US2883284A - Molybdenum base alloys - Google Patents

Description

United States Patent MOLYBDENUM BASE ALLOYS Dean D. Lawthers, Verona, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application July 30, 1956 Serial No. 600,701

6 Claims. (Cl. 75-211) This invention relates to molybdenum base alloys prepared by pressing and sintering powder mixtures of molybdenum and suitable alloying constituents, and to a method of preparing such alloys.

Molybdenum base alloys containing such alloying constituents as the transition elements are known and have been prepared by arc-casting a mixture of the components that have been pressed to the form of a consumable electrode. Molybdenum base alloys prepared by such methods are not homogeneous, and have coarse elongated grain structures. Furthermore, impurities in solution in arc-cast molybdenum base alloys precipitate at the grain boundaries. These factors make it very diflicult to work arc-cast molybdenum base alloys by conventional techniques of rolling, forging and swaging, and the recovery of material from working arc-cast molybdenum alloys is exceedingly low, frequently being less than 50 percent.

It is among the objects of the present invention to provide molybdenum base alloys that are homogeneous; that are characterized by a fine grain structure; that have high strength at elevated temperatures; that may be worked by conventional techniques; and that can be worked at satisfactory material recovery.

It is another object of the invention to provide a method for producing alloys characterized as in the foregoing object.

In accordance with the invention, molybdenum base alloys containing as alloying components one or more of zirconium, niobium, titanium, tantalum and vanadium are prepared by powder metallurgical techniques of pressing and sintering under conditions described below. The resultant alloys have an equi-axed fine grain structure, a low oxygen content on the order of below about 0.01 percent and are carbon-free except for traces that may be present as an impurity. The resulting alloys, contrary to the characteristics of arc-cast molybdenum base alloys of requiring very high temperatures of the order of 1315 C. or more for working, or the use of complex and tedious techniques to reduce the structure to workable form, may be'readily worked as prepared by conventional techniques, for the resultant alloys are homogeneous and fine grained and the oxygen, or other impurities, that may be present does not segregate at the grain boundaries. Furthermore, the material recovery upon working alloys of this invention exceeds SO percent and frequently exceeds 90 percent, a singular improvement as compared to the recovery of 50 percent or less of the arc-cast alloys.

Alloys of this invention contain one or more of the alloying constituents in an amount within the range of solubility of the alloying constituent in molybdenum, for alloys that contain the alloying constituents in excess of-its solubility are brittle and, therefore, may not be worked. Typical compositions contain, by weight, 0.1 to 2 percent of zirconium, or 0.1 to 10 percent of any of titanium, tantalum, vanadium, and niobium with the balance being essentially pure molybdenum. Other elements and impurities may be present as long as they do not deleteriously affect the resultant alloys.

The molybdenum base alloys are produced by powder metallurgical techniques. In a typical procedure a commercial molybdenum powder, usually of average particle size that will pass a minus 325 mesh screen, is treated to eliminate, essentially, the oxygen content of the molybdenum, for example by hydrogen reduction in a furnace at a suitably elevated temperature so that the reaction can proceed and the oxygen can be removed. Heating in hydrogen for about 1 to 6 hours at a temperature of about 500 to 1000 C. will normally be satisfactory, with the optimum conditons being determined primarily by the amount and nature of the oxygen content of the molybdenum. The resultant oxygen-free molybdenum powder is then mixed with the alloying constituent in a suitable blender. When the powders are mixed, they are pressed at a pressure of the order of 10,000 p.s.i. or higher to compact the powders sufiiciently to permit further handling. The pressed article is then sintered at a temperature of at least 1700 C., preferably above 2000 C., but below the melting point of the alloy; about 2 to 30 or more hours at sintering temperature is sufificient to insure satisfactory alloying. The density of the resultant alloy must be at least percent of theoretical to insure good workability. During sintering, uniform diffusion of the alloying constituent through the molybdenum occurs and a true alloy ingot results.

