US2976194A - Methods for improving the properties of refractory metals - Google Patents

Description

United States ate'nt METHODS FOR Ill/IPROVING THE PROPERTIES OF REFRACTORY METALS Maurice R. Commanday, Los Angeles, and James-R.

Darnell, Reseda, Calif., assignors, by mesne assignments, to Chromizing Corporation, a corporation of Delaware No Drawing. Filed July 10, 1959, Ser. No. 826,112

8 Claims. (Cl. 148-131) This invention has to do generally with an improved process for improving the properties of metals by their surface treatment, and relates particularly to the conditioning of the refractory metals tungsten and molybdenum as and for the purposes herein explained.

The refractory metals molybdenum and tungsten are subject tocontamination when heated in air or other non-inert gases to the temperatures required to relieve stresses induced by mechanical working (in the 1700 to 2000 F. range). It is generally believed that this contamination greatly reduces the important property of ductility at temperatures around the normal ambient range. In addition, these refractory metals are notoriously reactive with air at annealing temperatures, to the extent that some means must be taken to prevent contact with air or other oxidizing gases; otherwise complete destruction of the material results. Presently, and heretofore, stress relief annealing of these materials is understaken in furnaces in which high purity hydrogen or argon atmospheres are maintained; These atmospheres areeither reducing or neutral to a considerable degree and are capable of preventing oxidation and, in the instance of hydrogen as applied to annealing of molybdenum,'surface oxides are reduced at this temperature. p

The construction, maintenance and operationof'large furnaces employing these special atmospheres is both expensive and, in the case of hydrogen, hazardous. One object of this invention is to eliminate the need for complex, expensive equipment, and is a further object of this invention to eliminate the hazards associated with the use of hydrogen atmosphere furnaces.

It appears that room temperature ductility of many metals is directly related to freedom from contamination, and that the impurities responsible for-thisapparent lack of ductility may exist in the form of interstitial compounds such as oxides, nitrides, etc. 'I nterstitial contamination can be greatly alleviated through the use of special annealing atmospheresthat employ mixtures of hydrogen with halidegases, but the employment 'of'these highly specialized annealing atmospheres is' expensive. and cumbersome. In addition, since all'halides" are highly toxic, obvious hazards are involved.

A further purpose of this invention is to provide halide gas annealing atmospheres without the use of expensive equipment and under conditions assuring complete safety to working personnel and equipment.

As illustrative, for purposes of presenting the invention in its application to the refractory metals molybdenumand 1 tungsten, we may refer to a material such as .5 titaniumbalance molybdenum alloy, a refractory metal in current popular use, the interstitial contamination in which can be measured in terms of its ductile-to-brittle bend transition temperature. If one attempts to bend a given piece of this material at a temperature below the transition temperature for that particular piece, it will break with little or no plastic deformation as though it were glass or ceramic. If the same attempt to bend is made at a temperature above the transition temperature for the piece ice 2 in question it will bend and exhibit reasonable plastic deformation. The temperature at which this change takes place (i.e., from ductile-to-brittle bend characteristics) is extremely well defined within a matter of four or five degrees Fahrenheit. There is no gradual change, but rather a very abrupt one.

of tungsten This powdered mixture is-in the 'nature'of u a pack which may contain an additional powdered'co'm t y Studies have been made of bend transition temperatures for various lots of .5% titanium-balance molybdenum alloy from various suppliers, and values have been established ranging from minus 30 F. to plus 200 F. In addition, considerable variation in transition temperature has been observed on pieces cut from the same sheet. This can be understood in terms of a variance in the distribution of interstitial contamination over the surface of a given piece of material. This is a particularly awkward condition from the point of view of fabrication techniques such as bending, forming, shearing, etc., since the variation in transition temperature occurs Within the normal room temperature range. For this reason, shop personnel may find that they may safely shear and bend a piece of .5% titanium-balance molybdenum alloy at room temperature, or the specimen may break, depend-- ing upon its specific transition temperature (i.e. purity). This has led to the adoption of the practice of hot'bending and hot shearing, which is expensive and awkward, and, in fact, may add to the contamination. Moreover, many refractory metal items of manufacture contemplated would be far more valuable if they could be considered ductile over a broader range of temperatures. Such items may be subject to temperatures below freezing as well as in the range of 3000 F. and higher. Obviously, any accidental or service impact loads imposed on a refractory metal part at temperatures below its transition temperature would be likely to cause complete failure.

it has been shown that the transition temperature of .5% titanium-balance molybdenum alloy sheet can be signficantly reduced by the order of 20 degrees Fahrenheit by removal of the outer .005" of material by grinding. It is generally accepted that the 'portion of material most clearly responsible for loss of ductility is the outer layer, which is evidently more heavily contaminated, It

is a current practice, therefore, to surface grind material prior to working. This operation is both costly in the grinding as well asin the loss of expensive material; frequently of the original supplied weight of the'mate:

rial. 1

The present invention aims to so purify the outer layers of material as to eliminate the necessity of surface grind It appears possible that the application of, this.

these metals-alloyed with smaller percentages of other metals such as titanium,-nickel, chromium and others depending upon particular properties desired in the'alloy'; i

As' generally contemplated, the present process involves heating them'etal (molybdenum, tungsten or their alloys) in contact with a powdered mixture of an ammonium halide salt, and a powdered quantity of the same refractory metal, i.e. powdered molybdenum where the metal 1 is molybdenum or an alloy thereof, or powdered tungsten where the metal being treated is tungsten or an alloy ponent serving as an inert solid diluent. I

