US3467516A - Wrought nickel base alloy - Google Patents

Description

Sept. 16, 1969 J. F. BARKER 3,467,516

WROUGHT NICKEL BASE ALLOY Filed May 2, 1966 United States Patent O Fcice 3,467,516 WROUGHT NICKEL BASE ALLOY James F. Barker, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed May 2, 1966, Ser. No. 546,647 Int. Cl. C22c 19/00 U.S. Cl. 75-171 3 Claims ABSTRACT F THE DISCLOSURE A wrought nickel ibase alloy having good strength properties up to about 1800 F. is provided with improved weldability in sheet form through a careful balance of the precipitation strengthening elements Al and Ti and the solution strengthening elements Mo and W in the presence of carbon to form the desired carbides along with desired intermetallic compound structure. The element Cr is included at a level sufficiently low to stabilize the allow structure and Co has been included to contribute toward hot workability.

This invention relates to nickel lbase alloys and, more particularly, to an improved fabricable nickel base sheet alloy having improved weldability and resistance to subsequent weld cracking along with a good balance of stable tensile and stress rupture properties between room tem- .perature and about 1800 F.

The invention described and claimed in the United States patent application herein resulted from 'work done under United States Government contract FA-SS-64-1. The United States Government has an irrevocable, nonexclusive license under said application to practice and have practiced the invention claimed herein, including the unlimited right to sublicense others to practice and have practiced the claimed invention for any purpose whatsover.

The advance of technology in power producing apparatus, for example gas turbine engines, continuously identifies new needs for improved materials. One type of alloy more widely used in such apparatus has become known as the nickel base supperalloy because of its general high strength and oxidation resistance at elevated temperatures.

Tensile strength and stress rupture strength of nickel base super alloys along with their associated ductilities are important considerations for designers in the development of components. However, a very serious weldability problem can exist when the alloy form is that of sheet material from which components are fabricated. Along with weldability is the problem of the capability of the material to be heat :treated after welding as well as to be repair welded `without cracking.

One emphasis in the development of nickel base super alloys has been on high tensile strength and high stress rupture strength. It is now recognized :that a practical combination of these strengths along with inherent oxidation resistance and a solution to the welding problem is a practical answer to the use of improved nickel base 3,467,516 Patented Sept. 16, 1969 superalloys in components for advanced power production apparatus.

Therefore a principal object of this invention is to provide a nickel base alloy which can be reduced to sheet form and which has an improved balance of tensile and stress rupture properties between room temperature and about 1800 F. along with the capability of being welded, heat treated after welding and repair `welded Without detrimental cracking.

This and other objects and advantages will be more readily recognized from the following detailed description and the drawing which is a graphical comparison between stress rupture properties of the alloy4 of the present invention and similar known alloys.

It has been found that the above object can be realized from a nickel base alloy composition consisting essentially of, by weight, 0.07-0.17% C; 0.005-0.02% B; 13-16% Co; l2-15% Cr; 5.5-7% Mo; 3.5-4% A1; 2-2.7% Ti, with the sum of Al and Ti being 5.5-6.5%; 1.4-3.5 W with the balance nickel and incidental impurities.

In general two major Vstrengthening mechanisms are considered with nickel base super alloys. One of these involves the solid solution strengthening of the alloy through the use of such elements as tungsten and molybdenum. The other is the precipitation strengthening mechanism involving the use of such elements as titanium, aluminum, columbium, etc. in the presence of carbon in the nickel ibase to precipitate various carbides or various complex phrases. The present invention recognizes a critical relationship and effect between the individual solution strengthening elements and the individual precipitation hardening elements as separate groups as well as complex balance and interrelationship between these two groups of elements.

Although tensile properties and stress rupture properties are important considerations, a practical alloy cannot be designed solely with these properties in mind if usable structures are to be made from them. This is particularly true when the alloys are to be reduced to sheet form for subsequent fabrication by welding into a structure. Thus ductility, weldability and repair weldability can be equally important considerations if the alloy is to be useful in such wrought form.