Handling, storage and processing of the molybdenum powder until it has been pressed to a compact most suitably is accomplished under inert conditions, for the rate of oxygen pick-up of molybdenum powder is quite rapid and the presence of significant quantities of oxygen in the resultant alloys adversely afiects their properties. An atmosphere of helium or argon, and the use of a vacuum have been found to be satisfactory. While the presence of oxygen should be minimized, I have found that it is not essential to exclude it entirely, for it can be tolerated for up to about five minutes without deleterious oxygen pick-up occurring. For analogous reasons, the sintering of the compact is accomplished under a non-reactive atmosphere. A vacuum of the order of less than 0.1 micron is satisfactory. An inert gaseous atmosphere can also be used; however, it is ditficult to obtain an inert gas of a purity sufiicient to avoid oxygen pick-up at the conditions of temperature and time that are used in the sintering step.

Alloys prepared as just described, in addition to being workable by conventional practices, are more uniform than arc-cast alloys for the powder mixture can be homogenized prior to pressing and sintering, and normal and inverse segregation during solidification does not occur for the metal is never molten. Uniformity also is promoted by sintering the compositions for several hours to permit substantially complete diffusion of the components. The small grain structure of the alloys indicates the presence of a large grain boundary area which, together with the absence of grain boundary segregation, contributes to their working characteristics.

Several alloys were prepared and tested for their high temperature strength. During processing contact between oxygen and the oxygen-free molybdenum powder was entirely excluded by blending and pressing the powder mixtures in a helium atmosphere and conducting the sintering step in a vacuum. Stress relieved samples of the various alloys were then tested for their stress characteristics at high temperature. In the following table are the data obtained; the amount of alloying constituent in the alloys is by weight percent.

Table Hours at Stress of Molybdenum Base Alloys at 982 0. Stress, p.s.i.

1.89 1.75 1.457 1.647 v. Nb? T1. Ia-.

All of'the data on the zirconium alloy were obtained on a-single sample, it being used at the successively higher stresses until it finally ruptured after 23 hours at 100,000 p.s.i.

These data demonstrate the unusual strength of alloys prepared according to the method of the present invention. The extraordinary high strength of the zirconium alloys is especially apparent. Indeed, these zirconium alloys have the highest strength at elevated temperatures of any alloys of which I am aware. This unusual aspect of these alloys was further demonstrated by testing an 0.4 percent zirconium-molybdenum alloy that was prepared by the described process at 40,000 p.s.i. at 982 C. During the test the furnace overheated and the sample was at 40,000 p.s.i. for 12 hours at a temperature of 1093 C. This sample ruptured after 170 hours. These data and that included in the table demonstrate the singular strength of this alloy, especially when it is considered that unalloyed molybdenum that is stress relieved ruptures in about 3.6'hours at 982 C. at a stress of only 18,000 p.s.i.

A molybdenum base alloy of this invention containing 1.89 percent vanadium has a 100 hour rupture strength of 17,000 p.s.i. at 982 C., and this, too, is a significant improvement over unalloyed molybdenum. Moreover, this alloy may be cold worked at a 90 percent reduction in area with a material recovery of over 90 percent. When a MoV alloy of the same vanadium content is prepared by arc-casting, working results in a recovery of less than 50 percent.

Similarly favorable comparisons occur upon considering the other alloys of my invention. Thus, a 1.75 percent niobium alloy in the stress relieved condition has a 100 hour rupture strength of 35,000 p.s.i. at 982 C.; a one percent niobium alloy was tested at 35,000 p.s.i. at 982 C. and did not rupture after 500 hours, the test being stopped due tofurnace failure. A stress relieved 1.45

percent titanium alloy has a 100 hour rupture strength of over 30,000 p.s.i. at 982 C., as does a stress relieved 1.64 percent tantalum alloy. It is thus apparent that all of the alloys of this invention have significant high temperature strength.

