US3619183A - Nickel-base alloys adaptable for use as steam turbine structural components - Google Patents

Abstract

Nickel-base alloys containing correlated amounts of aluminum, titanium, columbium, molybdenum and tungsten, adaptable for use as structural components (notably steam turbine bolts) at temperatures on the order of 1000* F. Alloys, which also contain iron and usually carbon, afford specified minimum room and elevated temperature yield strengths and other desirable metallurgical characteristics.

Description

United States Patent on 3,619,183

72 Inventors John H. Olson [56] Relerences Clted Franklin Lakes N44 UNITED STATES PATENTS I 2]] No 2,994,605 8/l96l can et al. 75/171 Filed Mar I968 3,046JO8 7/l962 lusclstcm 75/! 7| [45] Pat nt d Nov, 9, 1971 Primary ExaminrrRlchard 0. Dean [73] Assignee The lnternatlonal Nickel Company, Inc. L Pin l New York, NY.

[54] ABSTRACT: Nickel-base alloys containing correlated SC I N D I amounts of aluminum, titanium. columbium. molybdenum o and tungsten. adaptable for use as structural components [52] US. Cl 75/l7l. (notably steam turbine bolts) at temperatures on the order of l48/32.5. l48/l62 l000 F. Alloys, which also contain iron and usually carbon [5]] lnt.C| C221: 19/00 afford specified minimum room and elevated temperature [50] Field of Search 75/] H, yield strengths and other desirable metallurgical characteristics.

NICKEL-BASE ALLOYS ADAPTABLE FOR USE AS STEAM TURBINE STRUCTURAL COMPONENTS As is generally known, nickel-base alloys have found considerable acceptance in innumerable commercial and industrial applications, applications requiring adherence to critical service requirements. For example, such materials have been gainfully utilized in the production of various components for steam turbine environments, including both rotors and blades. Nonetheless and as is so often the case, advanced structural designs brought about by increasingly severe operating conditions necessitate the development of new alloys capable of withstanding the more stringent demands imposed. Steam turbine bolting is illustrative.

Not long ago the 12 percent chromium stainless steels were extensively, if not exclusively, used as the bolting material in fastening the outer shell sections of steam turbine assemblies. But with the introduction of larger size turbines designed to operate at temperatures circa l,000 F. such steels were deemed wanting, particularly in respect of strength capabilities. In this regard and as contemplated herein, bolting alloys should be capable of consistently delivering in the aged condition a minimum room temperature yield strength (0.02 percent offset) of at least 85,000 pounds per square inch (p.s.i.) and a l000 F. minimum yield strength of better than 70,000 p.s.i. And provided other desired properties are not sacrificed, it is most beneficial that these minima be at least 90,000 p.s.i., and 75,000 p.s.i., respectively. 1

One of the currently used bolting materials has on occasion manifested notch sensitivity," i.e., its load carrying ability is considerably impaired by the presence of a notch. As a practical matter, it is virtually impossible to absolutely preclude the formation of a notch, crack, or similar flaw, flaws which may be internally or externally induced either during or subsequent to processing. Once present, a flaw ostensibly serves as a focal point for concentration of stresses. This localization of stress concentration promotes the notch as a point of self-propagtion. How well a material resists this propagation is a reflection of its notch sensitivity or notch toughness.

A drawback of the above-mentioned notch sensitive alloy concerns the coefiicient of thermal expansion (CTE) parameter. Usually the turbine shell is formed from a ferritic steel having a CTE of between about 7.75Xl0 to 8Xl0 in./in./ F. at the operating temperature of l,000 F.; however, at such temperature this notch sensitive alloy has a CTE on the order of about 8.4Xl0" in./in./ F. Quite naturally, a bolting alloy should exhibit a CTE as compatible as practicable with the shell material and to this end a CTE of below about 8 would be of considerable advantage. Steam turbines are often "torn down for-purposes of inspection, repair, etc. On cooling the bolts (and nuts) tend to contract and thus tighten. At best the bolt is difficulty removable, and the wider the gap between the CTEs of shell and bolt the more acute and tedious the problem becomes.