As will be shown in the following tables, combinations of solution strengthening and precipitation hardening elements can be added to a Ni-Cr-Co base material in different proportions to result in unusually high .tensile and stress rupture strengths. Nevertheless, the alloy of the present invention within the .much broader scope of the alloys of this type dene an improved alloy which has the unusual combination of good tensile and stress rupture properties along with good ductility, fabricability and weldability in an inherently oxidation resistant and stable alloy.

Typical of the alloy forms within the scope of the present invention are those, the compositions of which are shown in the following Table I.

TABLE I.-WEIGHT PERCENT, BALANCE Ni AND LNCIDENTAL IMPURITIES These alloy forms as well as those other alloys the compositions of which are shown in the following Tables IV and VI-I were vacuum induction melted in approximately pound heats and were cast into 1/2 X 3" x 6 slab ingots except for alloy forms 3 and 4 of Table I. The ingots were conditioned on all sides by grinding and converting to .0S-.06" thick sheet by hot rolling. Alloy form 3 was a 100 pound vacuum induction melted heat and alloy form 4 was a 2400 pound vacuum melted heat.

After reduction to sheet, the various alloy compositions in Table I as well as those other alloys shown in the various tables which follow were evaluated first for solution heat treatment to find the gamma prime phase solution temperature. With the exception of the alloys outside the scope of this invention including aluminum and titanium at higher levels, the gamma prime solution temperature was found to lie within the range of 2000-2150 F. and primarily 2050-2l50 F. Therefore, such range was selected as the solution heat treatment temperature for the hot rolled sheet material specimens prior to property evaluation. Following the solution heat treatment was a typical aging treatment for nickel base super alloys. In this case it was selected to be at about 1400 F. for a normal period of time such as 16 hours. The following Table II lists the tensile and stress rupture properties of the alloy forms of Table I both at room and elevated 3 templ'atlll's aS ShOWIl.

TABLE IL STRENGTH PROPERTIES used in the fabrication of nickel base and other alloys and by electron beam (EB) welding. The following Table III summarizes the results of the weld restraint patch test conducted on the alloys of Table I.

TABLE HL-wE-LD RESTRAINT PATCH TEST STATUS AFTER WELDING, IN CONDITION INDICATED Type Stress Repair weld relief Age weld Alloy form:

1 TIG No cracks... No cracks--. No cracks. 2 TIG do do Do.

"In the above Table HI the annealing heat treatment was 2025* F. for 10 minutes, furnace cool to 1950* F. in four hours then water quench prior to welding into the fixture. The stress relief treatment was accomplished by inserting the patch test assembly into a furnace preheated 3 hours at 2100 F. It was held there for 10 minutes and then furnace cooled at the rate of 30 F. per minute to 1200" F. Aging was performed at 1400 F. for 16 hours after which it was air cooled. As can be seen from the above table, the alloys of the present invention pass successfully this critical weldability evaluation.

Contrasted with the alloy of the present invention are a number of other alloys some of which have either tensile or stress rupture strengths or both slightly better than some forms of the present invention. However, none of these other alloys tested successfully passed the weld restraint patch test. Thus such other alloys outside the scope of the present invention do not provide the unusual combination of good strength properties combined with weldability so important to the use of an alloy in sheet form.

Stress rupture 1,400 F. tensile UTS (K si.) 0.2 YS (K si.)

Alloly form:

As was emphasized before, even though the alloy forms of Table I shown in Table Il a good balance of strength properties an equally important property is the ability of a sheet alloy to be welded, stress relieved, aged after stress relief and repair welded so that practical use can be made of the alloy.

The weldability evaluation used in connection with the alloy of the present invention involved welding to a strong retaining back member -a 31/2 diameter first disc of 0.05-0.06" sheet having a 1%." diameter central opening. In this case the back member was a 1/2 thick square plate each side of which was 6%" long and including a 3 diameter central opening. After welding the first disc of test material at its outer periphery symmetrically over the central opening in the restraining back member, a central circular 11/2 diameter patch or second disc of the same test material was welded in the 111/2 diameter opening of the first disc.

