Microstructure and tribological behaviour of liquid nitrocarburised tool steels

In the present work, industrial-scale liquid nitrocarburising in cyanide salt bath (Tufftriding) at 580 °C was employed to improve the wear resistance of three tool steels: the AISI D6, the AISI H13 and a new special-grade Cr–Mo–V tool steel, suitable for cold work and plastic mould applications. It was observed that the microstructural characteristics of the starting material have an important influence on the nitrocarburising case depth. The tribological behaviour of the nitrocarburised tool steels was studied and compared to the behaviour of the same tool steels after annealing. Pin-on-disk sliding wear tests were carried out under severe conditions (normal force varying from 50 to 265 N, sliding speed: 2.68 m s−1 and average relative humidity: 60%), using quenched AISI D6 tool steel as counterbody. For the three tool steels examined with and without surface treatment, the evolution of both the friction coefficient and of the wear coefficient were determined for a total number of 30 000 sliding cycles. The wear mechanisms involved were investigated by microscopic observations of the worn surfaces and the subsurface degradation of the tested materials was determined by microscopic observations of the corresponding cross-sections. It was found that the friction coefficient depended only on the applied load, whereas the wear rate and the degradation mechanisms were strongly influenced by the different microstructures of the three nitrocarburised tool steels.