In addition to the foregoing, alloys suitable for steam turbine bolting (or other high temperature fasteners) should also manifest a low creep rate at temperatures of about l,000 F. to l,200 F., otherwise, should a bolt undergo substantial deformation (extension) in use, serious consequences could ensue. More specifically, such alloys should be characterized by high resistance to what is commonly termed relaxation." As treated in excellent fashion at page 604 of the Eighth Edition of THE METALS HANDBOOK, turbine bolts are tightened initially to a certain elastic strain and corresponding elastic stress. While in service at elevated temperature, bolt creep does occur. It is considered that some portion of elastic strain is thus transformed into plastic strain. As a result, there is reduction in stress and this is given the apt description relaxation. lf the degree of relaxation is too much, what was a leakproof joint is no longer.

Too, it would be particularly desirable if the same alloys developed for bolting were characterized by good stress rupture properties at temperatures of l,000 F. to 1,200 F including stress rupture strength, ductility and resistance to impact such that the alloys would be useful for other applica-- tions, e.g., tubing, piping, valves, and the like. The subject invention is addressed to attaining these objectives while obviating difficulties heretofore experienced.

It has now been discovered that nickel-base alloys of special composition and containing interrelated amounts of aluminum, titanium, columbium, molybdenum, tungsten. etc., display an excellent combination of room and elevated temperature characteristics rendering the alloys particularly useful in the production of various articles of utility, particularly steam turbine bolts.

It is an object of this invention to provide a new and improved nickel-base alloy.

Another object is to provide nickel-base alloys capable of manifesting yield strengths of at least 85,000 p.s.i. at room temperature and of at least 70,000 p.s.i. at l,000 F the alloys also being substantially notch insensitive and characterized by good ductility.

Other objects and advantages will become apparent from the following description.

Generally speaking, alloys contemplated herein are age hardenable and, in accordance with the invention, contain (percent by weight) from 17.5 percent to 22 percent chromium, about 2.3 percent to 3.3 percent of columbium, about 2.5 percent to about 3 percent molybdenum, about 2.5 percent to about 3.25 percent tungsten, about 0.4 percent to about 0.75 percent aluminum, about 0.35 percent to about 0.7 percent titanium, up to about 0.l2 percent e.g., about 0.01 percent (advantageously 0.04 percent) to 0.l percent carbon, about 3 percent to about 12 percent, e.g., 5 percent to 9 percent, iron, up to about 0.1 percent, e.g., 0.0l percent to 0.05 percent, magnesium, up to about 0.01 percent, e.g., 0.003 percent to 0.008 percent, boron, up to 0.l percent, e.g., 0.0] percent to 0.05 percent, zirconium, up to about 0.4 percent silicon, up to 0.75 percent manganese, and the balance essentially nickel. The use of the expression balance" or essentially" in referring to the nickel content of the alloys, as will be understood by those skilled in the art, does not exclude the presence of other elements commonly present as incidental constituents, e.g., deoxidizing and cleansing elements, and impurities normally associated therewith in small amounts which do not adversely affect the basic characteristics of the alloys. In this regard, elements such as oxygen, nitrogen, phosphorus, sulfur, and the like should be kept as low as is practicable. Tantalum, as is known, is usually found in commercially produced columbium and can be present, for example, up to 0.5 percent.

In carrying the invention into practice, it is important in consistently achieving highly satisfactory results on a production basis that the chromium content not fall below 17.5 percent, for, as will be illustrated herein, inadequate tensile strength can easily be the result. The efiect of chromium in this regard is deemed somewhat surprising. Usually, chromium is considered to perform as a simple solid solutioning element without much effect on age hardening response. Although a complete theoretical explanation may not yet be at hand, chromium in accordance herewith, appears to impart a rather strong influence on the age hardening characteristics of the alloy. Whatever its exact role, it markedly contributes to achieving the necessary strength levels at both room and elevated temperatures. There are certain instances in which chromium can be lowered to about 15 percent, possibly l4 percent, as will be illustrated herein. But this opens up an area of risk. In any event, however, percentages above 22 percent are to be avoided lest alloy stability be impaired. In consistently achieving a sufficient strength plateau and with the view of minimizing loss of stability, it is, accordingly, quite beneficial that the alloys contain not less than l9 percent nor more than 21 percent chromium.

