DE1608242A1 - High temperature nickel alloys - Google Patents

Description

concerning
High temperature resistant nickel alloys.

The present invention relates to nickel alloys with alloying metal additives that make up the resulting alloy Give stability to the uptake of sulfur and make them suitable for use under high stresses make it suitable within a wide temperature range.

Numerous nickel alloys have previously been found to be useful for use at high levels Temperatures, as they occur in gas turbines, dio applied to plug-in tools or in other areas been, DLo high operating temperature temperabureu created by dLö

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Use of these alloys in the turbine part of the engine are permitted, require an improvement in the Work performance because the high gas temperatures lead to high pressure and / or low fuel consumption lead per printing unit.

While it initially appeared that these nickel alloys met the more stringent requirements of stability to high operating temperatures and high stresses to which the wing portion of the turbine blade was subjected, the temperature conditions brought other factors into play which impaired the full exploitation of the high temperature resistance advantage of the alloy. One of these factors is the susceptibility of the lüurbinenschaufein to the uptake of sulfur at high temperatures, ie ^! a form of catastrophic corrosion. Another factor is the instability of the alloys at operating temperatures which, in service, results in a loss of good properties in terms of high stress fracture, thereby affecting the use of nickel alloys in the first place. Another factor is the url reliability during operation due to the brittleness in the very highly stressed root part of the turbine

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shovel as a result of their work at medium Temperatures, i.e. at 6 * 1-8 - 8l6 ° C. Such middle ones Temperature conditions exist because the hotter ones Wing parts at temperatures in the range of 871 - 1093 ° have to work"

The uptake of sulfur is a form of high temperature corrosion, which occurs when the combustion gases contain products resulting from the reaction arise from sulfur contained in the fuel with sucked sea salt. These products that one generally responds as sulfates and those in the combustion process formed, touching the turbine components and destroying the protective oxides, so that the Schviefel can penetrate the material. There is evidence of such intrusion the formation of dark protruding bubbles on the wing parts. The bubbles reduce the cross-section thickness the blade or wing sub-body on which they are formed. In addition, this causes penetration von Schviefel a fast and destructive reduction of the effective cross-sectional area of alloy parts,

their original stress-rupture properties (stress-rupture properties). in the

Case of absorption of sulfur

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the components tend to operate prematurely and during operation unpredictable to break. This leads to increased maintenance problems, because in shorter time intervals, than is intended for the normal overhaul service, inspections are due to ensure that the blades are still in working order and have not yet been weakened so that the safety is seriously endangered.

A second factor which, during operation, reduces the initially adequate stress-rupture properties of some of the commercially available nickel-based alloys is the instability of the alloy at elevated temperatures. The instability of an alloy material results in the formation of intermetallic compounds which is the prädominierende known as "sigma phase", of about 816 forms at temperatures up to about 1038 ⁰ C (1500-1900 ⁰ P) plattchenartig. This "sigma phase" contains elements such as tungsten, molybdenum and chromium and, when it is formed, deprives these elements of their normal role as elements strengthening the solid solution. The "sigma phase" itself does not strengthen the alloy as a result of its platelike formation.

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A third factor which is a serious disadvantage with some of the commercially available nickel alloys is brittleness and notch sensitivity, as these phenomena contribute to unpredictable breakage and unreliability in operation. Most superalloy materials have adequate fracture toughness at the high temperature, ie at 871 to 109 ^ ⁰ C (1600 to 2000 ⁰ F) and are not notch sensitive. In contrast, all of the alloys show at moderate temperatures in the range of 6 ^ -9 to 816 ⁰ C (1200-1500 ⁰ F) based on nickel, a reduction of the ductility (ductility trough). Too great a loss of fracture toughness results in notch sensitivity. Higher operating temperatures, such as those used in the combustion chambers of, for example, advanced or advanced turbine engines, have the result that the turbine disk and the root parts of the turbine blades connected to it come to an equilibrium temperature in the middle range. The blade root parts made from alloys that have low ductility and are notch sensitive are highly susceptible to bin formation and subsequent breakage under the stresses developed during high speed rotation,

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Attempts have been made to reduce the absorption of sulfur by changing the composition of the Turbine blade surfaces by deposition of and diffusing in a protective layer on BEZW. to prevent into the surface of a material, e.g. by brazing a resistant sheet onto the material or by pouring aaee on Protective layers.

