Significance of heat treatment on the mechanical and fatigue properties of NiTi endodontic rotary files

The present investigation is conducted to address the relationship between microstructure and phase transformation with functional properties of NiTi rotary files. For this purpose, heat treatment at 460-500 °C for 90-120 min followed by cooling in water, air and furnace heat treatment was accomplished on grined rotary files from as-drawn wires with a chemical composition of Ti-50.8 at.% Ni. Prior to heat treatment, the rotary files exhibited an austenitic phase and demonstrated complete superelasticity during bending tests. Heat treatment resulted in a multiphase microstructure (B19’, R, and B2 phases) exhibiting shape memory effect and superelasticity, enhancing the flexibility of rotary files. The heat treatment at 460 °C for 120 min followed by furnace cooling revealed the maximum hardness value of ∼350 Hv and the best fatigue resistance with a number of cycles to fracture (NCF) of ∼1000 during rotation bending fatigue test in a simulated root canal. It was suggested that microstructural and phase engineering is feasible by controlling the heat treatment to find remarkable functional properties to produce high-performance endodontic rotary files.