Induction nitriding of grade 5 titanium alloy: process optimization

Conventional nitriding treatment on Ti alloys is a challenging technological process because of both Ti high affinity with N, that produces a barrier for the diffusion of gas in the bulk of material, and the need to perform the treatment at high temperatures, thus producing a degradation of the microstructure obtained by previous conventional heat treatment. In this work, the induction heating has been obtained in a nitrogen rich environment at atmospheric pressure in order to nitride Ti-6Al-4V alloy, thus reducing the microstructural alteration in the bulk of material caused by the treatment temperature of conventional nitriding. In this case the aim was to preserve, in the core, the solution and aging treatment of the material that was previously performed. The nitriding number cycles has been varied in order to produce treatments with different thicknesses and possibly different properties. Post-treatment surface morphologies and cross section microstructures have been studied by means of Stylus Profilometry and Scanning Electron Microscopy, in order to evaluate possible modifications of the original surface caused by the diffusion of N. The mechanical properties of the different treatments have been also evaluated using Vickers micro-hardness tests at different loading conditions.