The welding stresses produced in this configuration have been found to be equal to the yield strength of the test material and simulate the conditions found in a welded fabrication. The patch test assembly is then given the prescribed heat treatment for the alloy and later a repair test weld is placed radially inwardly from but adjacent the 1'1/2 diameter weld around the central plug.

This procedure was used in the evaluation of the alloys discussed here. Welding was accomplished by the well known tungsten inert gas (TIG) welding method widely 1,800 F./11 K s.i.

Percent el. Time (hrs.) Percent el.

TABLE IV.-0THER ALLOYS [Weight percent, balance Ni and incidental impurities] Cr Co M0 W Al Tl .AH-Tl TABLE V between the solution strengthening elements molybdenum and tungsten which are shown to have individual effects rather than to be interchangeable even on an atomic basis. A third aspect is the combined efrect of these two strengthening mechanism in various range.

Because of the intentional inclusion of carbon in the [Strength Properties after 2000-2150 F. solution and 1400 F. age] 1,400 F. tensile 1,800 F. stress rupture 0. 2 UTS YS Percent Time Percent (K s.i (K s.i el. (hrs.) K s.i el.

In Table V, UTS means ultimate tensile strength, 0.2YS means 0.2% yield strength, percent el. means percent elongation and K s.i. means thousands of pounds per square inc In Table IV, with the exception of Examples 14 and 15, the chromium level was maintained within the range of 12-15 in order to enhance stability. Alloys such as those evaluated here and to which the present invention relates would be expected to be unstable with chromium amounts in excess of about 15 weight percent and particularly as that percentage approaches However, if the chromium content is below about 12%, for example about 10% shown in Example 14, severe oxidation results at temperatures of about 1800 F. This was the case with Example 14 in the stress rupture testing. Also comparing Examples 15 and 21 shows the effect of lower chromium in similar alloys.

The other element which was maintained in the alloys of Table IV within the range of the present invention is boron. Much has been reported about the effect of boron on good stress rupture properties. Therefore, the alloy of the present invention includes boron in the range of at least 0.005% because less than that amount has little effect on such properties. However, the inclusion of boron at above about 0.02% tends to cause incipient melting of the alloy and hot shortness or poor ductility when being worked at elevated temperatures.

The element cobalt has been included in the alloy of the present invention in the range of about 13-16% for its benefit on hot workability. Some of the alloys of Table IV, all of which are outside the scope of the present invention, included cobalt within the range of the present invention for comparison purposes. In addition, cobalt levels between 10 and 25% are included to show that increased amounts of cobalt outside the present invention range at higher levels assist in improved stress rupture life but at the expense of much lower tensile ductility.

Several very important aspects of the alloy of the present invention are emphasized in the compositions of Table IV compared with the strength properties shown in Table V. One is the interrelationship ofthe precipitation strengthening'elements aluminum and titanium, the sum of which according to the present invention are limited to the range of about 5.5-6.5. Another is the interrelationship range of 0.07-0.17% carbides which are intentionally formed in the alloy of the present invention are specifically controlled. Carbon is purposely included at a level greater than 0.5 weight percent because at that level and below carbon functions primarily as a deoxidizer.

With insufficient additional carbon, a continuous carbide film can precipitate in the grain boundaries. On aging, this film would remain continuous and tend to embrittle the alloy. However, at levels of from about 0.07 to about 0.17%, there are formed discrete, agglomerated grain boundary carbides which destroy the continuity of the embrittling carbide films. This is particularly effective in the range of 0.07-0.12%.

To much carbon in the composition will not allow suficient grain growth unless increased solutioning temperatures are employed. However, solutioning temperatures greater than about 2150 F., used in the evaluation of this invention, have been shown to approach the incipient melting point of the lower melting alloy constituents.