These materials, the surfaces of which had been modified to provide a protective layer have, but have not found application everywhere, since a diffusion layer is generally under the operating conditions at high temperature diffuses further and therefore not as a protective layer with a constant Composition remains.

The alloys according to the invention can be used for Manufacture of components such as turbine blades are used that do not have a protective layer on their Surfaces require but are resistant to the absorption of sulfur and have a high strength and resistance at high temperatures as well as ductility at medium temperatures.

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A nickel alloy according to the invention consists of 3A - 18 % chromium I 2 -4.5 % tungsten I 5 - 20 $ cobalt; 0.5 - 3 $> molybdenum} 0.5 - 2.5 % niobium and / or 1 - 4 % tantalum, the percentage of tantalum plus twice the percentage of niobium being less than 5 % , 1 - 3 % titanium; 2.5-4.75 % aluminum; 0.01-0.2 % zirconium; 0.005-0.05% boron and 0.03-0.2% carbon; the rest consists of nickel and impurities, with the nickel content aging 50-75 % . The elements chromium, tungsten, molybdenum, niobium and / or tantalum, titanium, aluminum and nickel are present in such percentage amounts that the hardening factor of the following equation

lx% Cr + 1, IxJi "W + l, 8x # Mo + 3, MrX% Nb

Ta + · 4-, 3x # Ti + 6x% Al = 55 to 67

and the stability factor of the following equation

0.66. Ni matrix + 1.70 Co matrix + 4.66 Cr matrix + 5.66 (V / matrix + Mo matrix) = 2.50 or less

is equivalent to.

The alloys of this invention consist in essentially of a solid solution based on nickel, as a base, the refractory C.a'bide and a precipitated intermetallic compound of "gamma prime"

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(Ni-Al, Ti) contains, with specific weight ratios are required between the amounts of chromium, tungsten, molybdenum, titanium and aluminum if stability against the absorption of sulfur with simultaneous · formation of high temperature strength stay and the unstabilizing effect of the proportions of elements should be avoided, which the training favor the "sigma phase" during operation at high temperature.

Chromium is an essential element of this alloy, but required in very specific proportions. Stability against the uptake of Schllief el requires the presence of at least 1Λ % chromium »Higher amounts of chromium improve the stability against the uptake of sulfur. Above a content of 16 % , however, the disadvantageous effectiveness of chromium on strength begins to appear and above 18 % the reduction in strength is so great that the alloy can no longer be used for high-strength components. Chromium is therefore used in the range of Ik - 18 % _t, preferably in the range of 15 - 16.5 % .

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If the quantities of the specified alloy components can be varied within the specified ranges, the chromium being limited as specified above, the high temperature strength is retained when the alloy equations correspond to the equation given above for the ^ hardening factor:

lx% Gr + 1, IxJi W + l, 8x # Mo + 3, .4x # Nb + l, 7x #

Ta + ^, 3χ% Ti + 6x% Al = 55 to 67, preferably 58 to 6k,

wherein the wt -.% to the respective alloy components of the χ in place in the equation used.

High temperature resistance is only one of the required properties of the alloy. Short-term stress-rupture requirements can be met without any guarantee of the stability of the alloy. In fact, it has been found that only some alloys which have a hardening factor between 55 and 67 are stable, and that when the proportions of the alloy constituents are changed within the specified ranges, the stability is only maintained if the alloy equations are correct above equation for the stability factor

0.66 NL HabrLx + 1.70 Co matrix + k, 66 Cr matrix + 5t66 (W matrix + Mo matrix) = 2.50 or less

suffice.

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A preferred range for the stability factor is between 2 and 2.5.

This third parameter is based on a determination of the atomic percent of the elements present in the matrix. The atomic percentages of the basic mass are obtained by the following series of calculations. First, the composition of the alloy is converted from percent by weight to percent by atom. To do this, the weight percent of each element is divided by its atomic weight and the quotients are added. The quotient for each element is then divided by the sum of all quotients and the resulting number, multiplied by 100, gives the atomic percentage for the respective element in the gross composition. To calculate the individual atomic proportions in the basic mass, the amount of nickel that is consumed by the reaction with aluminum and titanium must first be determined based on the formula Ni ^ (Al, Ti). ¹ This number is three times the sum of the atomic percentages of aluminum and titanium and if this number is subtracted from the atomic percentages of nickel in the gross composition, the remaining atomic percentages of nickel are determined. The abomaren proportions of the matrix are determined by adding the remaining atomic percent nickel, the