Aluminum, even in the small amounts contemplated, exerts a most potent influence in imparting hardness and in conferring high tensile strength (at both room and elevated temperature) and stress-rupture strength. Reducing the aluminum much below 0.4 percent entails the objectionable risk of inadequate tensile strength. At the other end of the range, amounts much above 0.75 percent can undesirably detract from stress rupture ductility characteristics, particularly when the other essential constituents, columbium, titanium, molybdenum, tungsten, are at the higher end of their respective ranges. Moreover, the higher levels may give rise to an additional complicating factor. By way of explanation, alloys within the invention upon exposure to high temperature for extended periods of time tend to exhibit an increase in yield strength. This is deemed attributable to additional age hardening precipitate coming out of solution upon exposure to high temperature. Now, there may be applications in which this may not be objectionable, but for steam turbine bolting dimensional change might result with attendant difficulties. Thus, the mere presence of excess aluminum could possibly accentuate this condition, i.e., increase the possibility for additional age hardening constituent to come out of solution upon prolonged heating (and subsequent to usual age hardening treatments). While this affect can be minimized by special heat treatment, as will be discussed hereinafter, control of aluminum content is of advantage.

ln addition to aluminum, care must also be exercised in respect of the amounts of titanium, columbium, molybdenum, and tungsten. Titanium, for example, contributes'to tensile strength and hardness and also improves stress rupture life, although to a lesser degree than aluminum. Unlike aluminum, however, it significantly impairs alloy stability as evident from a not insubstantial loss in the capability of absorbing impact energy upon prolonged exposure to elevated temperature. For best results, including yield strength, stability and high temperature ductility, neither the lower nor the upper limits, respectively, of both of these constituents should be used simultaneously. In this regard, the sum of the aluminum plus titanium should be at least above 0.9 percent, advantageously at least 1 percent, and up to 1.4 percent.

Columbium, molybdenum and tungsten coact to confer hardness and strength. In addition, columbium enhances stress rupture life, but lowers stability, particularly in conjunction with molybdenum. In terms of loss in impact strength the copresence of columbium and molybdenum is synergistic in effect, the loss being greater than what it might be for these constituents individually. Accordingly, with the desideratum of reaching the best combination of strength and stability the combined columbium plus molybdenum should not exceed about 6 percent. Tungsten, although it exerts a positive influence in terms of strength, when present to the excess detrimentally affects high temperature ductility and undesirably raises alloy density. In view thereof and for highly satisfactory results, it beneficially should not exceed about 3 percent.

As a result of test data, it has been'determined that the total amount of the hardening and strengthening constituents (aluminum, titanium, columbium, molybdenum and tungsten) should not be concomitantly used at the upper end of their respective ranges if acceptable levels of both alloy stability and ductility are to be attained. On the other hand, there is the danger that if the minima of each of such elements are used, insufficient strength will result. Accordingly, these constituents should be correlated such that the strengthening and stability factor, SSF, expressed by the following relationship is satisfied:

about 5.25% to 6.4%.

Advantageously the SSF is from 5.4 percent to 6.2 percent.

A most highly satisfactory combination of properties is obtained with the alloys falling within the following ranges: about 19 percent to 21 percent chromium, about 2.5 percent to 3 percent each of columbium, molybdenum, and tungsten, about 0.5 percent to 0.7 percent aluminum, about 0.4 percent to 0.6 percent titanium, the sum of the aluminum plus titanium being at least 1 percent, about 0.04 percent to about 0.1 percent carbon, about 5 percent to 9 percent iron, about 0.003 percent to 0.008 percent boron, about 0.01 percent to 0.05 percent zirconium, and the balance essentially nickel.

