US2638426A

US2638426A - Method for heat-treating metals having difficultly reducible oxides

Info

Publication number: US2638426A
Application number: US127858A
Authority: US
Inventors: Porter H Brace
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1949-11-17
Filing date: 1949-11-17
Publication date: 1953-05-12
Anticipated expiration: 1970-05-12

Description

May 12, E

P. H. B METHOD FOR HEAT-TREATING METALS HAVING DIFFICULTLY REDUCIBLE OXIDES Filed Nov. 17, 1949 T0 HYDROGEN PURIFIER 33 1 AND SUPPLY I6 T0 vAcuuM PUMP 17 l2 TO ATMOSPHERE l4- 2/ l I 20- l r I '52 I l 1 23 I 24 2 27 HI HHH 27 IN VEN TOR. PoR TE}? H. BRAGE l/VVE/V TOR Patented May 12, 1953 METHOD FOR HEAT-TREATING METALS HAVING DIFFICULTLY REDUCIBLE OX- IDES Porter H. Brace, Pittsburgh, Pa., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application November 17, 1949, Serial No. 127,858

Claims. 1

The present invention relates to heat treating, such as furnac brazing or bright annealing or the like, of alloys containing metals having difficultly reducible oxides, and more particularly relates to a process and apparatus for furnace brazing or bright annealing such alloys or metals in a reducing and beneficiated atmosphere.

The present invention is concerned with the problems of furnace brazing, bright annealing and coating of metals or alloys when these metals comprise, as the sole element or as a major or minor constituent, readily oxidizable metals whose oxides are diificultly reducible. The following constitute examples of metals that are readily oxidizable and whose oxides are difficultly reducible: titanium, berylium, aluminum, chromium, magnesium, tantalum, columbium, cerium, and silicon. The foregoing elements are frequently alloyed with various base metals such as iron, copper, nickel or cobalt to impart desirable and advantageous properties to the resultant metal, but when present therein, being readily oxidizable metals Whose oxides are diflicultly reducible, result in brazing and bright annealing difliculties which are not overcome by the usual reducing atmosphere furnaces. For example, iron, cobalt, and nickel form an alloy of 10W expansivity known as Kovar and may be readily furnace brazed in a commercial tank hydrogen atmosphere. The addition of a small percentage of titanium to Kovar confers age hardening properties that greatly enhance the usefulness of the alloy; however, an attempt to braze or bright anneal titanium-Kovar in a tank hydrogen atmosphere results in the formation of a tough black adherent oxide film that prevents wetting thereof by a brazing metal, and obviously no bright anneal can be obtained. Even in vacuums of less than 10 millimeters of mercury and in hydrogen atmospheres of much better than ordinary purity, filming and tarnishing of the alloy occurs which effectively prevents brazing and bright annealing. The present invention contemplates a method whereby alloys such as titanium- Kovar and other alloys containing readily oxidizable metals Whose oxides are difiicultly reducible, such as those metals aforelisted, may be furnace brazed, bright annealed, or coated with a lower melting metal or alloy; and the present fied hydrogen gas beneficiated in reducing quality by the vapor of metallic chromium.

It is, therefore, one object of the present invention to provide for the furnace treating of metals or alloys comprising a readily oxidizable metal whose oxide is difiicultly reducible.

It is another object of the present invention to provide for the furnace brazing, the bright annealing, or the coating with lower melting metals or alloys of such metals as titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, and silicon, or alloys or mixtures of metals containing one or a group of such metals.

It is a further object of the present invention to facilitate the foregoing objects by providing a furnace with a beneficiated reducing atmosphere.

It is a further object of the present invention to facilitate the foregoing objects by providing a beneficiated reducing low pressure atmosphere comprising a highly purified reducing gas, such as hydrogen, beneficiated by the vapor of a readily oxidizable metal such as, for example, titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, or silicon.

It is a still further object of the present inven tion to provide a furnace for brazing, bright annealing, or the like, designed to facilitate the provision of a low pressure beneficiated reducing atmosphere as afore-mentioned.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description thereof.

In general, the process of the present invention contemplates introducing into a furnace an object or objects of, or containing, a readily oxidizable metal whose oxide is difficultly reducible, or several such metals, which it is desired to furnace braze, bright anneal, coat with a lower melting metal or alloy, or otherwise furnace treat, evacuating the enclosure while applying a relatively low heat thereto to remove a substantial portion of the oxygen contained therein by degassing the materials to be treated, introducing into said enclosure a preferably highly purified reducing atmosphere to preferably a relatively low pressure, vaporizing a supply of readily oxidizable beneficiator metal in said enclosure for beneficiating the atmosphere by the application of heat to said supply, and then heating the workpiece or pieces to complete the brazing, bright annealing, or other heat treating operation.

