US3230119A

US3230119A - Method of treating columbium-base alloy

Info

Publication number: US3230119A
Application number: US309920A
Authority: US
Inventors: Gordon D Gemmell; James E Mcnutt
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1963-09-17
Filing date: 1963-09-17
Publication date: 1966-01-18
Anticipated expiration: 1983-01-18
Also published as: DE1290727B; BE653157A; CH463799A; GB1014438A

Description

Jan. 18, 1966 G. D. GEMMELL ETAL 3,230,119

METHOD OF TREATING COLUMBIUM-BASE ALLOY 2 Sheets-Sheet l I Filed Sept. 17, 1963 E32: E 23: E 2 m Q 0 Q3: 2 35: 32 E @2256 E 82152: 2:22 352w =0 SE55: :5: US Swim (18d) S93E13 ATTORE 1966 G. D. GEMMELL ETAL 3,230,119

METHOD OF TREATING COLUMBIUM-BASE ALLOY 2 Sheets-Sheet 2 Filed Sept. 1'7, 1965 INVENTORS I... H T M W M D. E N NE RM 0A m G "U 0 0 W10 9/ I o 0 2 U. A 0 8 E a D "M W zu m m o. w %L G. 1. o 0 NM WW Z W M 1 MW. M 5B %A G H (1.5 M K SF. 50 G E 2A V 00 s u W V T 1 m H mm m E D nu HA 0 HF WTIG I. 0 T A I m m w w m E35: wwuzofi: 22;: 355; M H

0 6 m M m i I 2 2 F F E252 ma i: 223;; 953;

ATTORNEY United States Patent 3,230,119 METHOD OF TREATING COLUMBIUM-BASE ALLOY Gordon D. Gemmell and James E. McNutt, Wilmington,

Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sept. 17, 1963, Ser. No. 309,920 11 Claims. (Cl. 148-158) This application is a continuation-in-part of our copending application Serial No. 115,416, filed June 7, 1961, now abandoned.

This invention relates to the production of improved columbium-base alloys and more particularly to novel methods for imparting desired, improved physical properties to such alloys. More specifically, the invention relates to novel procedures for varying in a controlled fashion the mechanical properties of colurnbium-base alloys in order to impart to such alloys desired combinations of improved strength and ductility characteristics so that their usefulness in service under relatively high temperature conditions becomes desirably enhanced.

Due to its refractory nature, the metal columbium is finding increasing application as a base material for high temperature alloys. Since the metal in its unalloyed state lacks desired strength at high temperatures and is readily attacked by corrosive atmospheres, the addition of varying alloying elements has been undertaken to improve its strength and corrosion resistance properties. In some instances, the addition of such elements in quantity sufiicient to provide a desired improvement in respect to one property, for example, oxidation resistance, results in an undesired variation in another property, for example, high temperature strength. The addition of one or more of the elements titanium, molybdenum, tungsten, tantalum and vanadium proves particularly beneficial in increasing the resistance of columbium to corrosion by oxidizing gases at high temperatures. Also, valuable advantages in high temperature strength can be obtained when the columbium is alloyed with zirconium in the range of, say, 0.52% by weight when there is also present small amounts of interstitial elements such as oxygen, carbon or nitrogen, with which the zirconium combines to form small particles of a second phase or phases.

It has now been found that columbium-base alloys of differing compositions can be eifectively treated to impart further improvements in strength and other characteristics so that their usefulness in various applications will become further enhanced. It has also been discovered that the intentional variations in the properties of these alloys which this invention affords can be readily achieved by recourse to a novel heat treatment adapted to render the alioys readily fabricable at cold or hot working temperatures and adapted to exhibit under service conditions such valuable mechanical properties as, for example, desired high strength and low creep rate.

Columbian-base alloys found to be especially responsive to the benefits derived in this invention comprise those of the ternary type prepared by recourse to conventional alloying techniques, as, for example, are melting or sintering, and which will effectively alloy the elements making up the final compositions. Specifically, these alloys consist essentially of at least 50% by weight of columbium, from about 0.5% to about 12% by weight of zirconium, and at least one interstitial element from the group oxygen, carbon and nitrogen, which can be present either singly or in combination and in total amount ranging from about 0.02% to about 0.5% by weight. If desired, one or more of the following additional elements can also be present in the composition, the indicated amounts being by weight: Up to 35% tungsten, up to titanium, up to 25% molybdenum, up to 25% tantalum and up to 10% vanadium.

