WO1995027803A1

WO1995027803A1 - Modified nickel-chromium-iron-aluminium alloy

Info

Publication number: WO1995027803A1
Application number: PCT/US1995/003135
Authority: WO
Inventors: Bertie Forrest Hall, Jr.; Stanley James Valykeo
Original assignee: Hoskins Manufacturing Company
Priority date: 1994-04-08
Filing date: 1995-03-13
Publication date: 1995-10-19
Also published as: AU2158895A

Abstract

An oxidation-resistant alloy for use in a high-temperature or thermal cycling environment. The alloy comprises a nickel-based matrix having a solid solution of 19-23 % chromium and 3-6 % aluminum. A self-healing, thermodynamically stable oxide layer is formed upon a surface of the alloy which is exposed to an oxidizing atmosphere over a range of temperatures for extended periods of time. The oxide layer protects the alloy from the oxidizing atmosphere. Additions of calcium and yttrium are made to the matrix to substantially remove or stabilize oxygen and sulfur dissolved in the molten alloy. These additions result in retention of about 0.01-0.04 % of calcium and 0.01-0.04 % yttrium in the cast alloy. The matrix further includes about 2-8 % iron to inhibit nucleation and growth of a 'gamma prime' nickel aluminium intermetallic compound which would otherwise adversely harden the alloy and cause local disturbance of a uniform distribution of aluminium.

Description

MODIFIED NICKEL-CHROMIUM-IRON- ALUMINUM ALLOY

Technical Field

The present invention relates to a nickel- chromium-iron-aluminum alloy and, in particular, to such an alloy which is modified with yttrium and calcium.

Background Art

Nickel-chromium-iron alloys are used primarily for their oxidation resistance and strength at elevated temperatures. Such alloys may be used, for example, as sheathing for electric heating elements.

Electrical resistance alloys are found in instruments and control equipment to measure and regulate electrical characteristics and in furnaces and appliances to generate heat. In the latter applications, elevated temperature characteristics are of prime importance. In common commercial terminology, electrical resistance alloys used for generation of heat are referred to as resistance heating alloys.

Resistance heating alloys such as CHROMEL-A^®

(80 Ni; 20 Cr) are used in many varied applications -- from small household appliances to large industrial furnaces. In appliances, resistance heating elements are designed for intermittent, short-term service at about 100 to 1090° C (200 to 2000° F) . In industrial furnaces, elements must often operate continuously at temperatures as high as 1300° C (2350° F) ; as high as 1700° C (3100° F) for kilns used for firing ceramics; and occasionally as high as 2000° C (3600° F) for special applications.

The primary requirements of materials used for heating elements are high melting point, high electrical resistivity, reproducible temperature coefficient of resistance, good oxidation resistance in furnace environments, absence of volatile components, and resistance to contamination. Other desirable properties are elevated temperature creep strength, high emissivity, low thermal expansion and lower modulus (both of which help minimize thermal fatigue) , resistance to thermal shock, and strength and ductility at fabrication temperatures.

It is known that nickel-chromium-iron compositions are ductile alloys and thus are workable. They play an important role in heaters for the higher temperature ranges. Such heaters are constructed to provide more effective mechanical support for the heating element. However, many such alloys are readily oxidized and are restricted to use in non-oxidizing environments.

Oxidation resistance of nickel-chromium alloys at elevated temperatures is weakened by the limited adherence of an initial protective oxide layer to the base metal. The ability of an alloy to survive long exposures of 2000° F and above in air depends in large part on this protective layer remaining intact. Alloys presently used for electrical heating elements are made with the best techniques of melting, refinement and fabrication to maximize performance. However, variations in chemistry that occur between melts lead to differences in performance, as measured, for example, by accelerated life tests.

Various theories have been suggested to explain why some melts may show substantial improvement over the average performance. However, there has not been positive confirmation of any theory that has led to predictable improvement in life test results and consistently reproducible performance.

U.S. Patent No. 2,515,185 relates to nickel alloys, and more particularly to age hardenable nickel alloys. Such alloys, however, do not exhibit the requisite ductility. Nor does this patent envision the use of elements which promote oxidation resistance of the nickel alloys.

