Fatigue-resistant Nitinol instrument
A fatigue-resistant Nitinol instrument has a working portion in the deformed monoclinic martensitic state and an austenite finish temperature in the range of 40° to 60° C. Because the operating environment of the instrument is about 37° C., the working portion remains in the monoclinic martensitic state during its use. The relatively high austenite finish temperature and fatigue resistance is achieved by subjecting the nickel-titanium alloy to a final thermal heat treat in a temperature range of about 410° to 440° C. while the nickel-titanium alloy is under constant strain of about 3 to 15 kg. Further, the high austenite finish temperature is achieved without subjecting the alloy to thermal cycling to produce shape memory. Additionally, there are no intermediate processing steps occurring between obtaining a finished diameter of the wire or blank through cold working and the final thermal heat treat under constant strain.
1. An endodontic instrument comprising:
a working portion composed of a nickel-titanium alloy and configured for use in the human body, the working portion having cutting surfaces, an austenite finish temperature in the range of 40° to 60° C., and a shape memory restoration such that, after a 90° bend and release, the working portion when released returns less than 88°;
the working portion having been subjected to a first heat treat while under constant strain before the cutting surfaces are formed and subjected to a second heat treat after the cutting surfaces have been formed.
2. An endodontic instrument according to claim 1 further comprising the first heat treat is at a different temperature than the second heat treat.
3. An endodontic instrument according to claim 1 wherein the first heat treat is in a temperature range of 410 to 440° C.
4. An endodontic instrument according to claim 1 wherein the second heat treat is in a range of 248 to 500° C.
5. An endodontic instrument according to claim 4 wherein the second heat treat is in a range of 300 to 450° C.
6. An endodontic instrument according to claim 1 wherein the working portion when released returns no more than 75°.
7. An endodontic instrument according to claim 6 wherein the working portion returns to no more than 60°.
8. A method of manufacturing an endodontic instrument from a nickel-titanium blank to obtain improved fatigue failure characteristics, the method comprising the steps of:
(i) applying an elongational strain deformation to the blank within its reversible strain limit at a first heat treat temperature;
(ii) after step (i), forming an instrument from the blank by performing machining operations; and
(iii) after step (ii), heat treating the instrument at a second heat treat temperature.
9. A method according to claim 8 further comprising the first heat treat is at a different temperature than the second heat treat.
10. A method instrument according to claim 9 wherein the first heat treat is in a temperature range of 410 to 440° C.
11. A method instrument according to claim 9 wherein the second heat treat is in a range of 248 to 500° C.
12. A method according to claim 11 wherein the second heat treat is in a range of 300 to 450° C.
13. An method according to claim 8 wherein after step (iii) the endodontic instrument has an austenite finish temperature in a range of 40 to 60° C.
14. A method according to claim 8 wherein after step (iii) the endodontic instrument has a shape memory restoration such that, after a 90° bend and release, the working portion when released returns less than 88°.
15. An endodontic instrument according to claim 14 wherein the working portion when released returns no more than 75°.
16. An endodontic instrument according to claim 15 wherein the working portion returns to no more than 60°.