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In accordance with this invention, such columbiumbase alloys containing zirconium and an interstitial element within the limits stated are subjected to heat treatment under such conditions of time and temperature that solution of second-phase material and subsequently age hardening occurs as a result of the precipitation of at least one second phase in the alloy. The process of age hardening is preferably carried out at time and temperature conditions in excess of those which will produce recrystallization in the alloy. The term recrystallization as employed herein means the formation of a new, strain-free grain structure from that existing in coldworked metal. In general, the heating required for elfecting solution treatment of the alloys mentioned ranges from about 1600 C. to 2100 C. for a period of from 5 minutes to 9 hours, this treatment being followed by an additional heat treatment at temperatures ranging from about 1000 C.l500 C., and for a time period of from /2 hour to 40 hours to effect the desired age hardening. Preferably, temperatures ranging from about 1700 C. to 2000 C. are resorted to in the solution treatment step while temperatures ranging from about 1150 C.1300 C. are utilized in the age-hardening step.

As a refinement of this invention, the alloy is subjected to cold working between the solution heat treatment and the thermal aging step. In the case of preparing sheet, for example, the alloy is worked down to the penultimate sheet gage, solution heat treated, cold rolled to final gage and aged. The cold rolling step is rather critical and must not be over 40% reduction if improved tensile properties are to be developed. Preferably, this rolling reduces the sheet from 15 to 25% in thickness. In the cold rolling step between the penultimate and final gages, one or more passes may be used but in any case no intermediate heat treatment is desired during this reduction up to 40%.

In the solution heat treatment of the sheet form, minimum time is preferred because it permits solution of the second phase with a minimum of grain growth. From 5 to 15 minutes usually suffices in the preferred solution heat treating temperature range of from 1600 C. to 1800 C. Fairly rapid cooling from this temperature e.g. 300 C./min. or faster down to about 1200 C. is desired. This condition is easily accommodated with thin sections; hence, the process is well suited for the processing of sheet. After cooling the sheet is cold rolled as described above. During this cold working step strain induced hardening occurs. For the alloys of this invention the amount of strain imparted is proportional to the degree of cold reduction and has a critical effect on the precipitation hardening which occurs during the last thermal aging step. Too much rolling i.e. more than 40% reduction, destroys the advantages with respect to improved tensile strength obtained by the high temperature solution heat treatment. Less than about 15% reduction fails to give the strengthening effect. Between 15% and 25% reduction, a marked improvement in tensile properties is obtained after aging. In the final aging, temperatures between 1200 C. and 1425 C. are preferably resorted to. The time of aging is not very critical, being of the order of /2 hour or longer.

The alloy compositions preferred for use in preparing sheet material comprise from 0.5 to 3% Zr, 8 to 12% W, 0.02 to 0.15% C, up to 7% Ta, not more than .05% of other interstitials with the balance being Nb and incidental trace impurities.

It has been discovered that the presence of certain concentrations of tantalum in the alloys treated by this invention provides a still stronger alloy having good ductility and exhibiting improved thermal stability. From 2% to 7% of tantalum have proved most effective in giving added high temperature strength. Compositions of this type comprise an alloy consisting of from 2% to 7% Ta, 8% to 12% tungsten, 0.5% to 3% Zr, 0.02% to 0.15% carbon, .02% to 0.2% oxygen, the balance being columbium and traces of the usual elements.

The strengthening elfect of tantalum in these compositions is wholly unexpected since the addition of tantalum to colurnbium, the base metal, shows no such increase. A complex interaction with, the other constituents of the alloy appears to be involved which is at a maximum effect when amounts of tantalum near about 5 weight percent are present.

To a clearer understanding of the invention, the following specific examples are given, the. parts therein mentioned being by weight. These are aptly illustrative of the advantageous results achieved through application of the contemplated heat treating to various columbiumbase alloys in order to improve their physical properties, especially strength characteristics.

EXAMPLE I An alloy of zirconium and columbium was prepared by melting together in a water-cooled, copper crucible of an arc melting furnace, 396 parts of columbium containing 0.025% oxygen and four parts of zirconium containing 0.10% oxygen. When the metals had become completely molten, the furnace was turned off and the melt was allowed to solidify and cool in the inert gas atmosphere. The alloy composition was then remelted and resolidified six additional times to insure thorough mixing of the ingredient metals. The casting was then recovered from the crucible and was then machined to a round bar of /2" diameter by 5". Other alloys were similarly prepared and in accordance with the compositions listed in Table I below. Each of the thus-prepared melted and machined alloys was then swaged through a series of dies at 1000 C., each die reducing the cross section by l2 /2%. Standard tensile specimens were machined from each of the compositions. One sample of each composition was solution heat treated by heating the specimen above the recrystallization temperature for six hours at 1950" C. in a vacuum furnace, cooling the room temperature, and then aging the specimen for 30 minutes at 1100 C. in vacuum. Another sample of each composition was heat treated above the recrystallization temperature by heating at 1400 C. for 16 hours in inert atmosphere. The 0.2% yield strengths of the alloy compositions at 1100 C. in vacuum are compared in Table I, the term 0.2% yield strength as used herein being the stress at which the alloy sample exhibits an 0.2% deviation from the direct relationship of stress to strain.

