Comparative Study on Fatigue Resistance of Denco Files and Heat-Treated Machined Files

This study investigated the effects of heat treatment on the microstructure, phase transformation behavior, hardness, and fatigue resistance of NiTi rotary files. Conventionally machined rotary files fabricated from Ti-49 at.% Ni wire were heat-treated at 400–650 °C for 80–130 min under different cooling conditions. Microstructural features and phase transformation behavior were characterized using OM, FE-SEM, XRD, TEM and DSC. Bending, hardness, and cyclic fatigue tests were performed in a simulated curved canal. Before heat treatment, the files exhibited a predominantly austenitic structure and complete superelastic behavior during bending. Heat treatment produced a multiphase microstructure composed of B19′ martensite, R-phase, and B2 austenite, which led to the simultaneous manifestation of shape memory effect and superelasticity and enhanced the flexibility of the files. The best combination of properties among the heat-treated conditions was achieved at 450 °C for 130 minutes followed by furnace cooling, yielding a hardness of 335 ± 4 Hv and a cyclic fatigue resistance of approximately 1750 cycles to fracture, closely approaching that of the commercial Denco instrument (∼1800 cycles). Notably, increasing the treatment temperature to 470 °C further raised the hardness to 352 ± 6 Hv but sharply reduced the fatigue resistance to approximately 890 cycles, revealing a non-monotonic relationship in which higher hardness does not necessarily translate into improved fatigue performance. The results indicate that careful control of heat treatment can be used to engineer the microstructure and phase constitution of NiTi files, and that an optimal, narrow processing window rather than simply maximizing hardness is required to develop endodontic rotary instruments with improved functional performance.