The only equipment required for stress relief'anneal ing of the refractory metal consists of an appropriate furnace, such as a simple gas-fired, open-fire or semimuflle type furnace, and a treatment retort, chamber or box of the kind well known in the art of diffusing metals such as chromium, onto the surfaces of metals being treated. See e.g. Samuels Patent No. 2,536,774. The metal to be treated is packed in the retort box in surface contact with a finely powdered compound comprising approximately 50% to 95% by weight of an inert diluent such as clay, pumice, talc, silica or preferably tabular alumina (Alcoa T-61) 60 mesh and finer; to 50% molybdenum or tungsten (as the case may be) powder 60 mesh and finer, and a small quantity, .05 to .5 (more may be used but to no advantage) of ammonium halide salt such as ammonium bifluoride, ammonium bromide, ammonium iodide or ammonium chloride. This powdered pack material is compacted about the metal being treated so as to exclude as much air as possible. The treatment retort may be of the known box type having a top sealed to the body of the box by means of a fusible seal such as powdered glass, which melts during the heating period to allow gases to bleed out of the box, and which freezes upon removal from the furnace so as to prevent the ingress of air upon cooling. The loaded and sealed retort is introduced into a furnace heated to a temperature that may be in the range of about 1700 F. to 2200 F., or higher (typically about 1850 F.), and held there for a period of time ranging from four to twenty-four hours. During the initial period of heating, the ammonium halide salt decomposes to form cracked ammonia and halogen gas. The halide gas reacts preferentially with the powdered molybdenum or tungsten, since the powder affords a greater surface area than the metal being treated, so that a mixture of molybdenum or tungsten halide and cracked ammonia pervades the treatment zone.

The exact nature of the ensuing chemical reactions are not fully understood. However, we believe that the cracked ammonia acting in conjunction with the molybdenum or tungsten halide reduces the oxides and forms gaseous compounds with the unwanted interstitial contaminants at or near the surface, leaving a purified surface. This purified surface then is believed to set up an unstable zone into which subsurface interstitial contami' nants diffuse to be reacted and removed by the aforementioned gaseous mixture. The mechanism may be analogous to the reaction occurring in decarburization of carbon steel in which, through a surface chemical reaction, the solid compounds are caused to be removed from zones near the surface. 1

The following examples are illustrative of the invention:

Five specimens of cold worked .5 titanium-balance molybdenum sheet metal approximately 0.060" thick were treated for 8 hours at 1850 F. in contact with packs composed as follows:

Specimen #1:

No treatment (pack) Specimen #2:

Pack mixture 0% molybdenum approximately 100% tabular alumina, plus 3% ammonium bifluoride. Specimen #3:

Pack mixture 1% molybdenum, approximately 99% tabular alumina, plus .3% ammonium bifluoride 6 4 Specimen #4:

Pack mixture 3% molybdenum, approxnnately 97% tabular alumina, plus .3% ammonium bifiuoride. Specimen #5:

Pack mixture 5% molybdenum, approximately tabular alumina, plus 3% ammonium bifiuoride.

After treatment, the pack-treated specimens were removed from the retort in which they had been allowed to cool in still air, and visually examined. Specimen #2 was observed to be duller than the remaining treated specimens which were all bright in color. Subsequently, the specimens were sectioned transversely and the following Knoop micro-hardness determinations were made;

Nsuo Specimen No. 0.003" 0.000"

" Distance within :1 treated surface as measured on a cut for.

Softness of the metal is generally a function of its purity, i.e. freedom from interstitial contamination. From these data, it is observed that increasing softness follows increases in the molybdenum content of the pack.

We claim:

1. The method for surface decontamination and improvement of the properties of a refractory metal of the group consisting of molybdenum and tungsten and alloys of molybdenum and tungsten wherein the alloys consist essentially of molybdenum or tungsten, that includes heating the metal within a closed zone in contact with a powdered uniform mixture comprising about 50 to 95 percent (by weight) inert diluent, about 5 to 50 percent of the same refractory metal in powder form, and about 0.05 to 0.5% ammonium halide, for a period of about 4 to 24 hours at a temperature sufiicient to dissociate the halide and combine the released halogen with the powdered refractory metal.

2. The method according to claim 1, in which the diluent is tabular alumina.

3. The method according to claim 1, in which the halide is ammonium bifluoride.

4. The method according to claim 1, in which said temperature is between about 1700 F. to 2200 F.

5. The method according to claim 1, in which the refractory metal is essentially molybdenum.

6. The method according to claim 1, in which the refractory metal is essentially molybdenum and the halide is ammonium bifluoride. 7. The method according to claim 1, in which the said same metal is essentially molybdenum, the. halide is ammonium bifluoride and said temperature is between about 1700 F. and 2200 F.

8. The method according to claim 1, in which the refractory metal is essentially tungsten.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. THE METHOD FOR SURFACE DECONTAMINATION AND IMPROVEMENT OF THE PROPERTIES OF A REFRACTORY METAL OF THE GROUP CONSISTING OF MOLYBEDENUM AND TUNGSTEN AND ALLOYS OF MOLYBDENUM AND TUNGSTEN WHEREIN THE ALLOYS CONSIST ESSENTIALLY OF MOLYBDENUM OF TUNGSTEN, THAT INCLUDES HEATING THE METAL WITHIN A CLOSED ZONE IN CONTACT WITH A POWDERED UNIFORM MIXTURE COMPRISING ABOUT 50 TO 95 PERCENT (BY WEIGHT) INERT DILUENT, ABOUT 5 TO 50 PERCENT OF THE SAME REFRACTORY METAL IN POWDER FORM, AND ABOUT 0.05 TO 0.5% AMMONIUM HALIDE, FOR A PERIOD OF ABOUT 4 TO 24 HOURS AT A TEMPERATURE SUFFICIENT TO DISSOCIATE THE HALIDE AND COMBINE THE RELEASED HALOGEN WITH THE POWDERED REFRACTORY METAL.