US3476548A - Method for removing oxides from alloy powder - Google Patents

Description

United States Patent O flice 3,476,548 Patented Nov. 4, 1969 3,476,548 METHOD FOR REMOVING OXIDES FROM ALLOY POWDER Thomas S. Cloran, East Liverpool, Ohio, assignor to Crucible Inc., a corporation of Delaware No Drawing. Filed Dec. 9, 1966, Ser. No. 600,379 Int. Cl. B22f 1/00; C23g 1/02 US. Cl. 75-.5 6 Claims ABSTRACT OF THE DISCLOSURE It is known to produce metal-alloy articles, such as highalloy steel articles, by obtaining a powder of the desired alloy; placing the powder, which has been cleaned of contaminating surface oxides, in a protective atmosphere, such as a vacuum, and compacting the powder to form the desired article. Compacting is conventionally performed at elevated temperatures and consequently an inert atmosphere is required to prevent oxidation of the powder. Oxidation, of course, impairs bonding and more importantly results in a final compacted article having poor cleanliness.

For the purpose of cleaning metal powders prior to use in the above powder metallurgy operations, it is customary to clean them by removing surface oxides by operations substantially similar to those that would be used were the alloy in a form such as strip, sheet, or wire. For example, the alloy particles are subjected to a treatment in molten caustic, acid pickling and mechanical scale breaking, such as sonic vibration. These operations may be performed singly or in combination depending upon how difiicult the oxide of the particular alloy is to remove. The more difiicult or complex the alloy, the more severe must be the treatment.

Although the above-described cleaning technique has been found suitable for many alloy compositions, there are oxides of particular alloying elements, such as titanium, and aluminum, which are used as alloying elements in the so-called superalloys that present a particular problem. When the surface of these alloys is made substantially clean and free from oxide, upon exposure to an oxidizing atmosphere, such as air, immediate reoxidation will occur. Consequently, when using these alloying elements in powder metallurgy operations, it is necessary not only to effect removal of the complex surface oxides prior to compacting the powders of these alloys into articles, but more importantly it is also necessary to protect the powders once cleaned from reoxidation prior to compacting.

It is accordingly a primary object of the invention to provide a method whereby after cleaning by oxide removal the surfaces of alloy powders may be protected against reoxidation in the presence of oxidizing atmospheres prior to compacting.

This and other objects of the invention as well as a complete understanding thereof may be obtained from the following description and examples.

In the practice of the invention, a charge of alloy particles, which are intended for subsequent use in compacting into alloy articles, are placed in a bath of molten caustic. Any of the well-known caustics, such as sodium hydroxide and potassium hydroxide, may be used. The molten caustic is typically at a temperature of about 1000? F., and the powder is maintained therein for about 15 minutes. The particles are removed from the caustic and rinsed with water to remove excess caustic therefrom. Thereafter, the particles are transferred to an acid pickling bath, which may be, for example, hydrochloric acid or sulfuric acid. During acid pickling the powder is preferably subjected to sonic vibration to insure complete contact of the particle surface area with the acid. After pickling the particles are rinsed with water to remove excess acid. It is to be understood, of course, that more than one pickling operation, followed by water rinsing, may be employed.

After the particles have been pickledl and the excess acid removed by water rinsing, they are, of course, saturated with water. Upon exposure to the atmosphere in this condition, the surfaces of the powder, which have been cleaned of surface oxides by the treatment described above, will begin to reoxidize. Because these oxides, as explained above, are difficult to remove, adequate removal of oxides, particularly oxides of titanium and aluminum, thus formed will not result by merely placing the powder in an evacuated container and heating the same to an elevated temperature prior to compacting, which is a customary practice in powder metallurgy. Consequently, it is essential, if an oxygen-free article is to be produced, to protect the cleaned powders from reoxidation while exposed to oxidizing atmospheres prior to compacting.

In accordance with the present invention, the cleaned particles, after being subjected to the above-described oxide removal operation, are dried (water removed) and protected from further oxidation in the following manner. The wet particles are placed in a container, preferably that which is to be used during subsequent heating, evacuation, and compacting. The water is then displaced by the addition of an agent, such as methylene chloride. Methylene chyloride, or any drying agent substituted therefor, such as hexane or toluol, must have the properties of zero solubility for water, high vapor pressure, and be heavier than water. Upon displacement of the water, which may be removed by decanting, the methylene chloride saturated powder is protected against reoxidation by exposure to an oxidizing atmosphere, such as air. The methylene chloride provides a barrier between the surface of the powder and the surrounding air. The powder may then be placed in a chamber for evacuation and heating to elevated temperature whereupon, in the conventional manner, pressure may be applied to compact the powder into the desired alloy article. Since methylene chloride has zero solubility for water, no oxygen which would otherwise be present from any water dissolved in the methylene chloride is present during the heating operation. The high vapor pressure of the agent, as indicated by its low boiling point of 40 C., insures complete removal of the agent from the powder during evacation and prior to compacting. The vapor pressure of the drying agent must be sufficiently high so that at the temperatures and pressures prevailing in a particular operation, the drying agent will be completely removed prior to compacting. If the drying agent is not completely removed prior to compacting, such will result in contamination of the final compacted article. It may be seen, therefore, that in accordance with the method of the invention as outlined above dry, clean, oxide-free powders are protected against reoxidation prior to compacting to result in a compacted article having an extremely low oxygen content and a corresponding high degree of cleanliness.