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The atomic percent of tungsten and the atomic percent of molybdenum added and the atomic percentages for the various Elements are divided by this sum. The atomic fractions of the basic mass if they are for the respective χ are used in the above formula, give a measure of the stability of the alloy. Alloys with a stability factor greater than 2.50

the
form "sigma phase", which ver -.-

wrestles. -

Tantalum and niobium serve to stabilize the alloy the carbon content and are therefore predominant present as garbide; about f-three available Tantalum or niobium is so insignificant that it does not need to be taken into account in the above calculation «,

One method of determining stability is to take a sample of the alloy at 897 ° C Ages for 288 h, that is a period of time and a temperature at which the "sigma phase" forms if the composition is not stable. The sample is then cooled, divided, polished, etched and examined under the microscope for platelet-like components. If the structure is free of If this is the case, the alloy is considered to be stable.

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With regard to the stability factor, especially when operating within a narrow range of properties, control of the composition can be quite critical. A change of 1 wt -.% In the amount present of each of the following elements causes a change in the stability factor by a specified amount below :.

Tungsten around 0.04, molybdenum around 0.052, chromium around 0.06. The changes for titanium and aluminum are not linear and therefore do not have such a formal relationship; however, a 3 - k% change in aluminum causes a change in the stability factor of 0.125, and a change of Z - 3 % in titanium changes the factor by about 0.07.

In the production of the nickel alloys according to the invention, the amounts of certain elements are of exceptional importance. Tungsten is one of these important elements and should only be present in an amount of 2 to 4.5 % . A larger amount of tungsten may be advantageous from the point of view of hardening, but it causes a drastic reduction in strength and ductility in the medium temperature range,

Molybdenum is an essential Elenoni; the CeIbat with an "i-Jol fornmgehnlt from 2 - _} ·. ;; i wiir Ir

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the alloy, if no molybdenum were contained, tend to have a low fracture toughness in the medium .Temperature range. Molybdenum cannot therefore be replaced by tungsten as an equivalent. Alloys that contain molybdenum but no tungsten have good ductility at medium temperatures, but do not have as good high temperature strength as the alloys containing tungsten. Molybdenum is used in the alloys of this invention in an amount 0.3-3 % , the proportion of molybdenum being preferably no greater and generally less than the proportion of tungsten. In general, it is preferred that the Ge. The weight ratio of tungsten to molybdenum is in the range from 1.5 ι 1 to 3: 1, - The importance of molybdenum for the alloy is illustrated by a comparison of alloys which are essentially identical with the exception that one is not molybdenum but the other one contained 1.75% molybdenum. The molybdenum-free alloy showed after a 20-hour heat treatment at 87O ⁰ C (1600 ° P) a fatigue strength of 20 h at a temperature of 7 ^ 0 ⁰ G and a load of 60 kg / mm, while the alloy was 1.15% Molybdenum had a creep strength of 72 hours under the same conditions. This is an indication ο

° for the drastic reduction of the tension <jo

^ Fractional genchafton at medium temperatures at a ^. Reduction in the molybdenum content of the alloy.

* "Cobalt solder is also an essential element

der erf Lu * in / ^ ffeirir> fien alloys, a-in lack of Kq. BAD ORJGINAU

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bait has an adverse effect on breaking strength at elevated temperatures. For example, a comparison of sample alloys that are identical to one another with the exception of a cobalt content of 10% shows that the cobalt-free alloy at -60 ° C and 60 kg / mm load after a 20-hour heat treatment at about 870 ⁰ G (160 ⁰ F ) a ^ fatigue strength of 16 hours, the alloy with cobalt, however, had a fatigue strength of 53 hours. In a similar way, at 982 ° C and 1 ^, 1 kg / mm load, the cobalt-free alloy had a fatigue strength of only 39 h, compared to the? Fatigue strength of 9kh for the alloy with 10 % cobalt,

Aluminum serves a twofold role in the alloy. It contributes to the resistance to oxidation. and acts to form an intermetallic compound with nickel, which is an excellent hardener and strengthening the alloy.

