Effect of different surface states before plasma nitriding on properties and machining behavior of M2 high-speed steel

In the present work the effect of different surface conditions on the plasma nitriding response of AISI M2 high-speed steel was investigated. Samples and drills were prepared to different surface finishes prior to plasma nitriding: ground and sandblasted. Polished samples were used as a reference surface state. The plasma nitriding was performed at temperatures of 400 and 500 °C for two gas mixtures: 5 and 76 vol.% N2 in hydrogen. The surfaces were characterized before and after plasma nitriding concerning the microstructure, roughness, microhardness, chemical composition, phase composition and residual stress states. Machining tests were carried out with drills, during which drilling forces and flank wear were measured. A significant effect of the surface state prior to nitriding on the residual stress states and properties of the nitrided layer and untreated core has been observed. Thinner nitrided layers on ground and sandblasted samples were attributed to high compressive residual stress states and a stress-affected diffusion of nitrogen and carbon. In the machining tests, sandblasted drills exhibited the best performance. Lower nitrogen concentrations in the gas atmosphere gave the lowest drill flank wear for sandblasted surfaces, while higher nitrogen concentrations led to a reduction in drilling forces and torque.