U.S. Patent No. 4,460,542, which is incorporated here by reference, calls for the addition of yttrium to a chromium-aluminum-iron alloy which exhibits resistance to oxidation at high temperatures. However, this alloy only requires 14-18% chromium. U.S. Patent No. 4,671,931 abandons the concept of yttrium additions.

Disclosure Of The Invention

It is accordingly an object of the present invention to provide a modified nickel-chromium-iron- aluminum alloy having superior resistance to oxidation at elevated temperatures.

It is additionally an object of the present invention to provide such an alloy which is characterized also by its workability. The invention relates to an oxidation- resistant alloy for use in a high temperature or a thermal cycling environment. The alloy has a nickel- based matrix including 19-23% chromium and 3-6% aluminum. If a nickel-aluminum intermetallic compound, such as Ni₃Al were precipitated (an ordered FCC compound referred to as "gamma prime"), the alloy would become strengthened (i.e. workability is lessened), a property which is antithetical to ductility. To inhibit nucleation and growth of such precipitates, about 2-8% of iron is included in the nickel-based alloy.

To remove or stabilize oxygen and sulfur dissolved in the alloy, additions of calcium and yttrium are made to the melt so that about 0.01-0.04% of calcium and 0.01-0.04% of yttrium are present in the cast metal ingots. They permit the aluminum in the fabricated alloy to combine with oxygen in the environment to form a means for healing a thermodynamically stable oxide scale if the layer is damaged or spalls during thermal cycling.

Brief Description Of The Drawings

FIGURE 1 depicts the results of an accelerated life test of several alloys falling within the scope of the disclosed invention, wherein resistance change is plotted as a function of cycling time.

Best Mode(s) For Carrying Out The Invention

The alloy of the present invention is an oxidation-resistant alloy for use in a high temperature, or thermal cycling environment. The alloy has a nickel- based matrix, including, by weight, about 19-23% chromium and about 3-6% aluminum. The alloy also includes about 0.01-0.04% calcium and about 0.01-0.04% yttrium for stabilizing oxygen or sulfur dissolved in the alloy, while retaining calcium and yttrium in solid solution. As a result, the aluminum in the alloy may combine with oxygen in the environment to form a self- healing means for repairing a thermodynamically stable oxide layer if it becomes damaged or spalls in use.

Also present is about 2-8% iron for inhibiting nucleation and growth of nickel aluminum intermetallic compounds which would otherwise adversely harden the alloy and cause local disturbance of a uniform distribution of aluminum.

There is thus formed a self-healing, thermodynamically stable oxide layer disposed upon a surface of the alloy which is exposed to an oxidizing atmosphere over a range of temperatures for extended periods of time. The stable oxide layer protects the alloy from the oxidizing atmosphere.

The preferred amount of chromium is about 20-21% and that of iron is about 2.5-4.5%. Correspondingly, the preferred amount of aluminum is about 4-5%, while that of calcium is about 0.02-0.03%. The preferred amount of yttrium is about 0.02-0.04%. The balance of the alloy is nickel.

In the disclosed amounts, the presence of iron has been found to enhance workability of the alloy without compromising its oxidation resistance. Additionally, iron tends to reduce the formation of the gamma prime precipitate which would otherwise impair workability.

Aluminum is added for oxidation resistance. Its favorable resistance to oxidation renders the alloy suitable for use in ceramic kilns and heat treating furnaces.

To illustrate various aspects of the present invention, the following examples are disclosed.

Seven alloys were melted. Their chemistry, aside from impurities, is summarized in Table 1:

TABLE I

NOMINAL STARTING CHEMICAL COMPOSITION¹

(Weight Percent)

Allov I.D. Ni Cr Fe Al Y Ca

62 - 1 Bal. 19.71 0.22 3.78 NA² 0.0132

62 - 2 Bal. 19.55 2.27 3.73 NA 0.0109

62 - 3 Bal. 19.33 4.28 3.67 NA 0.0109

64 - 1 Bal . 20.50 0.15 4.10 NA 0.012

64 - 2 Bal. 20.23 2.30 4.00 0.012 0.0063

66 - 1 Bal. 19.90 6.00 4.20 None 0.0160

Detected

66 - 2 Bal 19.40 7.80 4.10 0.040 0.0060

Compositions are of the solid alloy.