Titanium is also an important element in the Establishing the strength of the alloy. Small amounts of titanium in interaction with aluminum are advantageous in the alloy due to a

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improved precipitating effect. However, amounts above 3 % are disadvantageous because alloys with a higher titanium content tend to crack. Preferably titanium is used in an amount of 1.5 to 2.5 % .

Niobium and tantalum can be used alternately, provided that the content of tantalum plus twice the content of niobium does not exceed a total of 5% based on the total alloy. Niobium and tantalum serve to stabilize the carbon with the formation of carbides, which serve to increase the strength at the grain boundaries. Niobium is preferred over tantalum in terms of density and price, while tantalum is advantageous in terms of oxidation resistance. -

Small additions of boron and zirconium are two-fold, as they have both the breaking strength as well as improve ductility. If the If the boron content of the alloy exceeds the limit given above, the alloy may be unsatisfactory, in particular in its fields of application,

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on which demands on the use Temperature changes are important,

The alloys according to the invention preferably contain the following proportions of the constituents: 15 Ms' 16.5 % Gr, 3 to k% VJo, 8 to 12 % Go, 1 to 2 % Hb and / or 2 to k% Ta, 3.5 to 4 , 75 % Al, 1.5 to 2.5 % Ti, 0.03 to 0.10 % Zr, 0.01 to 0.03% B, 0.1 to 0.17 % G, remainder nickel and impurities, the nickel content being 55 "to $ 69.

The testing of the alloys with regard to the fatigue strength at medium temperatures, e.g. at 76O ⁰ G (1 ^ 0O ⁰ F) and at high temperatures, e.g. at 982 ° C (180O ⁰ P) is carried out with the help of the usual stress-rupture test,

The stability of the alloys against the absorption of sulfur can be determined by immersing a weighed sample halfway in a melt in a crucible which contains a mixture of 9.5 g sodium sulfate and 0.5 g sodium chloride crucible is held in an oven at 927 ⁰ C for 2 h h, heated! laughing heating was dissolved in boiling water and the salts dio Le ^ ierungnproben by sandblasting, 'Vü'F-oiniftl. and again, "iowogon. En will dor

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Weight loss per unit of surface area

■ calculated and the attack is judged as severe if

2
the loss of 30 mg / cm exceeds, alloys, "the π a weight loss of less al 10 ing / cm. aufueisen, are well resistant to the absorption of sulfur. An inventive alloy with 1%%

2 Chromium showed a weight loss of about 30 mg / cm, while an alloy with 18 % chromium, identical in all other respects, showed a weight loss of between 1 hour and 5 mg / cm. A 12 % chrome alloy of a similar composition, on the other hand, showed a weight loss of around 200 mg / cm *

The invention is illustrated by the following examples, explained in more detail.

example 1

It vmrdeyi a 6.9 kg batch of a nickel alloy containing 15.5% chromium, 3.5% tungsten. 1.75 % molybdenum, IG % cobalt, 1.75 ^ niobium, 1.75 %. Titanium ,, % -, $ 25 aluminum, 0.05 % zircon, 0.015 % boron; 0.15 ^ carbon, ftest Hickel melted, first indent a Glirom-Niekel · Cobalt-Carbon Geminch melted under high vacuum and; then the tungsten. Molybdenum ^ niobium, aluminum ,, titanium, boron mvl zirconium zügen et ζ b was. \ - "

ο iitoBifHOzenbuale composition of the Legie nil xtie followsi. _r ^ -.V

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17.0 fo Cr, 1.12 % W, 1.04 % Mo, 9.70 % Go, 1.07 % Nb, 2.09 % Ti, 8.95 * # Al, 0.03 % Zr, 0.08 % B, 0.72 % C, 58.5 % Ni.

The molten alloy was turned into with the help a common lost machined casting process a cluster of 16 test bars was produced under high vacuum conditions. These test rods were each ■ 7.6 inches long and now 6.25 in diameter. The hardening factor of the alloy was 61.3 and its stability factor was 2.28.

The test bars had an elongation of 2.5 % with a creep strength of 75.5 h under a load of 60 kg / mm at 76O ⁰ C, an elongation of 5 % with a creep strength of 126.8 h under a load of 31, 7 kg / mm at a temperature of 871 ° C and an elongation of 1 ^ -% having a creep strength of 91.7 h under a BeIastung of lk _t 1 kg / mm at 982 C. The weight loss after 2 * 1 h at 926 ⁰ C (170 ⁰ P) of a sample which was half immersed in a melt of 95 # nabrium sulfate plus 5 ⁰ A sodium chloride was 5 mg / cm.

