US3194697A

US3194697A - Heat treatment of refractory metals

Info

Publication number: US3194697A
Application number: US227045A
Authority: US
Inventors: Winston H Chang
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1962-09-28
Filing date: 1962-09-28
Publication date: 1965-07-13
Anticipated expiration: 1982-07-13
Also published as: GB1051835A

Description

United States Patent 3,194,697 HEAT TREATMENT OF REFRACTORY METALS Winston H. Chang, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York No Drawing. Filed Sept. 28, 1962, tier. No. 227,045 7 Claims. (Cl. 148-133) This invention relates to a method for heat treating refractory metal based alloys and, more particularly, to a heat treatment for refractory metal based alloys having a carbon content greater than about 0.05 weight percent to control the amount, size and form of carbon present in the alloy.

The primary function of carbon in many refractory metal alloys is to form a metal carbide strengthener.

-" 'llhe precipitation of fine carbide particles lends high temperature tensile and rupture strength to many refractory metals such as those based on molybdenum, tungsten, columbium, tantalum and chromium. However, when more than about 0.05 weight percent carbon and particularly when more than about 0.1 percent carbon is included in such alloys to increase strength, initial processing requires procedures which raise the temperature of the alloy sufficiently high to place enough carbon in solution so that it later can be precipitated as fine carbides through additional processing. One of the dangers of intentionally including relatively large amounts of carbon in solution in such alloys is that relatively large quantities of carbon may remain in solution after processing because such processing cannot bring about full carbide precipitation. Large quantities of carbon in solution act to the detriment of low temperature ductility.

It is an object of the present invention to provide a method for heat treating refractory metal alloys including relatively large amounts of carbon so that the use and distribution of the carbon is optimized and the detrimental effects Which can result from too much carbon remaining in solution after subsequent processing is reduced.

- Another object of this invention is to provide a method of precipitating as a carbide strengthener, prior to the alloys complete processing, that portion of the carbon in solution in a refractory metal alloy which might not otherwise have been precipitated and which in solution would be detrimental to lower temperature ductility.

.These and other objects and advantages will be more readily understood from the following description and examples which are typical of, but not meant to be limitations on this present invention.

Briefly, the present invention in one form provides, in a method for heat treating a refractory metal based alloy, including carbon in excess of about 0.05 weight percent, the steps of aging the alloy prior to final processing, for a time and at a temperature suflicient to precipitate fine metal carbides, and then reducing the alloy to a finished ,7 form.

Excessive carbon retained in solution for the life of a finished article has been found to result in low room temperature ductility. According to known processing procedures, as a relatively high carbon-bearing refractory metal based alloy is first heated at high temperatures,

m various portions of the carbon are dissolved, the amount at'a somewhat lower temperature, small particles of carbides begin to precipitate. At this point the microstructure will have the appearance of a solution matrix in idfiififi? Patented July 13, 1965 which the larger particles of carbon still are present but there now exist smaller particles of carbides which have been precipitated at the processing temperature to strengthen the alloy. Of the carbon that originally was in solution, part has precipitated and part has remained in solution. The part which has remained in solution is that which has been found to be detrimental to low temperature ductility and that which the present invention attempts to control and use.

This invention recognizes that by introducing an aging step at a particular point in the-processing to precipitate carbides additional to those which normally would be precipitated during subsequent working, the carbon which would normally remain in solution to the detriment of low temperature ductility not only is removed but also is used beneficially. Thus the alloy is strengthened while at the same time low temperature ductility is improved.

The particular point referred to in the processing for the introduction of the aging step is a point prior to final processing such as rolling, swaging and the like. For example, if an article is to be finally processed or finished directly from a casting, then the aging step would be applied to the casting. If the article is originally cast, then extruded, forged and finally swaged, the aging step would be introduced prior to swaging. If the final processing of an alloy would be rolling into sheet, the aging step would be introduced prior to the rolling process. Thus the present invention controls the carbon content of refractory metal alloys to the greatest advantage.