Table I.Efiect of heat treatment on tensile yield strength of Nb-Zr binary alloys [1,100 C. 0.2% yield strength] Heat treatments Composition 6 hrs. at 1950 O.

16 hrs. at 1, 400 0., cooled to room p.s.1. temperature, and

aged 30 mins. at 1,100 0., p.s.i.

A. Nb lZr-0.02% O2 16,000 34, 000 B. Nb-5Zr-0.02% O 28,000 39, 000 C. Nb10Zr0.03% 0 36,000 41,000

EXAMPLE II Table II.-Efiect of heat treatment on strength and ductility of N-b-5Zr-10W-0.04O

[Tested at 1,100" 0.]

0.2% yield Ultimate strength tensile E, per- RA, per- (p.s.i.) strength 1 cent 2 cent 3 (psi.

16 hours at 1,4D0 C 40, 000 50, 000 20 40 Solution treated 9 hrs. at 2,000 O.,

cooled to room temperature and aged 30 mins. at

1 Ultimate Tensile Strength-Maximu.m tensile stress that a material can withstand.

2 Elongation (E) means the degree of permanent extension before fracture, expressed as a percentage of the original gauge length.

3 Reduction in Area (RA)The percentage reduction of the crosssectional area at the fracture as compared with the original cross-seetional area of the test piece.

EXAMPLE III An alloy of 5% Zr, 15% W, and 0.06% 0 was prepared in the manner in Example I. Two specimen pieces of the alloy were prepared by machining and swaging as described in Example I above. The alloy specimens so prepared were stress rupture tested at l C. in

By a tension test performed at constant load and constant temperature, the load being held at such a level as to cause rupture.

an atmosphere of argon. below.

Results are given in Table III Table lII.E/ject of heat treatment on stress rupture strength of Nb-Zr-O -W alloys EXAMPLE IV An alloy of 5% Zr, 15% W, 0.05% 0 balance columbium was conventionally prepared by consumable arc melting. A 2" diameter billet of this composition was extruded at 1450 C. at an extrusion ratio of 4 to 1. This extrusion was swaged at 1100 C. to /8" diameter. The accompanying FIGURE I is a graphical representation of the stress rupture data obtained on portions of this alloy treated in the following manners. Specimens of this swaged material were tested at 1100" C. for stress rupture in the as-worked condition (A of FIG. I). A second series of samples of the swaged material was heated for 18 hours at 1400 C. and tested also at 1100 C. for stress-to-rupture (B of FIG. I). A third series of samples was similarly tested following heat treatment comprising the two steps of solution treatment for 1 hour at 2000 C. and aging treatment for one hour at 1200 C. (C of FIG. I).

EXAMPLE V An alloy of composition 1% Zr, 0.027% 0 0.008% N 0.004% C, balance Nb, was prepared by are melting of the metal. The casting was machined into a 2" diameter billet which was cut into two pieces. One portion was extruded at a ratio of 4 to 1 at 1260 C. The other portion was solution treated for A hour at 16 2 Q C Table IV.Strength testing of heat-treated alloy: Nb-1Zr-0.027O -0.008N -0.004C

Stress-to-rupture test Ultimate Working and heat treatments tensile Stress Time to test, p.s.i. applied rupture (hrs) Extruded at 1,260" 0., swaged,

heat-treated 1 hour at 1,200 Cl. 10, 000 20 30,000 Solution-treated )4 hour at 1,620

C., swaged, heattreated 1 hour, at 1,200 C 10, 000 95 50,000

EXAMPLE VI Specimens of alloys of composition 1% Zr, 0.03% balance Nb; Zr, 0.04% 0 balance Nb; and Zr, 0.05% 0 balance Nb were made up in the same way as described in Example I. These specimens were machined into rods of A2" diameter by 5" and were swaged to 50% reduction in cross-sectional area. The swaged specimens were solution heat-treated for 8 hours in vacuum at 2000 C., cooled to room temperature, and then aged at 1100 C. in vacuum for varying periods of time of one to 16 hours.

It is an accepted metallurgical fact that the hardness of metals can be directly correlated with their strength (see R. T. Rolfe, Dictionary of Metal-lography, p. 247). In the accompanying FIGURE II are plotted the Vickers pyramid hardness numbers (VPHN) which were obtained upon room-temperature testing of the heat-treated alloy specimens of this example. The results show that the strength of the solution heat-treated compositions increases with aging time at 1100 C.