More specifically, when it is desired to accomplish one of the above-mentioned operations on a workpiece of titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, or silicon, or a similar metal having difficultly reducible oxide, or on an alloy of metals containing such a metal as one of its constituents, the workpiece is first cleaned to remove sensible contamination and is then mounted in a furnace enclosure which can be exhausted to a hi h vaoo um, preferably to a few microns of mercury. A supply of vaporizable and readily oxidizable metal such as, for example, one of those metals above enumerated as having a difficultly reducible oxide, and preferably of the more readily vaporim able metals of that group, is introduced into said furnace. The furnace is then exhausted to a high vacuum of preferably a few microns of rnencury, and the workpiece is lightly heated to drive out adsorbed gases, yet avoiding temperatures high en u h o cau e noticeabl ox izot on thereof, a t mp an e of o 3 0 C for a tow mi ut s usually be ng sa isf c o y. The heatin may be obtained by any des red m ans as, for X- ample, y ind t on of r s stan e heat or 'by electron bombardment, l/Vith the enclosure thus exhausted and t e workpiece de as ed, a s ply of u g as such as hyd gen o igh pur is introduced into the enclosure through an effioient p fi r, as for xample by diffusion thr u h a pa d um septum, up to a pressure f a f w centimeters of mercury. Thereupon the supply or readily x d able henefio a r me a is heat d by any suitable means to afford an appreciable vaporizat on t reof, thereby enefioiatin t d n atmospher by s a ngin nitrogen, x ge or o her on aminants ther T workpiece is then heated independently of or oncur h e eneil iatins meta t com p t t a ght an ea ing, o other he t treating operation.

One particular example of the present process is s o l ws; A t ta ium-F wer al oy containi g pproxi a y 5% itanium is to be furnace az he 101. 6 5 110 b tr a ed f le filSl Q1 1 a d then nt du ed int the fu na or enos re Whe eih th pe i n to be car ut- A q a t y of meta l c chr mium, the hone fio o r m al, havin app ximat ly 10 squar en meters of, surface is also intro u ed into the furnace. The furnace then eva uated to a few 3 .10 f m u y and hea ed app oximatel 100 to f ectuate the egass g operation. There pon, p lladium filt red h drog n is introduced to a pressure of approximately 4 centie ers f merc y, and he ohromimh is heated at from l200 to 140.0 C. for ap roximately minut to nefioia e he re cing atmosphere- The workpiece is then heated above the fusion point of the b azin a oy, wh ch may e, r xamp e, a pper base a y with 6% nicke 1% silicon, and 0.5% molybdenum. After suitable usion of he raz ng alloy has been obtained the rnace i o l n the braze ork e mov d, the entir opera ion r quiri aporoxh mately 40 m u s- In anoth r specific application of the prese proc ss, t fo i g steps were applied, with excellent success in the brazing of Steel and molybdenum parts to titanium containing alloys with high strength silicon-nickel-copper-molybdenum brazing alloy.

The pressure of the atmosphere in which the operation is carried out and the amount of vaporization of the beneficiating metal necessary to,

obtain a suitable atmosphere are interdependent factors, for it is desirable to adjust the pressure within the furnace to values which favor vaporization of the beneficiating metal, and yet it is desirable to cause an appreciable portion of the vaporized atoms to collide with the molecules of the atmosphere before encountering the walls of the furnace. Therefore, small variations in vacuum pressure may be compensated for by variations in beneficiating temperature, and vice versa. Further, with regard to the beneficiating action, the present invention also contemplates, when desired, the production of a glow discharge through the furnace to accelerate the action.

The present invention also contemplates a continuous process wherein the workpieces are brought successively into the system through a gas lock, moved countercurrent to a flow of beneficiated hydrogen or other reducing atmosphere through a, preheating and scavenging chamber, thence into an operation chamber wherein the brazing, bright annealing, or other heat treating operation is carried out in a bcneficiated atmosphere, and. thence moved into a cooling zone and out through a gas lock. The details of such a continuous process will be apparent to those skilled in the art,

The foregoing is a description of the process of the present invention; the following is a description of a furnace for carrying out the same made in onj n ion with the accompanyin drawing which is a longitudinal sectional View of the furnace.