The carbon range of the present invention has been selected specifically to provide more than required for deoxidization but less than that amount which will inhibit grain growth. Such range results in the formation of agglomerated carbides while at the same time allowing the use of practical solutioning temperatures well below the incipient melting point of any of the lower melting alloy constituents in order to maintain good properties.

One of the problems in providing a wrought material which can be made into sheet form is that its aluminum and titanium content be sufiiciently low for workability purposes. The alloys of Examples 9 and 10 including the sum of aluminum and titanium at 7.7 and 7.9 total weight percent respectively could not be converted into sheet because of cracking during reduction.

The alloys ofv Examples 24 and 26 could be reduced to sheet form because of the significantly greater amount of cobalt included in the alloy composition. However, the tensile properties of the resulting alloy is low, particularly the tensile ductility.

Example 8 which has slightly less of a total of aluminum and titanium could be reduced to sheet form but was very brittle as shown-by the tensile ductility of only 1% elongation and the substantially same ultimate tensile strength and 0.2% yield strength.

At the other end of spectrum of alloys the compositions of which are shown in Table IV is the alloy of Example 7 which is within the scope of the present invention except for the sum of the aluminum and titanium content and the carbon content. Referring to Table V, it is seen that Example 7 has good tensile and stress rupture properties. Nevertheless, as will be shown in the following Table VI, alloy 7 cracked in the stress relief step after welding and after being solution heat treated at 2l00 F. for 1/2 hour followed by air cooling. The mild nature of this cracking indicates the sum of the Al and Ti contents were slightly in excess of that desirable for the best weldability.

TABLE VI.-WELD RESTRAINT PATCH TEST 1 EB welded. 2 Severe transverse cracking on weldlng.

This reaction after welding of alloy 7 points out a critical relationship which exists between the total content of the precipitation hardening elements aluminum and titanium and carbon in the alloy of the present invention.

The precipitation hardening elements aluminum and titanium in quantities lower than that of the present invention are also shown'in Table IV. lFor example, alloy 18 includes a lower total amount of aluminum and titanium. As a result, the tensile properties are somewhat reduced and the stress rupture strength is significantly reduced. Other variations in aluminum and titanium content are shown in Table IV and the corresponding strength properties are shown in Table V.

Only those alloy compositions shown in Table IV having reasonably good strength properties were tested in the weld restraint patch test. Of the alloys tested, including alloy number 6 which is at the lower range of the sum after Welding and a severe crack upon aging. Thus it was not suitable for fabrication into a useful article.

The compositions shown in Table 1V were varied, in addition, to show the effect of the solution strengthening elements molybdenum and tungsten on the alloy of the present invention and to show that such elements are not interchangeable even on an atomic basis. The alloy of the present invention includes molybdenum and tungtens in the range of 5.5-7 weight percent molybdenum and 1.4-3.5 weight percent tungsten. For example, a com parison should be made between Example 7, including 5.5% Mo and 3.2% WWith Example 1l, including 3.4% Mo and 5.4% W with the same total of precipitation hardening elements aluminum and titanium. Table VI shows that Example ll could not even be welded because of severe transverse cracking on welding whereas Example 7 was at least weldable even though it cracked on stress relief.

'I'he elements molybdenum and tungsten are not interchangeable nor can they be substituted one for the other in the alloy of the present invention. The inclusion of amounts of tungsten greater than 3.5 weight percent results in poor weldability and severe cracking. Below 3.5% W the element can participate properly in solid solution strengthening. Example 8 which included as much as 10.1% W assisted the large amount of precipitation strengthening elements in providing a brittle alloy of very low stress rupture strength. Below about 1.4%, insutiicient tungsten is present to participate with Mo in the solution strengthening mechanism. In the absence of tungsten, as shown by AExample 2l, relatively good tensile properties can result but with unusually low stress rupture life, even with the proper range of the precipitation hardeners aluminum and titanium. Thus it is seen that the elements molybdenum and tungsten are critical with regard to the weldability of the alloy and play their own individual roles in providing solution strengthening along with good weldability.