EXAMPLE VII Specimens of alloys of 5% Zr, 10% W, 0.05% balance Nb, were prepared by are melting of the metal. Test specimens were prepared by extrusion and swaging as described in Example IV. The specimens were tested for stress-to-rupture at 1200" C. under a load of 25,000 psi. in inert atmosphere, following heat treatment under varying conditions. Results of these tests are recorded in Table V below.

Table V.Stress rupture testing of Nb-5Zr-10W-0.05% 0 alloys at 1200 C.

Treatment of alloy Time oi rup- Total elongature, hours tion, percent (a) "As-worked" condition: Extruded and 2. 3 5O swaged 1,450 O. at 4 to 1 ratio, swaged l,l00 C (b) Extruded and swaged plus solution heat treatment for 1 hour at 2,000 C., followed by aging heat treatment for 1 hour at 1,100 C 16 7 EXAMPLE VIII 6 heat treatment under a variety of conditions. The results are recorded in Table VI below.

Table VI.-Hardness of Nb-Zr-C alloy (with Ti, Mo, W) following various heat treatments VPHN at room Heat treatment (in vacuum): temperature (a) As-worked condition-extruded 353 (b) (a) followed by 2 hours at 1800 C. (solution heat treatment) 333 (e) (b) followed by 8 hours at 1400 C 363 (d) (b) followed by 8 hours at 1200 C 404 (e) (a) followed by 2 hours at 1900" C. (solution heat treamtent) 389 (f) (e) followed by 24 hours at 1400 C. 414

EXAMPLE IX An alloy composition of 10% Ti-10% Mo-0.5% Zr- 0.25% 0 balance Nb, was prepared as in Example I. This cast material was heated to 1000 C. and forged to a 50% reduction in cross-sectional area. The alloy sample thus treated was heated in vacuum for 8 hours at 2000 C., cooled to room temperature, and separate specimens were further heat treated in vacuum at 1100 C. for from one to 16 hours to determine the effect of various aging times. The results of hardness testing of these specimens thus heat-treated and aged are shown in the accompanying FIGURE III.

EXAMPLE X An alloy composition of 5% Zr, 5% V, 0.028% 0 balance Nb, was prepared by consumably arc melting the metals. The as-cast" hardness of the composition was 230 VPHN. Specimens of this alloy composition were heat-treated for varying times at several temperatures, and hardness values were determined. These conditions and hardness values are recorded below.

Table VII.Hardness value-Vickers pyramid hardness number [Nb-Zr-V-Oz Alloy] VPHN at room Heat treatment (in vacuum): temperature (a) As-cast condition 230 :(b) (a) followed by 4 hours at 2100 C. (solution treatment) 224 (c) (b) followed by 8 hours at 1000 C 242 (d) (h) followed by 8 hours at 1100 C 236 (e) (b) followed by 8 hours at 1300" C 240 EXAMPLE XI In the same manner as given in Example X, an alloy composition of 5% Zr-5% Ti-5% V-0.03% O balance Nb, was prepared and tested for hardness at room tem perature. The heat treatments and hardness values are recorded in the following table:

Table VlIl.Hardness of alloy of Nb-Zr-O (containing V and Ti) VPHN at room Heat treatment (in vacuum): temperature (a) As-cast condition 227 (b) (a) followed by 4 hours at 2100 C. (solution treatment) 207 '(c) (h) followed by 8 hours at 1000 C 245 -(d) (h) followed by 8 hours at 1100 C 251 (e) (h) followed by 8 hours at 1200 C 245 (f) (b) followed by 8 hours at 1300 C 266 EXAMPLE XII A series of alloy compositions as listed in the table below were prepared by arc melting. The alloy samples were solution heat treated 4 hours at 2000 C. in vacuum and aged an additional 8 hours at 1000 C. to 1400 C. in vacuum. Room temperature hardnesses are listed in Table IX.

Table IX.Efiect of solution heat treatment and aging on room temperature hardness of Nb-SZr-oxygen and nitrogen alloys Hardness values (VPHN) After solu- After solution heat treatment of 4 hrs. at 2,000 0. plus tion heat 8 hrs. at Alloy composition treatment of 4 hrs. at

2,000 C 1,000 O. 1,100 C. 1,200 C. 1,300 C. 1,400 C.

