Processes for producing low nitrogen metallic chromium and chromium-containing alloys and the resulting products
Processes for producing low-nitrogen metallic chromium or chromium-containing alloys, which prevent the nitrogen in the surrounding atmosphere from being carried into the melt and being absorbed by the metallic chromium or chromium-containing alloy during the metallothermic reaction, include vacuum-degassing a thermite mixture comprising metal compounds and metallic reducing powders contained within a vacuum vessel, igniting the thermite mixture to effect reduction of the metal compounds within the vessel under reduced pressure i.e., below 1 bar, and conducting the entire reduction reaction in said vessel under reduced pressure, including solidification and cooling, to produce a final product with a nitrogen content below 10 ppm. The final products obtained, in addition to low-nitrogen metallic chromium in combination with other elements, can be used as raw materials in the manufacture of superalloys, stainless steel and other specialty steels whose final content of nitrogen is below 10 ppm.
1. Processes for producing metallic chromium or chromium-containing alloys with a nitrogen content of below 5 ppm comprising:
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 backfilling the vacuum vessel with an inert gas to a pressure up to about 200 mbar;
ii) igniting the thermite mixture to effect reduction of the chromium compounds within said vessel under reduced pressure to generate a melt;
iii) solidifying the reaction products under reduced pressure; and
iv) cooling the reaction products to about ambient temperature under reduced pressure,
wherein igniting the thermite mixture and solidifying the reaction products are conducted under a pressure up to about 200 mbar, wherein the resulting alloy has a nitrogen content less than 5 ppm by weight.
2. Processes according to claim 1 , wherein the vacuum vessel is a ceramic or metallic container lined with refractory material.
3. Processes according to claim 2 , wherein the vacuum vessel is placed inside a vacuum-tight, water-cooled chamber for the entire reduction reaction.
4. Processes according to claim 1 , wherein the reducing agent is aluminum.
5. Processes according to claim 4 , wherein the aluminum reducing agent is in powder form.
6. Processes according to claim 1 , wherein the thermite mixture additionally comprises at least one energy booster.
7. Processes according to claim 1 , wherein the thermite mixture additionally contains an element selected from the group consisting of nickel, iron, cobalt, boron, carbon, silicon, aluminum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, rhenium, copper, and mixtures thereof in their metallic form or as compounds thereof capable of metallothermic reduction.
8. Processes according to claim 1 , wherein the inert gas is a non-nitrogenous gas.
9. Process according to claim 1 , wherein backfilling the vacuum vessel with the inert gas includes backfilling to a pressure of about 200 mbar.
10. Process according to claim 1 , wherein igniting the thermite mixture and solidifying the reaction products are conducted under a pressure of about 200 mbar.