Nitrided layer embrittlement due to edge effect on duplex treated AISI M2 high-speed steel

Plasma nitriding prior to TiN deposition on single-point turning tools made of high-speed steel (HSS) AISI M2 has been carried out in order to investigate the improvement of machining performance. Optical and scanning electron microscopy, microhardness test, glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD) were used to characterize the nitrided layer. Machining tests using a tool life criterion of 0.3 mm average width of the flank wear land were carried out to evaluate the service performance of the composite tools. The results have indicated that careful attention must be taken when comparing flat coupons to complex shaped substrates. In plasma nitriding systems without auxiliary heating, nitrogen incorporation depends on the plasma current density and, therefore, temperature, which can be significantly higher at the tool edges, forming an excessive deep and brittle diffusion case, reducing the service life of the duplex treated tool. The results also indicated that using brighter plasma nitriding, the TiN hard coating machining performance could be improved dramatically.