A particular advantage of these alloys as compared with pressed and sintered molybdenum is that the alloys have the higher recrystallization temperature, generally ranging from 100 to 300 C. higher than that of the molybdenum. Furthermore, the hardness of the alloys exceeds that of molybdenum at 1100 C., in some instances by as tion temperature of -170 C. in the cold worked condition and 40 C. in the recrystallized condition, the corresponding temperatures of an 0.1 percent zirconiummolybdenum alloy of this invention are 160 C. and 70 C., respectively. The other alloys of the invention, similarly to unalloyed molybdenum, evidence sufiicient ductility at about room temperature to permit conventional handling.

Bars or slabs of the sintered alloys can be readily hot worked at moderate temperatures far below the temperatures required to work arc-cast ingots. Thus, sintered bars wereprepared from each of the alloys in the table, the bars being 7 inches long by 1.5 inches wide and about 0.6 inch thick. The sintered bars were heated to temperatures of from 871 C. to 1204 C., in a non-oxidizing atmosphere, were hot rolled, with intermediate reheating if necessary, to strips of a thickness of 0.06 inch. Reductions of to 95% were etfected. The sintered bars can be readily forged, swaged or extruded. A recovery of usable strip of over 50% was realized in all instances, and in some cases the recovery of usable rolled metal was in excess of of the sintered bar. It will be appreciated that the high recovery and low hot working temperatures are outstanding benefits of the present invention.

The hot worked strip can be cold rolled or cold worked after reduction to a thickness of between about 0.05 and 0.06 inch. The hot working can be effected to produce strips, bars, rods, wire and the like. The shaped members produced by hot or cold working can be stress-relieved by annealing, at for example, 982 C. for one hour while in a non-oxidizing atmosphere.

It will be understood that ternary and other molybdenum base alloys may be prepared from two or more of the alloying components using the sintering and working procedures of the present invention. An'excellent alloy comprises 2% niobium, 3% titanium and thebalance molybdenum. Another ternary alloy comprises 0.5% zirconium, 1% niobium and the balance moylbdenum. A quaternary alloy comprises 2.5% titanium, 2% niobium, 0.4% zirconium and the balance molyb-. denum. v I

It will be understood that the alloys may be preparedv by simply admixing the molybdenum powder and the alloying component in powdered form if a highly ,de-'

oxidized molybdenum having less than 0.01% of oxygen is employed, and the alloying powder is similarly free from oxygen.

According to the provisions of the patent statutes, I

have explained the principle of my invention and have described what I now believe to be its best embodiment. However, I desire to have it understood that the invention may be practiced otherwise than as specifically described.

I claim:

1. In the process of preparing members from molybdenum base alloys, the steps comprising intimately admixing molybdenum powder having an oxygen content of below 0.01%. by weight with at least onesubstantiallyf oxygen free powdered metal in the given proportions se-' lected from the group consisting of from 0.1 to 2% by weight of zirconium, from 0.1 to 10% by weight of niobium, 0.1% to 10% by weight of tantalum, from 0.1 to 10% by weight of titanium and from 0.1 to 10% by weight of vanadium, compressing the resultant powdered mixture to a compact, and sintering the compact in the absence of a reactive atmosphere at a temperature above about 1700 C, but below the melting point of the mixture for a period of time to alloy the powders and to produce a sintered member having a density at least.

% of the theoretical density of the alloy, said powders being maintained substantially free from contact with reactive gases.

2. The process of claim 1, wherein the sintered member is heated to a temperature of from about 871 C- to 1204 C. in a non-oxidizing atmosphere and hot worked asaaaae to desired shape, the resultant usable worked alloy mem ber comprising over 50% of the sintered member.

3. That method of preparing a molybdenum base alloy that, as prepared, can be worked at a material yield in excess of 50 percent, that consists of deoxidizing a molybdenum powder with hydrogen at an elevated temperature, blending with the resultant reduced molybdenum powder a member selected from the group of powders consisting of 0.1 to 2 percent of zirconium, 0.1 to percent of vanadium, 0.1 to 10 percent of niobium, 0.1 to 10 percent of titanium, and 0.1 to 10 percent of tantalum, pressing the resultant powder mixture to a compact, and sintering the compact in a non-reactive atmosphere at a temperature below the melting point of the resultant alloy to alloy the powders and result in an alloy having a density of at least 95 percent of theoretical, said powders being maintained in an inert atmosphere prior to pressing for at least all the period of time involved that is in excess of five minutes.