Nb-Zr-0.03% O2-0.005% Nz 132 165 155 170 163 148 Nb-5Zr-0.05% Oz 135 153 152 156 156 143 Nb-5Zr-0.08% 132 156 153 156 141 134 Nb-5Zr-0.05% Nz 148 164 158 156 170 147 Nb-5Zr-0.08% N2 131 167 157 142 144 142 EXAMPLE XIII An alloy of composition 3.5% Zr, 0.07% C., balance Nb, was prepared by melting as described in Example I. A billet was machined from the casting and divided into two portions. One piece of the alloy was extruded at a ratio of 4:1 at 1870 C., swaged at 1100 C. to diameter rod, and heat treated for one hour at 1200 C. The second piece was heated treated one hour at 2100 C. prior to being extruded, swaged, and heat treated as described in the previous sentence. Both specimens were tested for ultimate tensile strength at 1100 C. Results are given in Table X below.

Table X.High temperature (1100 C.) strength of Nb-Zr-C alloys after heat treatments Ultimate tensile Heat treatment strength (1100 C.

(in vacuum): in vacuum) p.s.i.

(a) Extruded 4:1 at 1870 C., swaged at 1100 C. and heat treated 1 hour at 1200 C. 35,000 (-b) Heat-treated 1 hour at 2100" C. prior to treatment as in (a) 41,000

EXAMPLE XIV An alloy of the composition 3.5% zirconium, 0.1% carbon, 0.02% oxygen, 0.002% nitrogen, balance columbium, was prepared by the procedure described in Example I; Two specimens of the cast alloy were heated to 1000 C. and rolled. One of the rolled specimens was solution heat treated for one hour at 2100 C. A second rolled specimen was stress relieved for one hour at 800 C. Both specimens were then cold rolled to a total reduction (from the cast condition) of 95% and were then heat treated for /2 hour at 1100 C. On being tested for ultimate tensile strength, the results shown below were obtained:

Ultimate tensile strength (vacuum) (a) Cast alloy, rolled at 1000 C., stress relieved 1 hour at 800 0, cold rolled, and aged /2 at 1100 C. p.s.i 29,000 (b) Cast alloy rolled at 1000 C., solution heat treated 1 hour at 2100 0, cold rolled, and

aged /2 hour at 1100 C. p.s.i 42,000

EXAMPLE XV Example XIV was repeated except that the C content of the alloy was 0.005%. The resulting sample (which was not solution treated prior to cold rolling and aging) had an ultimate tensile strength of 33,000 p.s.i., while the specimen which had been solution heat treated for one hour at 2100 C. prior to cold rolling and aging, exhbited an ultimate tensile strength of 37,200 p.s.1.

EXAMPLE XVI A columbium base alloy containing 10% tungsten, 0.8% zirconium, and 0.11% C and other interstitial elements at the usual low level was prepared by vacuum arc melting the constituents to an 8 diameter ingot. This ingot was processed by the usual procedures, in volving extrusion followed by rolling at temperatures not over 1200 C., to give two sheets of 0.040" and and 0.030 thickness respectively. Each thickness was heat treated under vacuum both by the process of this invention at two solution heat treatments and by a lower temperature process for comparison. All were cooled rapidly after the first heat treatment. Processing variables and physical properties of the sheets are tabulated below:

Sample A B O D E- F Thickness, mils 30 40 30 40 3 4 Heat Treat; 0 0 Temp,

C

1, 200 1,200 1, 650 1, 650 1,750 1, 750 Time, hr 1 1 5 1 5 1 5 l 5 Cold rolled to- Mils 20 3O 20 30 20 30 ffercent reduction 33 25 33 25 33 25 Ag ng temp,

O

1, 200 1, 200 1,200 1,200 1, 200 1, 200 Aging time, hr 1 1 1 1 Tensile Prop.-

Room temp:

Yield str., K p.s.i 61 69 76 75 80 83 Ult. Y.S., K p.s.i 83 87 99 99 100 Percent elong... 14 12 73 12 12 11 At 1,200 O. vac:

Yield str., K p.s 19 26 30 36 40 42 Ult. Y.S., K p.s.i 23 29 34 40 44 47 Percent elong 29 21 10' 9 10 9 1 Minutes.

EXAMPLE XVII A plurality of alloy sampleswere prepared by melting the various components to ingots, and conventionally processing the ingots to 50 mil sheet at temperatures below 1200 C. These samples contained amounts of tantalum varying from 0 to 10%. Each sheet was then given a vacuum heat treatment for 10 minutes at 1650 C., cooled and cold rolled to 40 mils. The 40 mil sheet was then-aged 1 hr. at 1200 C. The essential chemical compositions and tensile properties of the ing sheets are given in the following table:

result- Composition, weight percent, bal. Nb Tensile properties Sample Room temperature 1,200 O. 1,315 0.

