Effect of heat treatment on transformation temperatures and bending properties of nickel-titanium endodontic instruments

Yahata Y, Yoneyama T, Hayashi Y, Ebihara A, Doi H, Hanawa T, Suda H. Effect of heat treatment on transformation temperatures and bending properties of nickel-titanium endodontic instruments. International Endodontic Journal, 42, 621-626, 2009.AbstractAim To investigate the effect of heat treatment on the bending properties of nickel-titanium endodontic instruments in relation to their transformation behaviour. Methodology Nickel-titanium super-elastic alloy wire (1.00 mm Ø) was processed into a conical shape with a 0.30 mm diameter tip and 0.06 taper. The heat treatment temperature was set at 440 or 500 °C for a period of 10 or 30 min. Nonheat-treated specimens were used as controls. The phase transformation behaviour was examined using differential scanning calorimetry. A cantilever-bending test was used to evaluate the bending properties of the specimens. Data were analyed by anova and the Tukey-Kramer test (P = 0.05). Results The transformation temperature was higher for each heat treatment condition compared with the control. Two clear thermal peaks were observed for the heat treatment at 440 °C. The specimen heated at 440 °C for 30 min exhibited the highest temperatures for M_s and A _f, with subsequently lower temperatures observed for specimens heated at 440 °C for 10 min, 500 °C for 30 min, 500 °C for 10 min, and control specimens. The sample heated at 440 °C for 30 min had the lowest bending load values (P < 0.05), both in the elastic range (0.5 mm deflection) and in the super-elastic range (2.0 mm deflection). The influence of heat treatment time was less than that of heat treatment temperature. Conclusions Change in the transformation behaviour by heat treatment may be effective in increasing the flexibility of nickel-titanium endodontic instruments.