Conventional processing techniques can be used in producing the alloys, but to achieve acceptable strength, they should be aged at a temperature of about l300F. to l400F. for a period of about 16 to 32 hours, e.g. 1300+F. for about 24 hours. To minimize the occurrence of a quite substantial increase in yield strength when the alloys are exposed to high temperature for periods of long duration, an additional aging step is deemed quite beneficial. When aged over the temperature range of 1300F. to l400F.(even for the considerable period of, say, 32 hours) and thereafter subjected to exposure at a temperature of about l000F. for an extended period of time, e.g. 1000 hours, it has been observed that the yield strength rather markedly increases. This behavior, as mentioned above herein, ostensibly is due to a greater amount of precipitating hardening phase coming out of solution whereby strength is increased. Since this might give rise to dimensional change, a second aging treatment is recommended at least for alloys intended for bolting applications. Thus, subsequent to the first aging treatment above set forth the alloys should be cooled to a temperature of 1,l00 F. to l,200 F. at a rate of about 20 F to 50 F. per hour and then held for about 15 to 30 hours. As a result of the second aging treatment, the percentage increase in yield strength that might otherwise be expected is considerably reduced. This, in turn, minimizes the possibility of dimensional change.

For the purpose of giving those skilled in the art a better appreciation of the invention, the following illustrative data are given:

A series of alloys both within and without the invention were prepared using vacuum melting techniques. The melt charges were made using the following type of ingredients: carbonyl nickel pellets, vacuum grade chromium, electrolytic iron, ferro-columbium, molybdenum pellets, tungsten powder, titanium sponge, aluminum rod, ferro-boron, and spectrographic carbon. Magnesium and zirconium were introduced in the form of nickel-magnesium and nickel-zirconium master alloys, respectively. Generally, the major charge components with about 0.05 percent carbon were melted in a magnesium oxide crucible and held about one-half hour at about 2,900 F. to effect oxygen removal. Additions of titanium, aluminum, boron, and zirconium were made and argon was introduced into the vacuum chamber (one-half atmosphere pressure) following which the nickel-magnesium master alloy was added. A final carbon addition was next made and ingots (30 lbs.) were thereafter poured at 2,850 F.

Usually the ingots were soaked at least 1 hour at 2,l50 F. and hot rolled to 2inches X 2 inches bar which was thereafter annealed at 2,l50 F. and hot rolled to 96-inch square bar using one intermediate anneal at about 2,l50 F. In some instances this exact practice was not followed during the early stages of the rolling. lngots which were not rolled down to 2 inches X 2 inches bar during the initial hot rolling were hammer-forged to produce stock that could be rolled further. In any case, it was considered that the final hot rolling procedures were similar for all heats, the average finishing temperature being estimated to be 1,750 F.