4. Method according to claim 3, said sintering being conducted in a vacuum of below about 0.1 micron pressure.

5. A molybdenum base alloy that, as prepared, can be worked at a material yield in excess of 50 percent, that consists of the alloy prepared by deoxidizing a molybdenum powder with hydrogen at an elevated temperature to an oxygen content of below 0.01% by weight, blending with the resultant reduced moylbdenum powder a member selected from the group of powders consisting of 0.1 to 2 percent of zirconium, 0.1 to 10 percent of vanadium, 0.1 to 10 percent of niobium, 0.1 to 10 percent of titanium and 0.1 to 10 percent of tantalum, pressing the resultant powder mixture to a compact, and

sintering the resultant compact in a non-reactive atmosphere at a temperature below the melting point of the resultant alloy to alloy the powders and result in an alloy having a density of at least 95 percent of theoretical, said powders being maintained in an inert atmosphere prior to pressing for at least all the period of time involved that is in excess of five minutes.

6. A molybdenum base alloy that, as prepared, can be worked at a material yield in excess of percent, that consists of the alloy prepared by deoxidizing a molybdenum powder with hydrogen at an elevated temperature to an oxygen content of below 0.01% by weight, blending 0.1 to 2 percent of zirconium powder with the resultant reduced molybdenum powder, pressing the blend of powders to a compact, and sintering the resultant compact in a non-reactive atmosphere at a temperature below the melting point of the resultant alloy to alloy the zirconium and molybdenum and result in an alloy having a density of at least percent of theoretical, said powders being maintained in an inert atmosphere prior to pressing for at least all the period of time involved that is in excess of five minutes.

References Cited in the file of this patent UNITED STATES PATENTS 2,491,866 Kurtz et al. Dec. 20, 1949 2,692,216 Baker Oct. 19, 1954 2,776,887 Kelly et al. Jan. 8, 1957 OTHER REFERENCES Pipitz et al.: Powder Metallurgy Bulletin, vol. 7, No. 2 (August 1955), pp. 53-59.

Claims (1)

1. IN THE PROCESS OF PREPARING MEMBERS FROM MOLYBDENUM BASE ALLOYS, THE STEPS COMPRISING INTIMATELY ADMIXING MOLYBDENUM POWDER HAVING AN OXYGEN CONTENT OF BELOW 0.01% BY WEIGHT WITH AT LEAST ONE SUBSTANTIALLY OXYGEN FREE POWDERED METAL IN THE GIVEN PROPORTIONS SELECTED FROM THE GROUP CONSISTING OF FROM 0.1 TO 2% BY WEIGHT OF ZIRCONIUM, FROM 0.1 TO 10% BY WEIGHT OF NIOBIUM, 0.1% TO 10% BY WEIGHT OF TANTALUM, FROM 0.1 TO 10% BY WEIGHT OF TITANIUM AND FROM 0.1 TO 10% BY WEIGHT OF VANADIUM, COMPRESSING THE RESULTANT POWDERED MIXTURE TO A COMPACT, AND SINTERING THE COMPACT IN THE ABSENCE OF A REACTIVE ATMOSPHERE AT A TEMPERATURE ABOVE ABOUT 1700*C. BUT BELOW THE MELTING POINT OF THE MIXTURE FOR A PERIOD OF TIME TO ALLOY THE POWDERS AND TO PRODUCE A SINTERED MEMBER HAVING A DENSITY AT LEAST 95% OF THE THEORETICAL DENSITY OF THE ALLOY, SAID POWDERS BEING MAINTAINED SUBSTANTIALLY FREE FROM CONTACT WITH REACTIVE GASES.