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The alloy was after aging for 288 hours at 899 ⁰ C (1650 ⁰ F) was examined under the microscope and was free of "sigma phase",

• Example 2 . .

A 6.9 kg batch of a nickel alloy containing 16 % Gr, Ur% W, 2 % Mo, 10 % Go, 2 % Nb, 2 $ Ti, 4.5 % - "Al-, 0.05 % Zv ₁ 0.015 i B, 0.15 -. $ Ο ', remainder Ni was melted by melting a chromium-nickel cobalt-carbon mixture together under high vacuum and then adding the tungsten, molybdenum, niobium, aluminum, titanium, zirconium and boron.

From the molten alloy XTOrde with the help Usual wax from sclimel pouring process under high vacuum conditions a cluster of 16 test rods manufactured. These rods were each 7.6 cm long and 6.25 mm in diameter.

The alloy had a hardening factor of 66, k and a stability factor of 2.45 -

The test bars showed an elongation of 4.5 % with a fatigue strength of 5715 h under a load '

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of 60 kg / mm at 76o C, an elongation of 6.0% and a creep strength of 92 h under a load of 31.7 kg / mm "at 871 ⁰ C and an elongation of 14% at a creep strength of 55 _f ^ 'h under a load of 1.1 kg / mm at 982 ° C. The weight loss was according to that described

e
h at 926 ⁰ C.

2 test for the absorption of sulfur 5 mg / cm

Microscopic examinations carried out as described showed that the alloy after a Aging at 8.99 ° C for 288 h free of "sigma phase" was. ·

Example 3

A 6.9 kg batch of a nickel alloy containing 15.5 % Cr, k-% W, 2 % Mo, 10 % Co, k-% ..Ta, .- 2z $ Ti, h -, 5% Al, 0.05 % Zr, 0.015 % B.,. 0.15 % C, S '© ^ .t. Ni was obtained by melting together a chromium-nickel ', ·,. Cobalt-carbon mixture melted under high vacuum and subsequent addition of tungsten, molybdenum, niobium, aluminum, titanium, boron and zirconium.

A cluster of 16 test bars was made from the molten alloy using a customary

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under-

tiare the rods. each long and natten one; Du-penmessep v @ ni _t g $ ira

The alloy had a heat rating QP of and a Sta "bil4tafesfalft <c> P" rom 2,48ν.

The test staff -wies.exi a Detaimg a Dauerstanäf estiglseifc wow, kh Ji elektrcex * ei.aei? I load iron 6ö kg / nim ^a at 760 ⁰ G ₁₅ a stretching "? Sn 11 ^ with a * permanent stiffness wn, S ^ -, 3 h sub-load of 1% l {: g / min"^; at 982 'ß _; on, Der öewients «· ¥ ei * lust after 2k h at S24 ^Q Q generaliB; elgm described about ^ tie recording van 'Senwefel fraud & - rog / cin ■ «·

The? Rie already possessed "piscne tote ^ suohung: shows that these buckets are aging at 899 ⁰ C wäteen" 288 Ii free viai *

Example; 4 - ... '; ■ - ^: ■' ■ "

line &, 3 Ug dteiFge ein ^ ifiekelle ^ iinang e 13 * 5 t Φ? * Ii % ^: tf, - a Jl iföi If M Qa * a 11 »4 a _r öi5i S> t * || # ß * Best Ni was

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addition; of tungsten, molybdenum, aluminum, titanium, boron and zirconium melted.

A cluster of 16 BrUf rods was made from the melted alloy with the help of a common Waohsaussphmel casting process under high vacuum conditions a manufactured;. These test bars were each 7.6 cm long and each was a diameter of 6 »25 rome«

The government had a liability factor of 66.9 and a stability factor van B, 60

The bridge rods had an elongation of 2 Ji with a structural strength of 28 _t 3 hours under a load of 6Θ kg / mm at? 6C3rS, an elongation of 2.5 Jf »it? a Daueretandfestigkeit of 12 h under a load of 34 ₁ 7 kg / mm at a door of Tewpe 871 ⁰ CJ and an elongation of 3.5 ^ with a Dauerstandfesi? igke4t of 67 "9 h under Elmar -ik load of ₁ kg X / mm at 982 ⁰ G, the loss of weight was n & ah 2 hours at 926 ^Q 0 according to the described eueh on the uptake of sulfur 2QQ mg / am