"Not added. " Oxidation tests were conducted at 2200°F for 80 hours and at 2300°F for 195 hours to compare oxidation resistance of the seven alloys.

The experimental procedure involved placing samples of the alloys in an electrically heated box furnace and exposing them to air flow created by convective thermal currents. The samples were cycled daily. After cooling to room temperature during each cycle, the specimens were examined. The results appear in Table II:

TABLE II TYPICAL ELEVATED TEMPERATURE PROPERTIES

Oxidation

Wt. Change

Allov I.D. (Gram/Cm²) ASTM Life (Hours)

62 - 1 - 0.00156 120

62 - 2 - 0.00153 212

62 - 3 - 0.001918 96

64 - 1 0.000860 224

64 - 2 0.000437 800

66 - 1 0.000112 444

66 - 2 0.000226 582

In Table II, a specific weight loss is caused by spallation of the oxide scale during cycling. However, Table II also shows that following an effective reduction in the sulfur content by combination with calcium and/or yttrium, the alloy may exhibit a slight weight gain through the formation of a protective scale of alumina. Desulfurized specimens tend to show a positive specific weight change throughout the duration of the life test.

The right-hand column of Table II follows the procedure set forth in ASTM designation B76-90 standard test method (2 min. on/2 min. off) at 2150°F for determining the accelerated life of nickel-chromium and nickel-chromium-iron alloys used for electrical heating. That publication is incorporated herein by reference.

Results obtained after 80 hours at 2200°F and 195 hours at 2300°F (31 cycles to room temperature) indicate that alloys such as 66-1 exhibited preferable performance because they incurred the lowest weight gain

(0.000112 gm/cm²) . However, alloy 64-2 yielded the longest life (800 hours) .

Although lifetime improvement over conventional alloys is not susceptible of easy quantification, it is likely that lifetime is increased by at least several fold.

Turning now to Figure 1, there is depicted the results of an accelerated life test wherein resistance change (percent) is plotted against hours of cycling.

The 64-2 alloy, as noted above, lasted almost 800 hours.

Table III summarizes the disclosed and preferred ranges of alloy composition, aside from impurities: TABLE III

ALLOY COMPOSITION RANGES³

(WEIGHT PERCENT)

Disclosed Preferred Ni Balance Balance

Cr 19 - 23 20 - 21

Fe 2 - 8 2.5 - 4.5

Al 3 - 6 4 - 5

Ca .01 - .05 .02 - .03 Y .01 - .04 .02 - .04

Without wishing to be bound by any particular theory, the inventors have observed that the advanced alloys and coatings disclosed rely on the formation and adherence of a thin and continuous aluminum oxide film to protect the base alloy from further oxidation attack at elevated temperatures. In order for the alumina scale to serve its protective function, it must remain adherent to the underlying alloy under prolonged exposure and thermal cycling conditions. It is known that segregation of indigenous sulfur to the metal-oxide interface induces premature scale spallation. This may occur through a reduction in the interfacial adhesion strength, with a resulting reduction in component lifetime.

The inherent reactivity of yttrium requires an exceptionally high degree of control over alloy chemistry during melting/casting. Control of the concentration of the reactive element additions is

Compositions are of the solid alloy. particularly important, since retention of a minimum amount in solution in cast alloys is required to impart acceptable oxidation resistance. However, when the concentration of the reactive element greatly exceeds that of the impurities with which it reacts, the formation of extraneous, low melting point phases can result. If the proportion of the reactive element is too high or too low, the oxidation characteristics of the alloy may be suboptimal .

The inventors have discovered that by the addition of aluminum to the nickel-chromium base alloy, oxidation resistance is enhanced by the formation of an impervious layer of aluminum oxide. To ensure that the surface oxide layer remains intact with a "self-healing" mechanism if the oxide is damaged or spalls, it is necessary to have aluminum dissolved uniformly in the alloy matrix to a level of about 4% by weight. At this level, it is thought that diffusion of the aluminum atoms in the matrix can occur quickly to replace aluminum depletion by alumina formation at the surface.

The limited solubility of aluminum in the nickel- chromium alloy can result in precipitation of some of the aluminum in the form of a nickel-aluminum phase referred to as "gamma prime." As noted earlier, these particles can cause severe hardening in the alloy and a reduction in the aluminum in solution in the surrounding matrix.

