Future trends in gaseous surface hardening of titanium and titanium alloys

An overview of existing surface hardening techniques applicable for titanium and titanium alloys with special emphasis on gaseous based processes is provided. New processes for gaseous surface hardening of titanium alloys and zirconium are presented. This entails low temperature oxidizing by chemically controlled partial pressures of oxygen resulting in deep and hard diffusion zones and the possibility for applying so-called mixed-interstitial phases for surface hardening. Mixed-interstitial phases (based on carbon, oxygen and nitrogen) can result in new intriguing microstructural features and properties in case-hardened titanium. Deep and very hard cases can be produced by these new techniques. Hardness values of metallic titanium of more than 1800 HV is possible by simultaneously dissolving three interstitial elements. Mixed-interstitial compounds based on oxygen and carbon can lead to hardness values in the range of 2000 to 3000 HV. Finally, examples of surface hardening of printed parts in titanium grade 5 are shown.