Ta W Zr O N YS, UTS, Percent YS UTS Percent YS UTS Percent K p.s.i K p.s.i. e1. e1. e1.

0 10. 0. 0 0. 1 0. 017 0. 008 79. 5 100 14 26. 6 35. 1 2. 8 10. 8 O. 9 0. 13 93. 0 112 11 35. 6 40. 6 3. O 10. 5 1. 0 0. 14 86. 4 102 17 39. 8 44. 8 4. 8 10. 4 0. 7 0. 09 92. 8 112 11 37. 4 42. 4 4. 8 10. 3 0. 9 0. 1O 0. 89. 9 111 14 30. 8 38. 2 4. 8 10. 3 1. 0 0. l1 0. 89. 6 109 18 34. 3 39. 8 4. 6 10. 4 0. 9 0. 20 0. 4 87. 3 107 14 20. 4 35. 5 6. 8 12. 4 1. 0 0. l1 0. 025 015 102. 5 124 13 43. 0 47. 4 6. 9 11. 5 1. 0 0. 12 0. 026 005 98. 5 116 17 39. 0 45. 5 BWH 296- 10. O 10. 3 1. 0 0. 0. 025 006 91. 8 112 37. 1 41. 0

The beneficial improvements in alloy properties manithe combination of heat treating the alloy at a temperafested by the products of this invention are derived from an internal reaction involving the combination of zirconium with the interstitial elements mentioned which advantageously impart to the columbium under the conditions of this invention a unique susceptibility to age hardening. No element other than zirconium has been found to function to a comparable degree in this capacity. The invention advantageously provides the conditions under which the second phase can be put into solution and subsequently precipitated in a controlled manner to provide the desired hardness, strength, duc tility and creep resistance properties which our improved alloys exhibit.

Where the additional alloying elements mentioned above are added to the Nb-Zrinterstitial alloy, such elements will impart certain specific, desired properties to the matrix, and will modify somewhat the solution and aging reaction which under the conditions of this invention determine the improved properties which our alloy product will possess. Obviously, many combina tions of elements are possible, and, within certain limits, the heat-treating procedure can be suitably varied, depending on the composition of the alloy under treatment, to achieve the properties desired in the ultimate alloy composition.

During solution treatment, two phenomena occur which promote subsequent age hardening when the alloy is heat treated at a lower temperature. First, re-solution takes place of any second-phase material, e.g. ZrO or carbides, present which has precipitated in an uncontrolled manner during the casting and cold working or prior fabrication. Secondly, the alloy becomes homogenized by diffusion of the alloying ingredients so that precipitation can occur more uniformly through the alloy than would be the case if the segregation resulting from the casting were not modified by heat treatment. Solution treatment prior to the aging treatment renders possible the precipitation of an optimum amount of second-phase material during the age-hardening step. When conditions conducive to putting a second phase into solution exist, the alloy is more readily fabricable. This solution of second-phase material comprises a first step in our process and renders more effective the subsequent age-hardening at lower temperatures than that at which the solution treatment has been carried out and effectively imparts desired high-temperature strength, hardness and creep resistance properties to the alloy. It has also been found, as certain of the examples demonstrate, that under some conditions the alloy being treated can be subjected to an advantageous mechanical working, e.g., swaging, extruding, rolling or forging treatment between the steps of solution heat treating and age hardening. This working treatment can be applied at any desired temperature, ranging from room temperature to not above that at which the alloy has been solution heat treated.

ture and for a period of time which will enable the second phase in the alloy to go into solution in the matrix of columbium or columbium-rich alloy, followed by an additional heat treatment at a lower temperature, but above the recrystallization temperature, to permit aging of the alloy, that is, precipitation of a second phase in addition to the matrix, with resultant hardening and increase in strength. The solution treatment may result in the complete or only partial dissolution of the second phase in the matrix. Such solution heat treatment and age-hardening heat treatment allow for many method variations, depending on the alloy compositions and the physical properties desired in the treated alloy piece. Examples of procedural variations which are possible and within the scope of the invention include the following:

(1) The alloy may be cooled directly from the solution-treating temperature to the lower aging temperature and held at this lower temperature for the necessary time to effect precipitation of the second phase in the matrix of the alloy.

(2) The alloy may be rapidly cooled to room temperature following the solution treatment, then reheated to the aging temperature and held at this temperature for the desired length of time.

(3) The alloy may be rapidly cooled from solutiontreating temperature to room temperature, cold worked, and reheated to the desired temperature for aging treatment, or alternatively, the s0lution-treated alloy may be cooled to aging temperature and worked while aging,

(4) The alloy may be rapidly cooled from the solutiontreating temperature to room temperature, then reheated to an aging temperature for a length of time required to effect some precipitation of the second phase material, then the temperature may be increased or decreased for an additional time to effect further precipitation of second phase material.

