructure and measurements of the microhardness. The conducted operational tests and analysis showed that, by means of the proposed methods of increasing durability, the tool life was extended. The best effect was obtained owing to the use of a hybrid layer composed of a nitrided layer (type α) and a coating of 12 μm thickness, because the tools were more resistant to abrasive wear in the analyzed key areas. Studies have confirmed the legitimacy of using hybrid methods combining many techniques simultaneously for increased durability of forging tools. •Microstructure and properties of hybrid layer have significant influence on the durability of tools in hot forging processes.•STy diversified type of substrate made of nitrided layer can affect the durability of dies covered with PVD coatings.•Hybrid layers consisting of diffusion alpha nitrided layer and CrN coating increased tools' durability by over 80%.•The use of the CrNx coating of non-stoichiometric chromium nitride allows to deposit coatings with a thickness of over 10 μm.