From time to time, other strengthening elements have been considered for addition to nickel base superalloys of the type to which the present invention relates. Typical of other such elements are columbium and vanadium. The alloy compositions shown in Table VII are typical of those other alloys, including such additional elements, evaluated in connection with the alloy of the present invention.

TABLE VIL-OTHER ALLOYS [Weight percent, balance Ni and incidental impurities] C Cr C0 M0 W A1 Ti A14-Tl B Other of aluminum and titanium and alloy 7 which is above the range of the total of aluminum and titanium, none passed the weld restraint patch test. Alloy 16, which had been tested before and passed was not retested in this evaluation because of its lower strength. As shown in Table VI, alloy number 6 formed a transverse crack on stress relief As can be seen from the strength properties of the following Table VIII, the addition of columbium and vanadium to bring the total amount of such hardening elements at least Within and in most cases greater than the range of the present invention, resulted in little if any improvement in strength properties.

TABLE VIII [Strength properties after 2,000-2,150 F. solution and 1,400" F. age] All of the compositions of Table VII were reducible to sheet form with the exception of alloy Example 34 which could not be converted to sheet. Alloys 27, 28, 29 and 30, which were the best of the alloys including columbium, were evaluated in the weld restraint patch test. Alloy 31 was not because it was too brittle: it fractured before the 0.02% yield point could be reached. All of the alloys 27, 28, 29 and 30 developed severe cracks in the welds during stress relief after welding so that none of them were capable of being evaluated further for weldability.

Although some alloys of the nickel base super alloy type can tolerate iron in relatively large amounts, iron is specifically excluded, except perhaps as a slight impurity, in the alloy of the present invention because of its tendency to reduce stress rupture properties.

Referring to the drawing, there is shown a comparison between the alloy form of Example 1 within the scope of the present invention as shown in Table I, that of another reported alloy A having a composition, by weight, of 0.05% C, 2.0% Al, 3.0% Ti, 6.0% Mo, 19.0% Cr, 12.0% Co, 1.0% W with the balance substantially nickel and a well known Alloy B having a composition, by weight, of 0.05% C, 3.0% Al, 3.0% Ti, 4.0% Mo, 17.5% Cr, 16.5% Co with the balance essentially nickel and incidental impurities and including no tungsten. The superiority of the present invention over a wide range is shown. The graph form is the Larson-Miller parameter type reported in the Transactions of the American Society of Merchanical Engineers, 1592, vol. 74, at pages 765-771.

Although this invention has been described in connection with specific examples and embodiments, it will be recognized by those skilled in the metallurgical art, the variations and modifications of which the invention is capable. It is intended yin the appended claims to cover all such modications and variations.

What is claimed is:

1. An improved, Wrought nickel base alloy characterized by improved weldability in sheet form along with good strength properties up to about 1800 F., the alloy consisting essentially of, by weight:

0.07-0.17% C, said carbon percentage being greater than that required for deoxidation and in addition being suicient for forming discrete agglomerated grain boundary carbides;

the sum of Al and Ti being 5.5-6.5;

with the balance Ni and incidental impurities.

2. The alloy of claim 1 in which the carbon range is 3. The alloy of claim 2 in which:

Co is 14-16%;

Cr is 13-15%;

Mo is 5.5-6.5%; and

W is 2.5-3.5%.

References Cited UNITED STATES PATENTS 2,809,110 10/1958 -Darmara 75-171 2,975,051 3/1961 Wilson et al. 75-171 2,977,222 3/ 1961 Bieber. 3,107,167 10/1963 Abkowitz et al. 3,110,587 11/1963 Gittus et al. 3,155,501 11/1964 Kaufman et al. 3,385,698 5/1968 MacFarlane et al. 75-171 OTHER REFERENCES Journal of Metals, February 1954, relied on pages 2li-218.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 14S-32

Images