IL45853A - Nickel-base alloys having a low coefficient of thermal expansion - Google Patents

Description

oij?D m ya ,"?p*J 79 nioolaon niAioao ITickel-base alloys having a low coefficient, of tiiemal expansion JAKES FRISJJCH BALDUII C/ 43863 FIELD OF INVENTION: The present invention pertains to nickel base alloy compositions consisting predominantly of nickel, chromium, molybdenum, and carbon.

Preferably, the alloys also contain boron. The alloys of the present invention provide a unique, and previously unavailable combination of properties including elevated temperature strength, resistance to oxidation, resistance to corrosion at elevated temperatures, and a very low coefficient of thermal expansion. The nickel base alloys of this invention are particularly useful for making hard facing welding rods both in cast wire and powder form; components for use in the glass forming industry; and components for use in hot sections of gas turbine engines, such as integral wheels, turbine shrouds, cases, seals, and the like.

BACKGROUND OF INVENTION: In recent years, there has developed a need for alloys having low thermal expansion characteristics coupled with elevated temperature capabilities. The need for such alloys, for the most part , has arisen in connection with gas turbine technology. With the growing demand for improved engine efficiency, attention has been focused upon increasingly sophisticated engine designs. Low thermal expansion characteristics of alloys from which gas turbine engine components are fabricated, is important! The chromium content must be decreased in order to maintain an overall alloy composition that possesses microstructural stability and high temperature strength. As chromium content is decreased, the resistance to oxidation and sulfidization necessarily decreases.

Despite the apparent dilemma of being able to select either a strong alloy or one with good resistance to environmental deterioration, a few compositions have evolved with a relatively good balance of both properties. However, even these compositions are suitable for use only in gas turbine engines employing high grade aviation fuels and operating conditions whereby hot corrosion and sulfidization are minimized, unless an oxidation and sulfidization resistant coating is applied to components formulated from such alloys.

Furthermore, despite the good combination of strength and corrosion resistance, such alloys are not well suited for applications in which low thermal expansion is a primary concern. Such alloys have high thermal expansion properties typical of nickel base superalloys.

Cobalt based superalloys rely on solid solution strengthening and a dispersion of primary carbides for elevated temperature strength. For this reason, cobalt based alloys will accommodate a significantly greater percentage of chromium than nickel base alloys. As a general proposition, cobalt base superalloys may be categorized as weaker , but more corrosion j resistant, than nickel base materials. The expansion properties of cobalt base alloys are generally higher than nickel base alloys, making cobalt base alloys even less attractive for applications which require low thermal expansion.

The present invention pertains to nickel base alloy compositions possessing a very low coefficient of linear thermal expansion and s ulfidation resistance adequate to enable use of uncoated components fabricated from the alloys in corrosive environments. In addition, the alloys possess elevated temperature strength characteristics adequate to permit the alloys to be employed for numerous high temperature applications.

The alloys of the present invention contain unusually high levels of [ chromium and molybdenum. In the vast majority of cases, chromium and ! i molybdenum containing commercial nickel base alloys contain concentrations · of chromium and molybdenum which are below the respective solubility limit ! of each element in nickel. In the alloys of the present invention, the concentra tion of chromium and molybdenum far exceeds normal solubility limits in nickel.

Excess chromium and molybdenum in the alloys are prevented from forming deleterious embrittling phases through the addition of boron and carbon. Boron and carbon react with chromium and molybdenum to form borides and carbides. Unusual and unexpected strength improvements result from the boride and carbide dispersions so produced.

High concentrations of chromium in both the metallic matrix and the strengthening dispersoid result in unusually high resistance to sulfidation and corrosion at , elevated temperatures. The presence of all four major alloying constituents (chromium, molybdenum, boron, and carbon) serve to lower the thermal expansion properties of the alloys. The expansivity of specific alloy compositions within the scope of the present invention is lower than any known commercial nickel, cobalt, or iron-base alloy.

The present invention provides a nickel-base alloy having a low coefficient of thermal expansion as well as elevated temperature strength and resistance to high temperature corrosion. In addition, the present invention provides a nickel base alloy composition having a high elevated temperature hardness and corrosion resistance suitable for use in high temperature hard facing applications. Furthermore, the present invention provides high strength nickel base alloys of sufficient chromium content to resist the fluxing action of molten oxides and thus is suitable for fabricating components useful in the manufacture of glass shapes.

