Effects of heat treatment conditions on the mechanical properties of orthodontic stainless steel wires

The purpose of this study was to evaluate mechanical properties of the orthodontic wires according to heat treatment environments and cooling methods. Four types of wires were heat-treated in air, argon and vacuum at 500°C, for 6 min, and were then either cooled in the furnace or a water bath. Increases in microhardness, maximal strength, yield strength and Young's modulus of the orthodontic wires after heat treatment were approximately 0.4-22.9%, 2.9-14.8%, 3.7-14.4% and 2.5-9.0%, respectively. This was considered to be because of mainly precipitation hardening and additionally subgrain formation by the heat treatment. Yield strength and microhardness of the heat-treated wires were also significantly dependent upon the cooling methods. The microhardness and yield strength of the water-cooled wires were lower than those of the furnace cooled wires. This was attributed to the precipitation degree of chromium carbides dependent on cooling rate. Heat treatment in an air oxidized the surface of the wires, and the mechanical properties and surface cleanness of the water-cooled wires were poorer than those of the furnace-cooled wires. Conclusively, orthodontic wires should be heat-treated in vacuum conditions or an inert gas environment after plastic deformation and should be cooled in a furnace in order to inhibit surface oxidation, maximize corrosion resistance and improve the mechanical properties.