In a preferred form, the aging step of this invention when applied to Mo-base or W-base alloys is conducted for at least about 5 hours and less than about hours, to avoid overaging to the detriment of high temperature strength, in a temperature range of about 2400-2800 F. When applied to Cb-based or Ta-based alloys, the preferred aging step is at about 200-2500 F. for about 1-10 hours; and when applied to Cr-based alloys, the preferred aging is at about l8002300 F. for about l-10 hours.

The annealing temperature for any of these alloys to solution carbon is preferred within the following ranges to control the carbon placed in solution: for Mo based and W based alloys-4 200 to 4000" F.; for Ta-based alloys-6000 to 4000 F.; for Cb-based alloys-3000 to 3500 F.; and for Cr-based alloys2500 to 3000 F. Annealing temperatures in excess of the higher temperature listed will tend to introduce more carbon into solution than can be precipitated effectively in subsequent processing. I

The following Table 1 gives the composition of some Mo-based alloys which have been studied in connection with the present invention.

Table 1 Com osition wt. ercent Alloy p p Ti Zr 0 M0 1 0. 1 0. 14 Bal. 1. 8 0. 13 Bal. 1. 6 0. 1 0. 13 Bal. 1. 6 0. 6 0. 13 Bal. 1 0. 1 0. ()1 Ba].

The following Table 2 presents processing conditions and resulting strength data for the alloys of Table 1 with conditions A-4, B-2, C-2 and D-2 including the particular aging process of the present invention prior to final working. Alloy V is included in Table 2 to show the effect of comparable heat treatment on a relatively low carbon bearing Mo-based alloy.

Table 2 Tensile properties 100-hr. Test rupture Alloy Processing conditions temp, strength,

F. U. E1. (1"), k.s.i.

k.s 1 percent I A. Extruded at 3,200 F., forged at 3,200-

2,800 E: A-l: Annealed at 3,000 F. 78 121.0 12

Swaged at; 2,5502,200 F., stress re- 2, 200 57.0 16 32.0 lieved (S.R.) at 2,200 I i/1 hr. A-2: Annealed at 3,500 I 78 132,

Swaged and S.R. as in A-l. 2, 200 63. 5 16 48. 0 A-3: Annealed at 3,750 F. 78 106. 0 0

Swagcd and S.R. as in A-l. 2,200 78.0 16 50.0 A-4: Annealed at 3,750 F. and aged at 78 125 2 2,750 F./l6 hrs. Swaged and SR. as in A-l. 2,200 73. 6 18 48. 0 II B. Extruded at 3,000 F;

B-l: Swaged 93% at 2,5002,100 F. 78 121. 8 1

S.R. 2,100 F. 1 hr. 3, 000 21. 5 37 13-2: Aged 2,500 F./50 hrs. 78 133.0 24

Swaged and S.R. as in 13-1. 3, 000 22. 7 31 III C. Extruded at 3,500 F.:

(1-1: Swaged 93% at 2,5002,l00 F. 78 139. 8 2

S.R. 2,100 F./1 hr. 3, 000 27.1 29 C2: Aged at 2,500 F./50 hrs. 78 133. 2 18 N Swaged and SR. as in C-1. 3, 000 28. 3 36' IV D. Extruded at 3,500 F.:

D-l: Swaged 93% at 2,5002,100 F. 78 162.3 13

8.3. 2,l00 F.1l1r. 3,000 31.3 '29 D-Z: Aged at 2,500 F./50 hrs. 78 148. 0 23 Swagcd and SB. as in D-l. 3, 000 30. 7 V E. Extruded at 2,900 I";

E-l: Annealed at 2,600" F.

Swaged 88% at 2,200 F.1,800 F. 78 98. 0 33 S.R. at 1,800 F./1 hr. 2,200 46.8 16 22. 0 12-2: Annealed at 3,500" F. 78 112. 0 36 Swaged and S.R. as in E-l. 2, 200 06.8 12 32. 0,

As used in Table 2 the terms U.T.S. means Ultimate Tensile Strength, k.s.i. means thousands of pounds per square inch, El means Elongation and S.R. means stress relieved. In Table 2, the conventional processing conditions for Alloy I is represented by A1.

