Processes for producing low nitrogen essentially nitride-free chromium and chromium plus niobium-containing nickel-based alloys and the resulting chromium and nickel-based alloys

1. Process for producing low nitrogen, essentially nitride-free chromium-containing nickel-based alloys comprising:

a) charging elements or compounds of elements which do not dissolve appreciable amounts of nitrogen in the molten state to a refractory crucible within a vacuum induction furnace and melting said elements or compounds therein under reduced pressure lower than 0.1 mbar;

b) effecting heterogeneous carbon-based bubble nucleation of the resulting melt in a controlled manner, thereby effecting substantial removal of nitrogen and oxygen from the melt;

c) upon cessation of bubble formation, adding low nitrogen chromium with a nitrogen content of below 5 ppm to the melt, said chromium having been prepared by:

i) vacuum-degassing a thermite mixture comprising chromium compounds and metallic reducing agents, contained within a vacuum vessel capable of withstanding a thermite reaction, to an initial pressure less than 1 mbar; and back filling the vessel with an inert gas to a pressure of about 100-200 mbar;

ii) igniting the thermite mixture to effect reduction of the chromium compounds within said vessel under reduced pressure;

iii) solidifying the reaction products under reduced pressure; and

iv) cooling the reaction products to about ambient temperature under reduced pressure,

wherein steps ii) through iii) are conducted under a pressure between about 100 to 200 mbar;

d) melting and distributing said added chromium throughout the melt under reduced pressure;

e) bringing the resulting combined melt to a temperature and surrounding pressure to permit tapping; and

f) tapping the resulting melt, directly or indirectly, to a metallic mold and allowing the melt to solidify and cool under reduced pressure, wherein the resulting alloy has a nitrogen content less than 5 ppm by weight.

2. Process according to claim 1 , wherein the reducing agent employed in the process to which the chromium compounds were previously subjected is aluminum.

3. Process according to claim 1 , wherein the thermite mixture employed in the process to which the chromium compounds were previously subjected additionally comprises at least one energy booster.

4. Process according to claim 1 where the heterogeneous carbon-based bubble nucleation of the resulting melts is effected by the controlled injection of graphite.

5. Process as defined in claim 1 wherein the resulting alloy is re-melted in a vacuum arc re-melting furnace, allowed to homogenize, and then forged into a desired shape.

6. Process as defined in claim 1 , wherein the carbon-based bubble nucleation is effected by controlled insertion of carbon in the form of a member selected from the group consisting of particulate carbon, carbon in the form of a bar, tube or cylinder, and combinations thereof.

7. Process for producing low nitrogen, essentially nitride-free chromium plus niobium-containing nickel-based alloys comprising:

a) charging elements or compounds of elements which do not dissolve appreciable amounts of nitrogen in the molten state to a refractory crucible within a vacuum induction furnace and melting said elements or compounds therein under reduced pressure lower than 0.1 mbar;

b) effecting heterogeneous carbon-based bubble nucleation of the resulting melt in a controlled manner, thereby effecting substantial removal of nitrogen and oxygen from the melt;

c) upon cessation of bubble formation, adding a low nitrogen chromium-niobium-containing master alloy with a nitrogen content below 5 ppm to the melt, said master alloy having been prepared by:

i) vacuum-degassing a thermite mixture comprising chromium and niobium compounds and metallic reducing agents, contained within a vacuum vessel capable of withstanding a thermite reaction, to an initial pressure less than 1 mbar; and back filling the vessel with an inert gas to a pressure between about 100 to 200 mbar;

ii) igniting the thermite mixture to effect reduction of said compounds within said vessel under reduced pressure;

iii) solidifying the reaction products under reduced pressure; and

iv) cooling the reaction products to about ambient temperature under reduced pressure, wherein steps ii) through iii) are conducted under a pressure between about 100 to 200 mbar;

d) melting and distributing said added chromium-niobium-containing master alloy throughout the melt under reduced pressure;

e) bringing the resulting combined melt to a temperature and surrounding pressure to permit tapping; and

f) tapping the resulting melt, directly or indirectly, to a metallic mold and allowing the melt to solidify and cool under reduced pressure, wherein the resulting alloy has a nitrogen content less than 5 ppm by weight.

8. Process according to claim 7 , wherein the reducing agent employed in the process to which the chromium-niobium-containing alloys were previously added is aluminum.

9. Process according to claim 7 , where the thermite mixture employed in the process to which the chromium-niobium-containing alloys were previously subjected additionally comprises at least one energy booster.

10. Process according to claim 7 , wherein the heterogeneous carbon-based bubble nucleation of the resulting melt is effected by the controlled insertion of graphite.

11. Process as defined in claim 7 wherein the resulting alloy is re-melted in a vacuum arc re-melting furnace, allowed to homogenize, and then forged into a desired shape.

12. Process as defined in claim 7 , wherein the carbon-based bubble nucleation is effected by controlled insertion of carbon in the form of a member selected from the group consisting of particulate carbon, carbon in the form of a bar, tube or cylinder, and combinations thereof.