(5 The alloy may be solution heat-treated and worked at an elevated temperature, then cooled rapidly to room temperature and further cold-worked, then reheated and age-hardened at an elevated temperature.

(6) The alloy may be solution-treated and cooled at a controlled rate to achieve properties especially desirable for subsequent aging or subsequent mechanical working and aging.

As noted, the heat treatments herein contemplated are effected under vacuum or an inert atmosphere of a rare gas such as argon, helium, etc., and can be applied to a wide variety of columbium-base alloys to improve their high temperature strength, resistance to creep and hardness properties. Because of the relatively wide variation in composition which such columbium-base alloys can exhibit, use of a wide range of temperatures and holding times is also contemplated. Thus, the solution heat treatment can be satisfactorily carried out at temperatures which generally range from about 1600 C. to 2100 C.

and over time periods ranging from minutes to 9 hours. Preferably, temperatures ranging from about 1650 C. to 2000 C. and times from about 1 hour to 4 hours are utilized for the more massive pieces and from 5 to 15 minutes for relatively thin sheets. When the solution treatment temperature used is in the higher portion of the ranges mentioned, shorter time periods of treatment can be resorted to while a longer period of treatment can be utilized when the temperatures employed are in the lower portions of the ranges mentioned. This also applies to the age-hardening step wherein use is contemplated of temperatures ranging from about 1000 C. to 1500 C. and preferably from about 1200 C. to 1400" C. with the times therefor ranging generally from A2 hour to 40 hours and preferably from /2 hour to 8 hours.

We claim:

1. A process for improving the tensile strength of a columbium-base alloy which comprises solution heat treating a columbium-base alloy consisting essentially of from about 0.5% to 12% zirconium by weight and from about 0.02% to 0.5% by weight of at least one interstitial element selected from the group consisting of oxygen, carbon and nitrogen at a temperature ranging from about 1600 C. to 2100 C. for a time period of from 5 minutes to 9 hours, and subsequently subjecting the alloy to a further but lower heat treatment for a time period of from /2 hour to forty hours at a temperature above recrystallization and ranging from about 1000 C. to 1500 C.

2. A process for improving the tensile strength of a columbium-base alloy which comprises solution heat treating a columbium-base alloy consisting essentially of by weight from about 0.5% to 12% zirconium, from about 0.02 to 0.5% of at least one interstitial element selected from the group consisting of oxygen, carbon and nitrogen, and at least one element selected from the group consisting of up to 35% tungsten, up to 10% titanium, up to 25 molybdenum, up to 25% tantalum and up to 10% vanadium, at a temperature ranging from about 1600 C. to 2100 C. for a period of from 5 minutes to 9 hours, and thereafter subjecting the solution heat-treated alloy to an age-hardening treatment at a temperature above recrystallization and from'about 1000 C, to 1500 C. and for a time period ranging from /2 hour to forty hours.

3. A process for improving the tensile strength of a columbium-base alloy which comprises solution heattreating a columbium-base alloy consisting essentially of by'weight from about 0.5 to 12% zirconium, and about 0.02% to 0.5% of at least one interstitial element selected from the group consisting of oxygen, carbon and nitrogen, at a temperature ranging from about 1600 C. to 2100 C., effecting said heating over a period ranging from 5 minutes to 9 hours, mechanically working the solution heat-treated alloy, and thereafter heat-treating the mechanically worked alloy at temperatures above recrystallization and ranging from about 1000 C. to 1500 C. and for a time period ranging from /2 hour to forty hours.

4. A process for improving the tensile strength of a columbium-base alloy which comprises solution heattreating an alloy composition consisting essentially of by weight from about 3% to 5% zirconium, about 8% to 12% tungsten, from about 0.02% to 0.5% of at least one interstitial element selected from the group consisting of oxygen, carbon and nitrogen, balance essentially columbium, at a temperature ranging from about 1900 C- 2000 C. for about one hour, and subsequently heattreating the solution heat-treated alloy product for from about one hour to four hours at a temperature above recrystallization and ranging from 1100 C. to 1400 C.

5. A process for improving the tensile strength of a columbium-base alloy which comprises solution heattreating an alloy composition consisting essentially of by weight from about 3% to 5% zirconium, about 8%12% tungsten, from about 0.02% to 0.5 of at least one interstitial element selected from the group consisting of oxygen, nitrogen, and carbon, balance essentially columbium, at a temperature ranging from about 1900" C. to 2000 C. for about one hour, mechanically working the resulting alloy product, and thereafter heat-treating the mechanically Worked product for about three to six hours at a temperature above recrystallization and ranging from about 1200 C. to 1400 C.