In table l, alloys 1 through 8 represent alloys formulated in accordance with the invention whereas alloys A through H are beyond the scope thereof. These later alloys, however, afford a good basis for comparison with alloys 1 through 8 in terms of mechanical characteristics, the results being reported in table 11. Too, it should be noted that alloys 7 and 8 are included to demonstrate that it is possible to obtain an acceptable level of properties with alloys containing chromium percentages of about I5 percent to 16 percent, i.e., below 17.5 percent or 19 percent. The mechanical characteristics were obtained on specimens which had been annealed at l,800, F. for 1 hour and thereafter aged for about 24 hours at l,300 F. (Apart from the chemistry given in table I, each of the alloys contained about 5 percent to 9 percent iron, not more than (a) 0.04 percent zirconium, (b) 0.0l percent boron, (c) 0.03 percent magnesium, (d) 0.1 percent manganese, (e) 0.1 percent ployed as reflected by alloys 7 and 8. In this connection. the silicon, and (f) 0.31 percent tantalum, the balance being esaluminum content must be at least 0.5 percent and adsentially nickel and impurities.) vantageously at least 0.6 percent. the sum of the aluminum plus titanium being at least l.l percent, e.g., 1.2 percent. This 5 further restricts an already narrow aluminum range. Since TABLE 1. (OMlUSlTlON some percentage of aluminum is lost during processing, it thus can become difficult (with attendant risk) to consistentl y 8.3 1. achieve the necessary minimum in strength. etc. -lac- Ni. Cr Ch Ti Al Mo w Immm m Stress rupture tests were also conducted on \JHOUS alloys, the tests bein carried out usin combination t e s ecimens, g 8 VP P lfg. i.e., specimens having two reduced sections, one smooth and 20 3.0 0.60 0.7 2.55 '2. 0. 8 0. 001 31-1 one notched, the notched being a 0.007-inch root radius. a 1 51;: 01 01 :12; 2:0 3:8 00:2 21:; 11:; Th s prim ry f r h 0 th purp s r 20.0 0. 37 0.68 8.8311 B ascertaining notch sensitivity characteristics. The results are 1.. '...i' 3: 3 332 3 72 n3 given in table lll and it should be mentioned that the 0-065 1 s ecimens were room tem erature tensile tested after ex 0- P P P s..... 15.4 3.1 0. 50 0.50 2. .10 2. 0.085 5. .1. 110. a o .1. 3.15 0. as 0.10 2.50 a. 0 0.10 5 11 110. sure at 1,000 F. or 1,200 F. (Alloy Zhavmg been exposed at 3g? both temperatures). The time of exposure and stress are also I v t 11 I 1).... 15.8 3.0 0. 00 0.11 2.5 3.0 0.003 5. 311 1:0. 20 given together with room temperature yield strength (Y.S.), 16.1 2.45 0.36 0.50 3.85 2.55 0.086 5.31 110. M6 305 [L53 0'48 195 0' 11 5.64 no tensile elongation (El.), reduction of area (RA notch ten (1.... 14.0 3.0 0. 52 0. 43 3.1.- 3.: 0.017 5.00 110. Sll strength(N.T.S.), and the ratio ofnotch tensile strength to r- 051 ultimate tensile strength (NTS/UTS).

TABLE III Stress rupture conditions Smooth liar, percent 'lcm- Yivl ll l I 1 perm streii t i E (111- lvr l1t- Tiiin turv. 10.0. cation tioit ll NTS/ l10iii- 1 p.s.i. ttt'tfl NTS UTS Alloy No.1

X0111 X0110 Noiit- 103,100 33 5 (None X0110 X0110 8 7,500 11." "1 1,000 15, 000 1,200 107,700

