Effect of Heat-Treatment Conditions on Surface Properties and Corrosion Behavior of Orthodontic Stainless Steel Wires

Heat treatment is applied to orthodontic wires to inhibit the fracture after the orthodontist manipulates them to make loops, helical springs, and arch forms. We tried to investigate the effect of heat-treatment conditions on the surface properties of orthodontic wires. Four types of wires were heat-treated in air, argon, or a vacuum, and were either cooled in a furnace or water. The mechanical and surface properties of the heat-treated wires were investigated. Heat treatment increased the microhardness, strength, and Young’s modulus of the orthodontic wires. The heat treatment of stainless steel wires in air formed very thick and rough iron and chromium oxides on the surface, but the heat treatment in a vacuum or argon inhibited surface oxidation. After heat treatment, furnace cooling inhibited the surface oxidation of the wires better than water cooling did. Water-cooled wires had a rough surface, where oxide or hydroxide layers were formed by reactions with the cooling water. It is thought that water cooling formed layers that were not dense enough to play a key role in protecting the metals from corrosion. Although heat treatment can improve the mechanical properties of orthodontic wires, it reduces corrosion resistance when performed in air. Therefore, the orthodontic wires should be heat-treated in either a vacuum or inert gas and furnace-cooled to inhibit oxidation and minimize ion release.