Enhanced activation of self-passivating metals
A workpiece made from a self passivating metal and having one or more surface regions defining a Beilby layer as a result of a previous metal shaping operation is activated for subsequent low temperature gas hardening by exposing the workpiece to the vapors produced by heating an oxygen-free nitrogen halide salt.
1. A process comprising:
heating an oxygen-free nitrogen halide salt to produce vapors;
exposing to the vapors a workpiece made from a self-passivating metal and having a Beilby layer on at least a portion of a surface of the workpiece so that:
a temperature of the exposing is below a temperature at which nitride and/or carbide precipitates form in the workpiece;
the exposing activates the workpiece for nitriding or nitrocarburizing; and nitriding or nitrocarburizing the workpiece while the workpiece is activated.
2. The process of claim 1 , wherein the oxygen-free nitrogen halide salt is an ionic compound that:
(1) comprises a halide anion;
(2) has a room temperature solubility in water of at least 5 moles/liter;
(3) comprises nitrogen;
(4) is substantially free of oxygen; and
(5) vaporizes when heated to a temperature of 350° C. at atmospheric pressure.
3. The process of claim 1 , wherein the nitriding or nitrocarburizing utilizes gas derived exclusively from the oxygen-free nitrogen halide salt.
4. The process of claim 1 , wherein the nitriding or nitrocarburizing comprises contacting the workpiece with a gas derived from a nitrogen containing compound different than the oxygen-free nitrogen halide salt.
5. The process of claim 4 , wherein the contacting the workpiece with the gas derived from the nitrogen containing compound occurs during the activation.
6. The process of claim 4 , wherein the contacting the workpiece with the gas derived from the nitrogen containing compound occurs as the activation begins.
7. The process of claim 4 , wherein the exposing occurs simultaneously with the contacting the workpiece with the gas derived from the nitrogen containing compound.
8. The process of claim 4 , wherein the contacting the workpiece with the gas derived from the nitrogen containing compound occurs after the exposing.
9. The process of claim 4 , wherein the nitrogen containing compound comprises:
one or more compounds comprising:
an N/C compound,
at least one nitrogen to carbon bond,
at least four carbon atoms, and
the one or more compounds are solid or liquid at 25° C. and under a pressure of 1 atmosphere.
10. The process of claim 9 , wherein the one or more compounds comprise at least one of urea, acetamide and formamide.
11. The process of claim 1 , wherein the nitriding or nitrocarburizing further comprises:
contacting the workpiece with an additional gas different from the vapors, the additional gas comprising at least one of:
nitrogen derived from the decomposition of a compound during the nitriding or nitrocarburizing;
carbon derived from the decomposition of a compound during the nitriding or nitrocarburizing; and
nitrogen and carbon derived from the decomposition of a compound during the nitriding or nitrocarburizing.
12. The process of claim 11 , wherein the additional gas comprises vapors from the decomposition of an N/C compound.
13. The process of claim 1 , wherein the exposing occurs for at least about 15 minutes.
14. The process of claim 13 , wherein the exposing occurs for at least about 30 minutes.
15. The process of claim 1 , wherein the oxygen-free nitrogen halide salt is a particulate solid and wherein the exposing further comprises:
encapsulating the workpiece with the particulate solid; and
heating the workpiece to a temperature high enough to vaporize the particulate solid.
16. The process of claim 1 , wherein the oxygen-free nitrogen halide salt comprises at least one of ammonium chloride, ammonium fluoride, guanidinium chloride, guanidinium fluoride, pyridinium chloride, and pyridinium fluoride.
17. The process of claim 16 , wherein the oxygen-free nitrogen halide salt comprises at least one of ammonium chloride and guanidinium chloride.
18. The process of claim 1 , wherein the self-passivating metal is a stainless steel.
19. The process of claim 1 , wherein the self-passivating metal is one of:
10 to 40 wt. % Ni and 10 to 35 wt. % Cr;
a nickel-based, cobalt-based or manganese-based alloy containing 10 wt. % or more chromium; and
a titanium-based alloy.
20. The process of claim 1 , wherein the self-passivating metal is a titanium-based alloy.
21. The process of claim 1 , wherein the nitriding or nitrocarburizing comprises:
a first nitriding or nitrocarburizing step during the exposing; and
a second nitriding or nitrocarburizing step after the exposing.
22. A process comprising:
heating an oxygen-free nitrogen halide salt to produce vapors;
exposing to the vapors a workpiece made from a self-passivating metal and having a Beilby layer on at least a portion of a surface of the workpiece so that:
a temperature of the exposing is below a temperature at which nitride and/or carbide precipitates form in the workpiece;
the exposing activates the workpiece for nitriding or nitrocarburizing; and
nitriding or nitrocarburizing the workpiece concurrently with the activating.