The microscopy marriage as described

that nasty lifgierung near

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Aging at 899 ⁰ C 'during 288 h a moderate proportion of "sigma phase" containing. "

Example 5 '

A 6.9 leg batch of a nickel alloy was produced. holding 16 % Cr, 6 $ W ₁ 2.5 % Mo, 10 % Co, 2 Ji Nb, 2 $ Ti, - * fc, 5 $ "&, 0.05 ^ '" Zr, 0.015 ^ B, 0, 15 ^ C, remainder Ni was melted by melting a chromium-nickel cobalt-carbon mixture under high vacuum conditions and then adding tungsten, molybdenum, niobium, aluminum, titanium, boron and zirconium.

• A cluster of 16 test rods was produced from the molten alloy using a common Manufactured under high vacuum conditions by the lost wax casting process. These test bars were each 7.6 era long and each had a diameter of 6.25 now. ·.

The alloy had a hardening factor of 69.5 and a stability factor of 2.62. -

The test bars had an elongation of 2.5% and a creep resistance of 1 ?, J hunter a

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Load of 6o kg / mm ^at ? ⁶ ° ^G » ^an elongation of 3 % with a fatigue strength of

2 18.3 h under a load of 31 »7 kg / mm. at a temperature of 871 G and an elongation of 5 % with a creep strength of 37.6 h under a load of 14.1 kg / mm at 982 ° G. Loss of weight was after 2 h at 926 ⁰ G according to the test on the incorporation of sulfur 20 mg / cm.

The as described carried out microscopic studies revealed that this alloy after aging at 899 ⁰ C for 288 h a moderate proportion of "sigma phase" contained.

A comparison of Examples 1, 2 and 3 (according to the invention) with Examples h and 5- (comparative alloys with e.g. to achieve the desired stability against the uptake of sulfur as well as the desired stability.

BAD0RI6INAL

Claims

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Claims (5)

P a t e η b a η s ρ r ü c h e 1. Heat-resistant nickel alloy, containing chromium, tungsten, cobalt, molybdenum, niobium and / or tantalum, titanium, aluminum, zirconium, boron and carbon, the remainder nickel and impurities, geke η η ζ calibrated by a content of 50 "- 75 % Ni, I ^ - 18 % Cr, Λ 2 - k, 5% W, 5 - 20 % Co, 0.5 - 3 % Mo, 0.5 r 2.5 % Nb, 1 - 4 % Ta, ' where Ta + 2 Nb "K 5%, and 1-3 '%. Ti, 2.5 - ' ¹ V ₁ 15% Al, 0.01 - 0.2 % Zr, 0.005 - 0.05 % B and 0.03 - 0.2 % C, the proportions of the alloy constituents of the equation of Hardening factor lx $ Cr + 1, Ix ^ W + 1 13y $ Mo + 3, kxfo -Nb + l, 7xj £ Ta + 4,3x ^ Ti + 6x ^ Al = 55 to 61 and the equation of the stability factor 0.66 Ni matrix + 1.70 Co matrix + k _t 66 Cr matrix + 5.6 (W matrix + Mo matrix) = 2.50 or less. ,> 2. Nickel alloy: according to claim 1, geke η η ζ eich η et, by a content of 55 - 69 fo Ni, 15 - 16, -5 % ..Cr -, - 3 - ^ ^ W, 8 - 12 % Co , 0.5 - 3 ^ Mo, 1 - 2 ^ Nb and / or 2 - ^ ^ Ta, 1.5 - 2.5 % Ti, 3.5 - ^ι ϊ,? 5% Al, 0.03 - 0 , 1 ^ Zr, 0.01-0.03 % B and -0.1-0.17 % C. BATH 009883/0564 • 16082 <t2 3. Nickel alloy according to claim 1 or 2, ge ~ characterized by a hardening factor of 58 Mr Sk. k. Nickel alloy according to claims 1 to 3 »characterized by a stability factor of 2.1 to 2.45. 5. Nickel alloy according to claim 1 to 4, characterized by a weight ratio from tungsten to molybdenum in the range from 1.5: 1 to 3: 1 ' 009883/0564