Although the high temperature tensile and rupture properties of a relatively high carbon alloy such as Alloy I of Table 2 can be improved by high temperature annealing, it is to be noted that this gain is at the expense of room temperature ductility. For example the higher temperature processing of A-2 and A-3 results in more carbon being taken into solution for subsequent precipitation and hence strengthening of the alloy. However, more carbon remains in solution as well and its detrimental effect is shown by the ductility data; A comparison of the room temperature elongation for conditions A-2 and A-3 in view of the progressively increasing annealing temperatures shows that as the annealing tempreature is increased and more carbon is taken .into solution without being subsequently precipitated, the more brittle is the alloy at lower temperature. More carbon is in solution after swaging and stress relieving to affect the low temperature ductility, which in case of condition A-3 is about 0%. However, comparing the data for con dition A-3 with that for A-4 it is seen that the introduc-- tion of an aging step according to this invention to eliminate solutioned carbon by precipitating additional carbides, prior tofinal swaging and stress relief results in an alloy of improved low temperature strength and ductility with comparable high temperature strength. This Alloy-I heat treated according to the present invention, as represented by condition A-4 of Table 2, would be useful at room temperature as well as at 2200 F. Thus Alloy I,

processed according to A-4, presents a ditferentkind of alloy, with regard to microstructure, for subsequent working. That same alloy heat treated in a conventional manner, as represented by A-1, would not be as strong. Heat treated at high temperatures to dissolve more carbon for strengthening, as shown by A3, it would be too brittle at lower temperatures. I

A comparison of the data for alloys II, III and IV of Table 2 with and without the aging step in the heat treatment of the present invention shows the substantial increase in low temperature ductility possible from use of this invention. These alloys are particularly designed for use with the aging heat treatment of this invention.

Alloys II and III, specifically designed to be treated by the heat treatment of this invention, showin particular the significant effect of the aging step. Alloy IV is a unique. and unusual alloy in that it is strong and ductile at room temperature and 3000 F. with or without the aging step of the present invention. However, data for Alloy IV is includedin Table 2 to show that its ductility can be further increased through the processing of this present invention. Alloys II and III are more sensitive to the heat treatment of the present invention. As shown by the data for conditions B2 and C-2, they exhibit adequate low temperature strength alloy with goodductility after such heat treatment. At the same time their high temperature strength is improvedby the aging step of'the present invention.

Thus the data of Table 2 shows the effect of appropriate aging to precipitate excess dissolved carbon as carbides before final processing of relatively high carbon-bearing alloys so that it is not available to act in a manner detrimental to low temperature ductility. This was accomplished in the examples of Table. 2 by introducing the aging step prior to the final processing of swaging;

As a result of the aging process of the present invention, a fine precipitate of carbide is present in the structure of a processed alloy. Therefore a different kind of structure is available for such subsequent secondary processing as swaging and stress relief than is available after conventional hea-t'treatrnents. The aging step results in a very large volume: fraction of precipitated carbides which, upon subsequentswaging-or working, increases the effectiveness of such working. Therefore, the comparison after swaging of a relatively high carbonbearing alloy aged according to this invention, and thus having'more precipitated carbides and less carbon in solution, with one not so processed shows the aged alloy to be significantly more ductile while of comparable strength. This is shown by the .data in Table 2.

The aging process of the present invention does not have the same beneficial effect on leaner carbon alloys for example, those. having less than about "0.05 Weight percent carbon. As shown by Alloy V inTable 2, an

alloy which is the same as Alloy I, except for the carbon content, when annealed or aged at 2600 F. (E1) is lower both in strength and ductility than the alloy when annealed at 3500 F. (E-2). With higher carbon content alloys, the higher annealing temperature of E2 type processing would dissolve more carbon, thus to increase rather than decrease strength but at a sacrifice of lower temperature ductility. This is not true of lower carbon content Alloy V because the smaller amount of carbon in Alloy V does not create the problem of carbon in solution, which problem is solved by this invention.

Although the present invention has been described in connection with specific alloys as examples, particularly molybdenum base alloys, and with specific times and temperatures, it will be recognized by those skilled in the art of metallurgy and heat treatment the variations and modifications of which the present invention is capable.