To modulate this mechanism, iron and other metals may be added. To enhance the adherence of the surface oxide to the base metal, small additions of calcium and yttrium with other rare earth metals have been found to be effective. The results of alloying and testing show that reproducible results can be assured if the following chemistry controls are exercised:

TABLE IV DETAILED COMPOSITION RANGES (WEIGHT PERCENT)

Chromium 19 - 23

Aluminum 3 - 6

Iron 2 - 8

Zirconium 0 - 0.10 Calcium 0.01 - 0.05

Sulfur 0 - 0.008

Yttrium 0.01 - 0.04

Boron 0 - 0.005

Carbon 0 - 0.03 Silicon 0 - 2.0

Manganese 0 - 0.5

Titanium 0 - 0.25

Nickel Balance

Since yttrium is highly reactive and expensive, it is added last to the melt after calcium and other reactive additions have substantially reduced the oxygen and sulfur content of the molten metal.

The inventors have developed the disclosed alloy so that it may serve as a tube, wire, or strip for use as a heating element or as a tubular member in applications which are exposed to an oxidizing atmosphere at high temperatures.

The disclosed alloys provide for favorable oxidation resistance at the highest temperatures of intended use without spalling of the surface oxide.

Yttrium, calcium, and zirconium in the proper relative amounts effectively reduce the oxygen and sulfur content of the resulting alloy. As a result, these highly reactive additions are present uniformly in the matrix of the alloy. This ensures initial reaction of aluminum in the matrix at the hot surface with the ambient air/atmosphere and provides a base for bonding subsequent layers of aluminum oxide. The control of sulfur in the alloy by calcium and yttrium tends to neutralize the potential of this undesirable impurity to interfere with oxide layer formation.

The chemistry of the alloys disclosed requires refining the melt to neutralize the sulfur and oxygen contained in the alloy. Hot fabricability is promoted through the addition of boron and zirconium in the disclosed amounts and sequence during melting. The precipitation of "gamma prime" is retarded by increasing the solid solubility of aluminum in the nickel-chromium base alloy. As a result, cold working processes are facilitated.

Further, the disclosed alloys promote formation of a continuous protective layer of alumina. Thus, adherence of the oxide layer to the base alloy is ensured during thermal cycling, thereby promoting longer life at higher temperatures over comparable alloys which are presently available.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein, in connection with specific examples thereof, will support various other modifications and applications of the same. It is accordingly desired that, in construing the breadth of the appended claims, they will not be limited to the specific examples of the invention described herein.

Claims

What Is Claimed Is:

1. An oxidation-resistant alloy comprising, in weight percent : a sheath for use in a high temperature environment, the sheath including a nickel-based matrix, the matrix including about 19-23 chromium and about 3-6 aluminum; a self-healing thermodynamically stable oxide layer formed upon a surface of the matrix, the surface being exposed to an oxidizing environment over a range of temperatures for extended periods of time, the layer protecting the sheath from the oxidizing environment; about 0.01-0.04 calcium and 0.01-0.04 yttrium for removing or stabilizing oxygen and sulphur dissolved in the matrix, said calcium and yttrium permitting the aluminum in the matrix to combine with oxygen in the environment to supplement the oxide layer and to heal the surface if the oxide layer is damaged or spalls; and about 2-8 iron for inhibiting nucleation and growth of a nickel aluminum intermetallic compound that would adversely harden the sheath and cause local disturbance of a uniform distribution of the aluminum.

