Effect of heat treatment on cyclic fatigue resistance, thermal behavior and microstructures of K3 NiTi rotary instruments

Abstract

Objective. The aim of this study was to investigate the effect of heat treatment on the cyclic fatigue resistance, thermal behavior and microstructural changes of K3 NiTi rotary instruments. Materials and methods. Twelve control (as-received) and 12 experimental (heat-treated) K3 NiTi rotary instruments were compared in this study. Those experimental K3 instruments were heated in a furnace for 30 min at 450°C and then quenched in water. The cyclic fatigue resistance was measured with a fatigue tester. The thermal characteristic and the microstructures of both instruments were investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), respectively. Results. There was a significant increase in the cyclic fatigue resistance between the heat-treated instruments and the as-received instruments (T-test, p < 0.05). DSC showed that the as-received and heat-treated samples were different, with an increased A_f (austenite-finish temperature) for the latter. TEM analysis revealed that both as-received and heat-treated instruments were composed mainly of an austenite phase. However, the heat-treated samples had an increased appearance of larger grains, twinning martensite, TiO₂ surface layer and a Ni-rich inner layer. Conclusions. Heat treatment increased the cyclic fatigue resistance of NiTi files and changed the thermal behavior of the instruments without marked changes in the constituting phases of NiTi alloy.

Key Words::

• cyclic fatigue resistance
• heat treatment
• K3 NiTi rotary instrument
• microstructural changes
• thermal behavior

Acknowledgment

Yu-Chan Kim acknowledges the financial support of the Industrial Strategic Technology Development Program (10042703) funded by the Ministry of Knowledge Economy (MKE, Korea). The authors' gratitude is extended to C. H. Shim at Advanced Analysis Center, KIST, for the microstructure characterization using TEM. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (2011-0014231: Dr. SW Chang & 2009-0086835: Dr. KY Kum).

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.