Chemical Heat Treatment of Modified Titanium Alloy Surface Structures

Abstract

Titanium alloys due to their properties are of great interest for use as a structural material for the parts of friction units, pump plungers, shaft-bush parts operating in harsh environments. But high tendency of titanium alloys to the contact seizure under friction restricts wider application of titanium and its alloys. One of the main ways to improve the anti-friction properties of titanium alloys is a chemical heat treatment. The most common type is nitrogen hardening that allows us to achieve high values of micro-hardness on the surface. However, the biggest drawback of the method is small depth of the hardened layer that is up to 0.16 mm. To increase a depth of the hardened layer the paper proposes the preprocessing of the part surface by deformational cutting, which allows creating a macro-relief of different shape as the alternate vertical or inclined layers of the part material of a specified thickness and height with a gap or no gap between the layers. Doping of this structure, when nitriding, is provided over the entire macro-pattern surface, thereby enabling to obtain a thickness of the hardened layer, which depends on the processing parameters in deformational cutting. The paper presents the results of the microstructure study and the micro-hardness measurements of the VT1-0 titanium alloy sample after deformational cutting process followed by nitriding. The influence of the deformational cutting parameters on the total thickness of the hardened surface layer is analyzed. It is found that the entire surface of macro-pattern has been azotized. The total thickness of the nitrogen hardened modified surface structure became 4 times larger as compared to the flat surface that was azotized. The results can be used as a theoretical basis to design the titanium alloy parts operating in conditions of friction.