6. A process for improving the tensile strength of a columbium base alloy which comprises solution heattreating a columbium-base alloy consisting essentially of by weight from about 3%5% zirconium, from about 0.02% to 0.5 of an interstitial element selected from the group consisting of oxygen, nitrogen and carbon, up to about 5% tungsten, and up to about 5% molybdenum, for from one to four hours at a temperature ranging from about 1650 C. to 2100 C., and thereafter age-hardening the resulting alloy by heat treatment for from hour to 8 hours at a temperature above recrystallization and in the range of from about 1100 C. to 1200 C.

7. A process for improving the tensile strength of a columbium base alloy which comprises solution heattreating a columbium-base alloy composition consisting essentially of by weight from about 3% to 5% zirconium, from about 0.02% to 0.5% of an interstitial element selected from the group consisting of oxygen, carbon and nitrogen, up to about 5% tungsten, and up to about 15% molybdenum, for from one to two hours at a temperature in the range of 1650 C. to 2000 C., mechanically Working the resulting solution heat-treated alloy and subsequently heat-treating the mechanically worked product 2 to 8 hours at a temperature above recrystallization and in the range of 1100 C. to 1200 C.

8. A process for improving the tensile strength of a columbium base alloy in sheet form which comprises forming an alloy sheet at its penultimate gage consisting essentially of, by weight, from about 0.05% to 12% of zirconium and from 0.02% to 0.5% of at least one interstitial element selected from the group consisting of oxygen, nitrogen and carbon, solution heat-treating the sheet for from 12 hours in the temperature range of from 1600 C. to 2100 C., cooling, cold rolling the heat-treated cooled sheet to final gage without reducing said sheet by more than 40% of its cross sectional area, and subjecting the cold rolled sheet to age hardening for from 1 to 4 hours at temperatures in the range of from 1100 C. to 1500 C.

9. A process for improving the strength of a columbium base alloy sheet consisting essentially of, by weight, from 0.5% to 5% of zirconium, from 8 to 12% of tungsten, up to 7% tantalum, from 0.02% to 0.15% carbon, and not more than 0.05% of other interstitial elements and incidental trace impurities, which comprises solution heat-treating said sheet for from 5 to 15 minutes in the temperature range of from 1650 C. to 1800 C., cooling, and cold rolling the cooled sheet to final gage without intermediate heating, and then aging the cold rolled sheet from /2 to eight hours in the temperature range of from 1200 C. to 1425 C.

10. An improved high strength and low creep rates alloy composition readily fabricable at cold and hot working temperatures and exhibiting improved thermal stability, consisting essentially of from 2 to 7 weight percent of tantalum, from 0.5 to 3% of zirconium, from 8% to 12% of tungsten, from 0.02% to 0.15 of carbon, from 0.02% to 0.2% oxygen, up to 0.05% of nitrogen, and the balance being columbium and incidental trace impurities.

11. A process for improving the tensile strength of a columbium base alloy consisting essentially, by weight, of from about 0.53% zirconium, 8-12% tungsten, 0.02- 0.15 carbon, balance columbium, which comprises forming said alloy into sheet form, solution heat-treating said sheet for a period of from 5 to 15 minutes in'the 13 14 temperature range of from 1600-4800 C., cooling the References Cited by the Examiner solution heat treated sheet and cold-rolling it to effect UNITED STATES PATENTS from 15 to 25% reduction of its cross-sectional area, and

subjecting the cold-rolled sheet to age hardening for a g ai 1 d of from about 1 hour to 4 hours at a temperature 5 e en n 3,113,863 12/1963 Chang et a1 75-174 above recrystallization and in the range of from 1100" C. to 1500 C. DAVID L. RECK, Primary Examiner.

Claims

1. A PROCESS FOR IMPROVING THE TENSILE STRENGTH OF A COLUMBIUM-BASE ALLOY WHCIH COMPRISES SOLUTION HEAT TREATING A COLUMBIUM-BASE ALLOY CONSISTING ESSENTIALLY OF FROM ABOUT 0.5% TO 12% ZIRCONIUM BY WEIGHT AND FROM ABOUT 0.02% TO 0.5% BY WEIGHT OF AT LEAST ONE INTERSTITIAL ELEMENT SELECTED FROM THE GROUP CONSISTING OF OXYGEN, CARBON AND NITROGEN AT A TEMPERATURE RANGING FROM ABOUT 1600*C. TO 2100*C. FOR A TIME PERIOD OF FROM 5 MINUTES