Influence of the phase structure of nitrides and properties of nitrided layers on the durability of tools applied in hot forging processes

This article presents beneficial results of surface layer modification by gas nitriding. The main purpose of the work is to demonstrate the influence of nitriding parameters on phase structure of iron nitrides and therefore on the durability of nitrided forging dies. First, the laboratory tests determined the parameters of thermo-chemical treatment - nitriding by the ZeroFlow method to obtain the desired phase structures: alpha, gamma' or epsilon, nitrides. Then the microstructure and properties of the obtained layers were examined. Next, an industrial test was carried out on real tools in the hot forging process. The effect of nitriding on durability was evaluated by comprehensive analysis of used tools including macroscopic observations, laser surface scanning, SEM observations and microstructure and microhardness testing in the surface layer. The results showed a significant effect of improving durability. There were also clear differences in the wear mechanisms depending on the nitride layer used. Alpha-type layer is less prone to cracking, while gamma prime-type layers crack most, which may be due to their crystallographic structure and higher hardness. The results confirmed the rightness of controlling the nitriding process in order to obtain the proper layer, especially in the case of nitriding of forging tools.