2. The alloy of claim 1, wherein the matrix comprises 20-21% chromium.

3. The alloy of claim 1, wherein the matrix comprises 4-5% aluminum.

4. The alloy of claim 1, wherein the matrix comprises 0.02-0.03% calcium.

5. The alloy of claim 1, wherein the matrix comprises 0.02-0.04% yttrium.

6. The alloy of claim 1, wherein the matrix comprises 2.5-4.5% iron.

7. The alloy of claim 1, wherein the matrix comprises 0.001-0.005% boron.

8. The alloy of claim 1, wherein the matrix comprises 0.05-0.10% zirconium.

9. An oxidation-resistant alloy for use in a high temperature or thermal cycling oxidizing environment, the alloy consisting essentially of, in weight percent: a nickel-based matrix including about 19-23 chromium and about 3-6 aluminum; a self-healing thermodynamically stable oxide layer formed upon a surface of the matrix which is exposed to the oxidizing environment over a range of temperatures for extended periods of time, the layer protecting the alloy from the oxidizing environment; about 0.01-0.04 calcium and 0.01-0.04 yttrium for removing or stabilizing oxygen or sulphur dissolved in the matrix, thereby permitting the aluminum in the matrix to combine with oxygen in the environment to form a means for healing the layer and the surface if the layer is damaged or spalls; about 2-8 iron for inhibiting nucleation and growth of a nickel aluminum intermetallic compound that would adversely harden the alloy and cause local disturbance of a uniform distribution of the aluminum; and a balance of impurities.

10. A heating element comprising, in weight percent: a nickel-based matrix including about 19-23 chromium and about 3-6 aluminum; a self-healing thermodynamically stable oxide layer formed upon a surface of the matrix which is exposed to an oxidizing environment over a range of temperatures for extended periods of time, the layer protecting the alloy from the oxidizing environment; about 0.01-0.04 calcium and 0.01-0.04 yttrium for removing or stabilizing oxygen or sulphur dissolved in the matrix, thereby permitting the aluminum in the matrix to combine with oxygen in the environment to form a means for healing the layer and the surface if the layer is damaged or spalls; and about 2-8 iron for inhibiting nucleation and growth of a nickel aluminum intermetallic compound that would adversely harden the alloy and cause local disturbance of a uniform distribution of aluminum.

11. The alloy of claim 10, wherein the matrix comprises 20-21% chromium.

12. The alloy of claim 10, wherein the matrix comprises 4-5% aluminum.

13. The alloy of claim 10, wherein the matrix comprises 0.02-0.03% calcium.

14. The alloy of claim 10, wherein the matrix comprises 0.01 [0.04] -0.04% yttrium.

15. The alloy of claim 10, wherein the matrix comprises 2.5-4.5% iron.

16. The alloy of claim 10, wherein the matrix comprises 0.001-0.005% boron.

17. The alloy of claim 10, wherein the matrix comprises 0.05-0.10% zirconium.

18. An oxidation-resistant alloy for use in a high temperature or thermal cycling oxidizing environment, the alloy comprising:

Weiσht Percent

Chromium 19 - 23

Aluminum 3 - 6

Iron 2 - 8 Zirconium 0 - 0.10

Calcium 0.01 - 0.05

Sulfur 0 - 0.008

Yttrium 0.01 - 0.04

Boron 0 - 0.005 Carbon 0 - 0.03

Silicon 0 - 2.0

Manganese 0 - 0.5

Titanium 0 - 0.25

Nickel Balance

19. The alloy of claim 18, consisting essentially of:

Weight Percent

Chromium 19 - 23 Aluminum 3 - 6

Iron 2 - 8

Zirconium 0 - 0.10

Calcium 0.01 - 0.05

Sulfur 0 - 0.008 Yttrium 0.01 - 0.04

Boron 0 - 0.005

Carbon 0 - 0.03

Silicon 0 - 2.0

Manganese 0 - 0.5 Titanium 0 - 0.25

Nickel Balance

20. The alloy of claim 18, wherein the ratio of chromium to iron is within the range 1:2 to 1:12, and wherein the ratio of chromium to aluminum is within the range 1:3 to 1:8, the ratios providing the alloy with an optimum combination of corrosion resistance in a variety of corrosive media and hot and cold working properties to permit production of thin sheet, tubing, and other commercial forms.

21. A thermocouple sheath comprising, in weight percent : a nickel-based matrix including about 19-23 chromium and about 3-6 aluminum; a self-healing thermodynamically stable oxide layer formed upon a surface of the matrix which is exposed to an oxidizing environment over a range of temperatures for extended periods of time, the layer protecting the alloy from the oxidizing environment; about 0.01-0.04 calcium and 0.01-0.04 yttrium for removing or stabilizing oxygen or sulphur dissolved in the matrix, thereby permitting the aluminum in the matrix to combine with oxygen in the environment to form a means for healing the layer and the surface if the layer is damaged or spalls; and about 2-8 iron for inhibiting nucleation and growth of a nickel aluminum intermetallic compound that would adversely harden the alloy and cause local disturbance of a uniform distribution of aluminum.