A charge of a mesh powder of a precipitation hardenable, nickel-chromium base alloy having in percent by weight- 12.50 chromium, 4.20 molybdenum, 2.20 columbium and tantalum, 6.10 aluminum, 0.80 titanium, 0.12 carbon, 0.012 boron, 0.10 zirconium, and balance nickel in a 100 gram quantity was poured into a sealed end of a A in. pipe about 3 ft. in length. Steel wool was then tapped on top of the charge to secure it at the bottom 2 inches of the pipe. The open end of the pipe was connected to a vacuum pump and pumping was begun and continued until the outgassing rate stabilized (about 1 micron per minute). About 12 to 14 in. of the closed end of the pipe were placed in a furnace at a temperature of 2200 F. and maintained therein until the outgassing rate stabilized at about 100 microns per minute. The pipe was removed from the furnace and the end containing the charge was compressed by a ZOO-ton press operated at about 25-ton per square inch. This produced a compact approximately A1 in. thick, 2 in. wide, and 3 in. long. Sections of this compact were obtained and olished specimens were prepared therefrom. The polished specimens were examined at magnifications of about 400 Prior to the above-described treatment, the powder was cleaned by treatment in a hot sodium hydroxide bath (about 850900 F.) for approximately 15 minutes, water rinsed to remove excess caustic, placed in bath of hydrochloric acid (160180 F.) for approximately 5 minutes while being subjected to sonic vibration, and water rinsed.

The powder was divided into a number of 100-gram charges, which were air dried and subjected to the above treatment. Polished specimens of this material exhibited 85-90 percent oxide-free particle boundaries.

The operation was repeated, except that the air-drying step was replaced by an operation wherein the wet powder was placed in a container, and covered with methylene chloride. The water was displaced and removed by decanting. The owder saturated with methylene chloride was then treated as described above. Polished specimens of this compacted material exhibited 95-100 percent xide-free particle boundaries.

The above experimental work indicates that by drying and protecting the powder after cleaning and prior to oxidation by using methylene chloride, reoxidation of the powder is prevented to result in a final compact having oxide-free particle boundaries, which is not the case if the particles are air-dried in the conventional manner.

Although various embodiments of the invention have been described herein, it is obvious that other adaptations and modifications may be made by those skilled in the art without departing from the spirit and scope of the appended claims.

What is claimed is:

1. A method for producing a charge of alloy powder free of complex surface oxides for use in compacting to form an alloy article comprising treating said powder with molten caustic, water rinsing said powder to remove excess caustic, acid pickling said powder, water rinsing said powder to remove excess acid, removing the water from said powder by displacing the same with a dry ng agent, said drying agent being heavier than water, havmg substantially zero solubility for water and high vapor pressure, whereby said drying agent provides a barrier between the chemically clean surfaces of the powder and the oxidizing atmosphere.

2. A method according to claim 1 wherein said drying agent is methylene chloride.

3. A method according to claim 1 wherein said alloy powder contains an amount of at least one alloying ele- 0 ment selected from a group consisting of titanium and aluminum.

4. A method according to claim 1 wherein said drying agent is selected from a group consisting of methylene chloride, hexane, and toluol.

5. A method according to claim 1 wherein said powder is subjected to sonic vibration during acid pickling and displacing of the water with said drying agent.

6. A method for producing a charge of alloy powder free of complex surface oxides for use in compacting to form an alloy article comprising treating said powder with molten caustic, rinsing said powder to remove excess caustic, acid pickling said powder, water rinsing said powder to remove excess acid, removing the water from said powder by displacing the same with a drying agent, said drying agent being heavier than water, having substantially zero solubility for water and high vapor pressure whereby said drying agent provides a barrier between the chemically clean surfaces of the powder and the oxidizing atmosphere, charging said powder saturated with said drying agent to a gas-tight container to remove said drying agent by vaporization thereof, whereby said charge of alloy powder is dry and free of complex oxides and prepared for compacting.

References Cited UNITED STATES PATENTS 2,027,532 1/1936 Hardy 75-211 2,458,661 1/1949 Webster et al. 1343 2,567,456 9/1951 Webster 134--3 2,630,393 3/1953 Francis 1343 2,638,424 5/1953 Hansgirg 1343 L. DEWAYNE RUTLEDGE, Primary Examiner T. R. FRYE, Assistant Examiner US. Cl. X.R.