TABLE It The results depicted in table lll illustrate that alloys within Yield Strength Elongation, Reduction in the invention are notch insensitive. Actually, where failure ocpercent area. pe curred, it was in the smooth portion of the test specimen and Rm 1,000 D R313 R311. L000 under a stress quite above that normally encountered in ser- A N vice for steam turbine bolting. By way of explanation and with W reference to the data in table Ill, no failure was ex erienced with alloy 2 even after 1,800 hours exposure at l,O00 F. under the exceptionally high stress of 100,000 p.s.i., failure not occurring until another specimen was tested at l,200 F. In this latter instance, failure came about at 190 hours exposure at the still very high stress of 80,000 p.s.i. The failure that l5. did occur was ductile in manner, the elongation being'37 perg: cent and the reduction of area being 62 percent. The notched E. portion of this specimen was put back in test and was exposed for an additional 1,502 hours (making a total of 1,692 hours) 11... whereupon the test was discontinued. Subsequent testing revealed the specimen was not weakened as a consequence of 0 the notch since the room temperature tensile test showed the n be noted from the data given in tables I and H that ultimate tensile strength to be l89,900 p.s.i. versus the each of the alloys within the subject invention manifested 167,500 .s.i. obtained in the initial aged condition. yield strengths at room temperature in excess of the required A third specimen of alloy 2 was exposed at the higher temminima of 85,000 p.s.i. at room temperature and 70,000 p.s.i. per-alum f 1,200 F. but nd a mor re li ti tres of at l,000 F. ln m rke Con ra h r t n Of the oy 65 55,000 p.s.i. No failure followed the exposure period of 1,248 Sidelhe invention afforded y Strengths fy g hours and the test was discontinued. Nor was failure exthe same minimum requirem nts. A oys A and 3 gh perienced in respect ofalloys 4 and 5 after exposure periods of sufficiently high in chromium exhibited inferior strength due 1050 and 1,000 hours, respectively, at l,200 F. (tests disconto the low total of aluminum plus titanium. Also, the Strengthtinned). Stability Factor, SSF, was low. On the other hand, alloys such As referred to herein, a second aging treatment is beneficial as D through H having an adequate total of aluminum plus in minimizing the tendency for the alloys to exhibit increased titanium were still unsatisfactory, the reason being attributed ield strength upon prolonged exposure at elevated temperato low chromium levels. With higher aluminum contents and t re. Thus, to illustrate this aspect, alloy 2 was double aged, an SSF of at least 5.5, beneficially at least 5.7, alloys containi.e., after aging at 1,300 F. for2 4 hours, it was furnace cooled ing down to 15 percent or 14 percent chromium can be emto 1.150" F. at a rate of about 25.20 F. per hour and held (aged) for 24 hours. After this second aging treatment, alloy 2 exhibited a yield strength (0.02 percent offset) of 118,000 p.s.i., approximately 15,000 p.s.i. above that obtained with the single aging treatment conducted for 24 hours at l,300 F. This value of 1 18,300 p.s.i. represents a reduction of one-half in the difference in yield strength obtained prior and subsequent to long time exposure. The differential in yield strength generally would not be expected to be as great with most other compositions within the invention. Alloy 2 (also alloy 1) was the most heavily alloyed in terms of aluminum. titanium. columbium, molybdenum. and tungsten and. thus. would be compositionally most susceptible to manifest increased strength upon prolonged exposure at elevated temperature.

1n the double aged condition, alloy 2 was tested for notch sensitivity by exposing the alloy for approximately 1,2 hours at l,000 F. under a stress of 1 15,000 p.s.i. No failure was encountered whereupon the stress was raised to 125,000 p.s.i. and the test was continued for an additional 1 19 hours. When failure had not yet occurred, the stress was raised to 135,000 p.s.i., failure being brought about after 95.4 hours at temperature. Again failure was experienced in the smooth section. This cycle of testing confirmed that the alloys within the invention exhibited excellent resistance to notch sensitivity.

As indicated above herein, the coefficient of thermal expansion for alloys used for turbine bolting purposes should be as close as possible to the coefficient of thermal expansion of the metal from which the turbine shell is formed. In the double aged condition above described, Alloy 2 manifested a coefficient of thermal expansion, after heating to 1,000 F. and holding for minutes, of 7.8 in./in./ F. This value compares quite favorably with the value of 7.75 l0" to 8 l0 in./in./ F. discussed previously herein.

In an effort to determine stress relaxation characteristics, alloy 2 was compared against an alloy known to exhibit satisfactory resistance to relaxation as a bolting material. In this connection, three separate determinations were made using three different strain values. The initial strain was 0.15 percent which was maintained over the full course (1,000 hours) of the first test. The stress at the beginning of this test was about 38,080 p.s.i., the temperature being 1,1 12 F. (same temperature used in all three tests). After exposure for 1,000 hours, the final stress value was then determined, a level of approximately 30,200 p.s.i. being obtained. This value was virtually identical with that manifested by the standard alloy of comparison under the same conditions of test.

