Effect of surface plastic deformation of tool steel on ion-plasma nitriding in glow discharge

Abstract

In this paper the influence of surface plastic deformation of P6M5 tool steel on the characteristics of the hardened layer after ion-plasma nitriding in the glow discharge is investigated. P6M5 tool steel annealed at 850 °C was used for the study. The annealed samples were preliminarily subjected to severe plastic deformation by torsion, with cold settling by 43 % and torsion by 1.5 turns, under hydrostatic compression of 4 GPa. Ion-plasma nitriding was carried out in a gas mixture of 50 % argon, 35 % nitrogen and 15 % hydrogen, at a gas pressure of 200 Pa and a temperature of 450 °C, for 2, 4 and 6 hours. The microhardness of the surface layer of the samples was investigated by the Vickers method on inclined slides. To reveal the microstructure, the studied samples were subjected to chemical etching for 10 s with a solution of acids: C2H5OH (80 ml), HNO3 (10 ml), HCl (10 ml) and C6H3N3O7 (1 g). The thickness of the nitride zone was estimated from the obtained optical images of microstructures. It was found that surface plastic deformation of steel contributes to an increase in the rate of nitrogen diffusion into the depth of the material due to an increase in the density of dislocations and the formation of microdefects due to a highly fine-grained structure. It was found that the surface plastic deformation of steel before ion-plasma nitriding leads to an increase in the thickness of the hardened layer of tool steel P6M5 in a2 times, due to an increase in surface free energy, which contributes to the increase in adsorption of the saturating element and the formation of nitrides in the near-surface layer of nitrided material.