What is claimed is:

1. In a method for heat treating a metal alloy based on war refractory metal selected from the group consisting of M0, W, Cb, Ta and Cr including carbon in the range of about 0.1-0.2 weight percent, the steps of: annealing the alloy at a temperature of about 2500-4000 F. for a time sutficient to place carbon in solution with the base metal; aging the alloy prior to final processing for about 1-50 hours at a temperature of about 1800-2800 F. to precipitate fine metal carbides in the microstructure of the alloy; and then reducing the alloy to a finished form.

2. In a method for heat treating a Mo based alloy including carbon in the range of about 0.10.2 weight percent, the steps of: heating the alloy at a temperature of about 3200-4000 F. for a time sufficient to place carbon in solution with the base metal; aging the alloy prior to final processing for about 5-50 hours at about 2400- 2800 F. to precipitate fine metal carbides in the microstructure of the alloy; and then reducing the alloy to a finished form.

3. In a method for heat treating a W based alloy including carbon in the range of about 0.1-0.2 weight percent, the steps of: heating the alloy at a temperature of about 3200-4000 F. for a time sufiicient to place carbon in solution with the base metal; aging the alloy prior to final processing for about 5-50 hours at about 2400- 2800 F. to precipitate fine metal carbides in the microstructure of the alloy; and then reducing the alloy to a finished form.

4. In a method for heat treating a Cb based alloy including carbon in the range of about (1.1-0.2 weight percent, the steps of: heating the alloy at a temperature of about 3000-3500 F. for a time sufiicient to place carbon in solution with the base metal; aging the alloy prior to final processing for about 1-10 hours at about 2000-2500" F. to precipitate fine metal carbides in the microstructure of the alloy; and then reducing the alloy to a finished form.

5. In a method for heat treating a Ta based alloy including carbon in the range of about 0.1-0.2 weight percent, the steps of: heating the alloy at a temperature of about 3000-4000 F. for a time sufiicient to place carbon in solution with the base metal; aging the alloy prior to final processing for about 1-10 hours at about 2000- 2500 F. to precipitate fine metal carbides in the microstructure of the alloy; and then reducing the alloy to a finished form.

6. In a method for heat treating a Cr based alloy including carbon in the range of about 0.1-0.2 weight percent, the steps of: heating the alloy at a temperature of about 25003000 F. for a time sutficient to place carbon in solution with the base metal; aging the alloy prior to final processing for about 1-10 hours at about 1800-2300 F. to precipitate fine metal carbides in the microstructure of the alloy; and then reducing the alloy to a finished form.

'7. In a method for heat treating a metal alloy based on a refractory metal selected from the group consisting of Mo, W, Cb, Ta and Cr, including carbon in the range of about 0.050.2 weight percent, the steps of: heating the alloy at a temperature of about 2500-4000 F. for a time sufficient to place carbon in solution with the base metal; subsequently, prior to final processing, aging the alloy for about 1-50 hours at a temperature of about 1800-2800" F. to precipitate fine metal carbides in the rnicrostructure of the alloy; and then reducing the alloy to a finished form.

References Cited by the Examiner UNITED STATES PATENTS 2,467,675 4/49 Kurtz et al -174 2,628,926 2/53 Ramage et al. 148-11.5 2,678,272 5/54 Ham et al. 14811.5 2,698,892 1/55 Hardin 75176 4/58 Ruthardt 148'-11.5

Claims

1. IN A METHOD FOR HEATING TREATING A METAL ALLOY BASED ON A REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF MO, W, CB, TA AND CR INCLUDING CARBON IN THE RANGE OF ABOUT 0.1-0.2 WEIGHT PERCENT, THE STEPS OF: ANNEALING THE ALLOY AT A TEMPERATURE OF ABOUT 2500-4000*F. FOR A TIME SUFFICIENT TO PLACE CARBON IN SOLUTION WITH THE BASE METAL; AGING THE ALLOY PRIOR TO FINAL PROCESSING FOR ABOUT 1-50 HOURS AT A TEMPERATURE OF ABOUT 1800-2800*F. TO PRECIPITATE FINE METAL CARBIDES INT EH MICROSTRUCTURE OF THE ALLOY; AND THEN REDUCING THE ALLOY TO A FINISHED FORM.