Multiple layer hardness ferrule and method

1. A ferrule for a tube fitting, comprising:

an outer layer having a hardness equal or greater than 40 HRc, the outer layer having a thickness of about 0.001 to about 0.002 inches;

an intermediate layer below the outer layer, the intermediate layer having a hardness that is less than the hardness of the outer layer and in the range of 80 HRb to 40 HRc, and the intermediate layer having a thickness equal or greater than 0.003 inches; and

a core below the intermediate layer, the core having a hardness that is less than the hardness of the intermediate layer.

2. The ferrule of claim 1 , wherein the intermediate layer has a thickness no greater than about 0.020 inches.

3. The ferrule of claim 1 , wherein the intermediate layer includes varying levels of hardness.

4. The ferrule of claim 1 , wherein the ferrule is made of an austenitic material.

5. The ferrule of claim 1 , wherein the thickness of the intermediate layer is equal or greater than 0.020 inch.

6. The ferrule of claim 1 , wherein the outer layer has a hardness equal or greater than 50 HRc.

7. The ferrule of claim 1 , wherein the intermediate layer has a hardness in the range of 95 HRb to 35 HRc.

8. The ferrule of claim 1 , wherein the intermediate layer and/or outer layer are not provided at some regions of the outer surface of the ferrule.

9. The ferrule of claim 1 , wherein the intermediate layer is formed by nitrogen infusion.

10. The ferrule of claim 1 , wherein the intermediate layer or core is hardened by dissolving at least nitrogen in the workpiece at a temperature T1, which is higher than the austenisation temperature and lower than the melting point of the passive alloy, and cooling the workpiece after the dissolution step to a temperature which is lower than the temperature at which carbides and/or nitrides form in the passive alloy.

11. The ferrule of claim 1 , wherein the outer layer is one of a carburized layer or a nitrided layer.

12. The ferrule of claim 11 , wherein the outer layer is the carburized layer.

13. A method of forming a tube fitting ferrule from a base material that has a first hardness, the method comprising:

hardening an outer layer of the ferrule such that the outer layer has a hardness that is greater than the first hardness, wherein the hardness of the outer layer is equal or greater than 40 HRc, and the outer layer has a thickness of about 0.001 to about 0.002 inches;

hardening an intermediate layer of the ferrule beneath the outer layer such that the intermediate layer has a hardness that is less than the hardness of the outer layer and greater than the hardness of the base material of the ferrule and in the range of 80 HRb to 40 HRc, and the intermediate layer having a thickness equal or greater than 0.003 inches.

14. The method of claim 13 , wherein the intermediate layer is hardened by nitrogen infusion.

15. The method of claim 13 , wherein the outer layer and/or intermediate layer are formed at selected regions of the ferrule.

16. The method of claim 13 , wherein the base material is an austenitic material.

17. The method of claim 13 , wherein the intermediate layer or a core of the ferrule is hardened by dissolving at least nitrogen in the workpiece at a temperature T1, which is higher than the austenisation temperature and lower than the melting point of the passive alloy, and cooling the workpiece after the dissolution step to a temperature which is lower than the temperature at which carbides and/or nitrides form in the passive alloy.

18. The method of claim 17 , wherein the cooling step takes place in an inert gas not containing nitrogen.

19. The method of claim 13 , wherein the outer layer is one of a carburized layer or a nitrided layer.