Lavrys S. Increasing the wear resistance of VT22 titanium alloy by surface deformation-diffusion treatment

The thesis presents a new solution to the scientific and technical problem of increasing the wear resistance of VT22 titanium alloy by intensification of gas nitriding by pre-modification of the surface structure by cold surface plastic deformation.The kinetic regularities of thermodiffusion saturation of VT22 titanium alloy by nitrogen were investigated and the basic kinetic constants of the process for a wide temperature range were calculated. Obtained data allow predict the nitriding parameters of VT22 alloy for obtaining regulated surface hardening. The correlation dependencies between the cold surface plastic deformation parameters and the level of surface hardening of VT22 titanium alloy with different structural states were established. It is shown that increasing load provides an increase in the level of surface hardening, and increasing number of passes – improving the quality of treated surface of VT22 titanium alloy. The influence of the initial structural state on the nitriding combined with strengthening heat treatment of VT22 titanium alloy was shown, which influences on the phase-structural state and the level of surface hardening of the modified layers. Thus, for the alloys with the initial equiaxed (α+β)-globular and β- structures, the level of surface hardening was higher than for the alloy with the initial (α+β)-lamellar structure. Instead, the most intense structural changes (α-phase growth) were observed in the alloys with the initial β- and (α+β)-lamellar structures.The thermal stability of the deformation hardened layer under conditions of nitrogen saturation up to 820°C due to the dominance of solid-soluble nitrogen hardening processes over recrystallization processes was confirmed. It was shown that the surface pre-deformation intensifies the nitriding combined with strengthening heat treatment of VT22 alloy, which was confirmed by a 50% higher surface microhardness and a 30% deeper hardened layer compared to the nitriding without pre-deformation. It was established that the deformation-diffusion treatment increases the wear resistance of VT22 titanium alloy in tribo-pair with BrAZhN 10-4-4 bronze twice and reduces the friction coefficient by 30% compared to nitriding without surface pre-deformation. The technological recommendations for combined deformation-diffusion treatment of VT22 titanium alloy were developed, which provide the regulated characteristics of hardened layers while maintaining the volumetric characteristics of the alloy.