The following description is given by way of xample of n embodim n of a f rnace ol signed to carry out the above-described process. Referring to the drawing, numeral ill indicates the main portion of the furnace, which is shown in the present embodiment as cylindrical in shape and is preferably constructed of some nonmetah lic heat resistant inert material such as silica glass. The cylinder It is closed at its lower end by a base H, and at its upper end with a cover dome l2, both being preferably constructed of the same material as the cylinder. The base H has the stand 22 extending upwardly therefrom which has the cup 2% formed on its upper end, the stand and cup likewise being preferably constructed of an inert material such as silica glass. Also, the base i l is provided close to its periphery with the pair of circular flanges 21 and form ing an annular groove about the base into which n end of the cylinder It i fit ed, the flang being preferably formed of a similar inert material. The cover dome it, like the base H, provided With a similar pair of circular flanges 28 and 3 I. forming an annular groove wherein the other end of the cylinder H] is positioned, and is further provided with the crossbar it from which depends the brackets is supporting the shelf 32, all or these elements being likewise formed from a similar inert material. The cover may be provided with a pressure gauge 33, and valves it, It, and ll for connection to the hydrogen purlfier and supply, to the vacuum and to the atmosphere, respectively, and may have an observation window formed of transparent material. The seating of the cylinder in the base and cover is made vacuum tight by inserting between the cylinder and the several circularfianges, 2i and SEE, and 28 and El, any suitable sealing compound 25, as is well known in the art. The present furnace is further provided with the inductor coil it, movable axially along -2,ces,42e

the cylinder, for heating the contents thereof as desired.

To briefly describe the operation of the present furnace in carrying out the foregoing process, a description is here made thereof in its application to the above-described first specific example of the process. With the cover removed, the cup 29 is filled with granular alumina. or other similar il for supporting the metallic chromium block placed thereon. The titanium.- Eiovar workpiece, indicated generally by the numeral 20, having the copper base brazing alloy '21 inserted in joint between the two vportions of the workpiece to be brazed, is positioned upon the shelf and the cover is placed upon the cylinder thus closing the furnace. The lead .pipe from the hydrogen supply and palladium puriher, not shown, is connected to the Valve l5, and the lead line to the vacuum pump, not shown, is connected to the valve it. Valve [7 vents to atmosphere.

With the hydrogen inlet valve I5 and the atmosphere valve i? closed, the furnace is evacuated while the workpiece 20 is gently heated at a temperature of about 100 C. by the inductor coil 19. For this degassing operation the inductor coil is first moved upwardly along the cylinder until it encompasses both the workpiece 20 and the chromium block 23. After the degassing operation is completed, the pressure within the furnace having been reduced to but a few microns of mercury, the vacuum valve It is closed and highly pu Tied hydrogen is permitted to enter the furnace through the valve I5 until the pressure within the furnace has reached about i centimeters of mercury, as indicated by the pressure gauge 33. Then inductor coil I9 is returned to its lowermost position as shown in the drawing, and the chromium block 23 is heated to a temperature of between 1200 and 1400" C. for approximately 20 minutes. After the hydrogen atmosphere of the furnace has been thus beneficiated, the inductor coil is again moved upwardly to encompass the workpiece and the temperature thereof is raised above the fusion point of the brazing alloy 21. After fusion occurs the furnace is cooled, the chamber vented to the atmosphere through valve I7, and the workpiece removed.

The herein described specific embodiments of the process and apparatus comprising the present invention are presented merely by way of example, and modifications thereof within the spirit and scope of the appended claims will be apparent to those skilled in the art and are within the monopoly there defined.

What is claimed is:

1. The process of furnace treating a metal workpiece comprising the steps of degassing the piece by evacuation of the furnace to a few microns of mercury pressure under moderate heating from 100 to 300 C., introducing a highly purified hydrogen atmosphere to a few centimeters of mercury pressure, beneficiating said atmosphere by vaporizing metallic chromium contained in said furnace under heating from 1200 to 1400 0., and then heating the workpiece to accomplish the desired furnace treatment thereof, the highly purified beneficiated hydrogen atmosphere operating to prevent the formation of a tarnish or oxide film upon the workpiece.

2. The process of furnace treating a metal workpiece comprising the steps of degassing the .piece by evacuation of the furnace to a few microns of mercury pressure under moderate heating from to 300 C., introducing a highly purified reducing atmosphere to a few centimeters of mercury pressure, beneficiating said atmosphere by vaporizing a readily oxidizable beneficiator metal contained in said furnace under heating from 1200" to- 1400 C., and then heating the workpiece to accomplish the desired furnace treatment thereof, the highly purified beneficiated reducing atmosphere operating to prevent the formation of a tarnish or oxide film. upon the workpiece.