In the second run the strain was increased to and maintained at 0.257 percent, the load in this instance being initially about 65,000 p.s.i. After approximately 144 hours, the stress was determined to be 56,100 p.s.i. 1n the last experiment the strain was maintained at 0.30 percent, the initial stress being 75,600 p.s.i. After nearly 170 hours, the stress was measured to be 65,500 p.s.i. As with the first, the second and third tests indicated that alloy 2 compared quite favorably with the standard alloy. These data, although ascertained by way of simulated test conditions, indicate that alloys contemplated within the invention will afford a more than satisfactory degree of resistance to stress relaxation.

Alloys of the subject invention can be produced in accordance with usual and conventional processing techniques as already indicated and as those skilled in the art will readily appreciate. it is preferred that vacuum induction techniques be employed although the alloys can be readily air melted. After forming ingots and prior to hot working, the ingots should be thoroughly homogenized at, say, a temperature on the order of about 2,100 F. This contributes to achieving uniform distribution of the alloying constituents and also better mechanical properties. The cast ingots can be initially hammered or press forged and subsequently hot rolled or the ingots can be hot rolled directly to plate or sheet with suitable intervening reheat treatments in order to maintain the temperature above about l,700 F. Where used, annealing treatments should be conducted within the temperature range of approximately 1,750 F. to l,850 F. as opposed to higher temperatures. It has been found that the lower annealing temperatures confer higher strength characteristics.

The alloys of the present invention can be produced in the form of bar, rod, sheet, plate, extruded tubing, and forgings and are useful at elevated temperatures on the order of about l,000 F. for such applications as steam piping, tubing, etc. Of course, the alloys are particularly adapted for use as fasteners in steam turbine assemblies, particularly bolting for fastening the outer shells or casings (usually flanged) of such assemblies. This follows from the excellent minimum yield Strengths (0.02 percent offset) afiorded at both room temperature and at 1.000 F. in combination with other desired characteristics discussed herein. (The aging treatment criterion used in determining the minimum yield strength is 24 hours at l,300 F. followed by air cooling.)

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium alloy adapted for use at elevated temperatures on the order of about l,000 F. and characterized in having a yield strength at room temperature of at least about 85,000 p.s.i. and a yield strength at l,000 F. of at least about 70,000 p.s.i. together with a high degree of resistance to notch sensitivity, said alloy consisting essentially of about 17.5 percent to 22 percent chromium, about 2.3 percent to 3.3 percent columbium, about 2.5 percent to 3 percent molybdenum, about 2.5 percent to 3.25 percent tungsten, about 0.4 percent to 0.75 aluminum, from 0.35 percent to 0.7 titanium, the sum of the aluminum plus titanium at least 0.9 percent and up to 1.4 percent, the aluminum, titanium columbium, molybdenum, and tungsten being correlated such that the strengthening and stability factor, SSF, expressed by the following relationship is satisfied 2.2 %Al+l.2 %Ti-i-O.6X%Cb+ about 5.25% to 6.4%, about 0.01 percent to 0.12 percent carbon, about 3 percent to 12 percent iron, up to 0.01 percent boron, up to 0.1 percent zirconium, up to 0.4 percent silicon, up to 0.75 percent manganese, and the balance essentially nickel.

2. An alloy in accordance with claim 1 in which the chromium is from 19 percent the aluminum plus titanium is at least 1 percent the SSF value is from 5.4 percent to 6.2 percent, the sum of the columbium plus molybdenum does not exceed about 6 percent, and the carbon is from 0.04 percent to 0.1 percent.

3. In a steam turbine assembly, a fastener for bolting the outer shell sections thereof and formed from an alloy consisting essentially of at least 14 percent to 22 percent chromium, about 2.3 percent to 3.3 percent columbium, about 2.5 percent to 3 percent molybdenum, about 2.5 percent to 3.25 percent tungsten, about 0.4 percent to 0.75 percent aluminum, from 0.35 percent to 0.7 percent titanium, the aluminum, titanium, columbium, molybdenum and tungsten being correlated such that the strengthening and stability factor, SSF, expressed by the following relationship is satisfied about 5.25% to 6.4%, with the further provisos that when the chromium content is less than 17.5 percent (a) the aluminum content is at least 0.5 percent, (b) the sum of the aluminum plus titanium is at least 1.1 percent and (c) the SSF is at least 5.5 percent, about 0.01 percent to 0.12 percent carbon, about 3 percent to 12 percent iron, up to 0.01 percent boron, up to 0.1 percent zirconium, up to 0.4 percent silicon, up to 0.75 percent manganese, and the balance essentially nickel, said alloy being further characterized in having a yield strength at room temperature of at least about 85,000 p.s.i. and a yield strength at l,000 F. of at least about 70,000 p.s.i.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 9 ,l83 Dated November 1971 JOHN H. OLSON and JERE H. BROPHY Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 40, for "10" read ----further-; line 43, for

"7.75 x 10 to s x 10 read --7.75 x 10" to s x 10 line 46, for "8.4 X 10 read 8.4 x l0 and line 52, for "difficulty" read --difficultly-.

Column 4, line 6, for "1300+F." read l300F.-; line 9,

for "temperature" read -temperatures;1ine 63, for "later" read -latter-; and line 71, for "l,800,F. read l,800F.-.

Column 5 TABLE I, under column heading "Cb" for Alloy No. 3

for "391" read --3.l-; under column heading "C" Alloy No. 6, for "0.72" read 0.072; for column heading "S.s.f. factor" read -S.S.F. factor; under column heading "S.S.F. factor", Alloy No. A, for "511" read 5.ll; and under column heading "C", Alloy No. C, for "0.84" read 0.084-.

Column 6 TABLE III for column heading g- 5%" read NTS and line 75, for "25.20 F." read --25F.. Column 7 line 32 for "7.75 x 10 to 8 x 10 read --7.75 X 10 to 8 x 10' Column 8, line 32 (Claim 1, line 10) before "aluminum insert --percent--; line 33 (Claim 1, line 11) after "titanium" insert -being--; and line 61 (Claim 3, line 11) for "%C+" read %Cb.

Signed and sealed this 6th day of June 1972. L .J

(SEAL) Attest:

EDWARD M.FLET( JHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

Claims (2)

1. 2. An alloy in accordance with claim 1 in which the chromium is from 19 percent the aluminum plus titanium is at least 1 percent the SSF value is from 5.4 percent to 6.2 percent, the sum of the columbium plus molybdenum does not exceed about 6 percent, and the carbon is from 0.04 percent to 0.1 percent.
2. 3. In a steam turbine assembly, a fastener for bolting the outer shell sections thereof and formed from an alloy consisting essentially of at least 14 percent to 22 percent chromium, about 2.3 percent to 3.3 Percent columbium, about 2.5 percent to 3 percent molybdenum, about 2.5 percent to 3.25 percent tungsten, about 0.4 percent to 0.75 percent aluminum, from 0.35 percent to 0.7 percent titanium, the aluminum, titanium, columbium, molybdenum and tungsten being correlated such that the strengthening and stability factor, SSF, expressed by the following relationship is satisfied 2.2 X %A1+ 1.2 X %Ti+ 0.6 X % Cb+ 0.6 X %Mo+ 0.3 X %W- 5 X %C is from about 5.25% to 6.4%, with the further provisos that when the chromium content is less than 17.5 percent (a) the aluminum content is at least 0.5 percent, (b) the sum of the aluminum plus titanium is at least 1.1 percent and (c) the SSF is at least 5.5 percent, about 0.01 percent to 0.12 percent carbon, about 3 percent to 12 percent iron, up to 0.01 percent boron, up to 0.1 percent zirconium, up to 0.4 percent silicon, up to 0.75 percent manganese, and the balance essentially nickel, said alloy being further characterized in having a yield strength at room temperature of at least about 85,000 p.s.i. and a yield strength at 1,000* F. of at least about 70,000 p.s.i.