3. The process of furnace treating a metal workpiece comprising the steps of degassing the piece by evacuation of the furnace under moderate heating, introducing a highly purified reducing atmosphere to a few centimeters of mercury pressure, beneficiating said atmosphere by vaporizing under high temperatures a readily oxidizable beneficiator metal contained in said furnace, and heating the workpiece to accomplish the desired furnace treatment thereof, the highly purified beneficiated reducing atmosphere operating to prevent the formation of a tarnish or oxide film upon the workpiece.

4. The process of furnace treating a metallic workpiece comprising the steps of degassing the piece by evacuation of the furnace to a few microns of mercury pressure under moderate heating from 100 to 300 C., introducing a highly purified reducing atmosphere to a few centimeters of mercury pressure, beneficiating said atmosphere by vaporizing a readily oxidizable beneficiator metal taken from the group comprising titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, and silicon under heating from 1200 to 1400 C'., and then heating the workpiece to accomplish the desired furnace treatment thereof, the highly purified beneficiated reducing atmosphere operating to prevent the formation of a tarnish or oxide film upon the workpiece.

5. The process of furnace treating a metallic workpiece comprising the steps of degassing the piece by evacuation of the furnace under moderate heating, introducing a highly purified reducing atmosphere to a few centimeters of mercury pressure, beneficiating said atmosphere by vaporizing under high temperatures a readily oxidizable beneficiator metal contained in said furnace taken from the group of titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, and silicon, and heating the workpiece to accomplish the desired furnace treatment thereof, the highly purified beneficiated reducing atmosphere operating to prevent the formation of a tarnish or oxide film upon the workpiece.

6. In a process for heat treating a metallic workpiece, the steps of vaporizing metallic chromium in a highly purified reducing atmosphere of a few centimeters of mercury pressure to beneficiate the atmosphere, and treating the workpiece in said atmosphere thus beneficiated.

7. In a process for heat treating a metallic workpiece, the steps of vaporizing a readily oxidizable beneficiator metal in a highly purified reducing atmosphere of a few centimeters of mercury pressure, said beneficiator metal being chosen from the group consisting of titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, and silicon, and treating said workpiece in said atmosphere thus beneficiated.

8. In a process for heat treating a metallic workpiece, the steps of vaporizing a readily oxidizable beneficiator metal in a reducing atmosphere of a few centimeters of mercury pressure, said benefieiator metal being chosen from the group consisting of titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, and silicon, and treating the workpiece in said atmosphere thus benefieiated.

9.111 a process for heat treating a metallie workpiece, the steps of vaporizing a, readily oxidizable beneficiator metal in an atmosphere v of a few centimeters of mercury pressure, said beneficietcr metal being chosen from the group consisting of titanium, beryllium, aluminum, chromium, magnesium, tantalum, columbium, cerium, and silicon, and treating said workpiece in said atmosphere thus benefieieted.

10. In a process for heat treating a Install-1e wcrkpiece, the steps of vaporizing a readily oxidizable beneficiatcr metal in a. highly purified reducing atmosphere of a few centimeters 0f References Cited. in the file of this patent UNITED STATES PATENTS Number Name Date 1,211,635 Smith v Jan. 9, 1917 10 1,478,365 Fisher et a1. Dec. 18, 1923 1,535.992 Fisher et a1. Apr. 28, 1925 1,731,268 fientsehler et al. v Oct. 15, 1929 2,126,074; Wissler Aug. 9, 1938 2,208,920 Allen s July 23, 194- 2,316,698 Ness Apr. 18, 194-42 2,3 0,202 Stroup July 10 1945 FOREIGN PATENTS Number Country Date 571,317 GreatBritain Aug. 20, 1945

Claims

1. THE PROCESS OF FURNACE TREATING A METAL WORKPIECE COMPRISING THE STEPS OF DEGASSING THE PIECE BY EVACUATION OF THE FURNACE TO A FEW MICRONS OF MERCURY PRESSURE UNDER MODERATE HEATING FROM 100* TO 300* C., INTRODUCING A HIGHLY PURIFIED HYDROGEN ATMOSPHERE TO A FEW CENTIMETERS OF MERCURY PRESSURE, BENEFICIATING SAID ATMOSPHERE BY VAPORIZING METALLIC CHROMIUM CONTAINED IN SAID FURNACE UNDER HEATING FROM 1200* TO 1400* C., AND THEN HEATING THE WORKPIECE TO ACCOMPLISH THE DESIRED FURNACE TREATMENT THEREOF, THE HIGHLY PURIFIED BENEFICIATED HYDROGEN ATMOSPHERE OPERATING TO PREVENT THE FORMATION OF A TARNISH OR OXIDE FILM UPON THE WORKPIECE.