SUMMARY OF INVENTION: In general terms, the present invention pertains to nickel base alloy compositions consisting essentially of nickel, chromium, molybdenum, carbon and boron. These alloys have good elevated temperature strength, resistance to oxidation, and resistance to hot corrosion, as well as a very low coefficient of thermal expansion. The invention also concerns componen for use in gas turbine engines and hard facing welding rod made from such alloys.

Table I sets forth a broad range, an intermediate range, and two different and narrower ranges, in terms of percent by weight, of elements employed in the alloys of the present invention. It should be understood that the tabulation in Table I relates to each element individually, and is not intended to solely define composites of broad and narrow ranges. Nevertheless, composites of the narrower ranges specified in Table I represent particularly preferred embodiments.

In addition to the alloying constituents specifically set forth in Table I, the alloys of the present invention may contain minor amounts of other elements ordinarily included in nickel base alloys by those skilled in the art which will not substantially deleteriously affect the important characteristics of the alloy or which are inadvertently included in such alloys TABLE I Intermediate Element Broad Range Range Narr Cr 24-42 28 -42 38 ■ - 42 Mo 8-22 12 -20 12 ■ - 16 C 0.1-1.4 0.15 -1.2 0.5 ■ - 1.2 B 0.04-0.7 0.04 -0.7 0.2 ■ - 0.7 Ni Balance Balance Balance by virtue of impurity levels in commercial grades of alloying ingredients. Impurities and incidental elements which may be present include titanium, manganese and silicon in amounts normally employed to achieve castability and melt deoxidation. Typically, these elements would be present in amounts less than 1% and preferably manganese and silicon would each be present in amounts of not more than 0. 5% while titanium would be present in amounts of not more than 0. 2%. Other impurities and incidental elements which may be present in the alloys of the present invention include copper in amounts of not more than 0. 5%, sulphur and phosphorous in amounts of not more than 0. 20% and iron and cobalt in amounts of not more than 2. 0%. Impurities such as nitrogen, hydrogen, tin, lead, bismuth, calcium and magnesium should be held to as low a concentration as practical.

BRIEF DESCRIPTION OF THE DRAWINGS: FIG. 1 is a graphical plot of thermal expansion properties of commercial iron, nickel and cobalt-base superalloys.

FIG. 2 is a graphical plot depicting 100 hour creep rupture life for various commercial alloys.

FIG. 3 is a plot of thermal expansion properties for commercial iron, nickel and cobalt base superalloys and for example alloys of the present invention.

FIG. 4 is similar to FIG. 2 but represents example alloys of the present invention rather than commercial alloys.

DESCRIPTION OF EXAMPLES AND PREFERRED EMBODIMENTS: As previously noted, commercially available high temperature alloys may possess some of the characteristics desired in an alloy useful for fabricating components of gas turbine engines, but such alloys do not possess all of the desired characteristics. This may be illustrated with reference to I several commercial alloys whose compositions are presented in Table II. ! As shown in FIG. 1, commercial alloys A and B of Table II show remarkably i ! low thermal expansion properties in comparison with typical high temperature j alloys. In FIG. 1, the shaded area designated 1 repr esents a range of ! mean coefficients of linear thermal expansion at various temperatures for ! 89 commercial iron, nickel and cobalt-base superalloys. Curves 2 and 3 ' j represent plots of mean coefficients of thermal expansion against tempera- j ture for, respectively, commercial alloys A and B, j In the case of both commercial alloy A and B, their low thermal ! ■j expansion is attributed to the presence of unusually high levels of molybdenum, i a refractory element with low expansivity. The total absence or very low J f >i chromium content in these alloys renders them unacceptable for service, i ; j in an uncoated condition, at temperatures over about 1400 °F. in a sulfidizing TABLE II Commercial Alloy Ni Co Fe Cr Mo W Al A* (1) 2.5 0.6 28 B* (I) 18 C (1) 1.5 19 22 9 0.6 D** (1) 20 29 1 E 10 (1) 24 (1) Balance No sulfidation resistance unless coated Vacuum Melted environment. Both alloys deteriorate catastrophically at temperatures of 1800 °F. and h igher under sulfidizing conditions, In addition to inadequate environmental corrosion resistance, the I elevated temperature strength of commercial alloy A is so limited that it cannot be employed for components subjected to high stress at temperatures of above 1600 °F. This is illus trated in FIG. 2 which plots 100 hour creep < rupture life in terms of temperature versus stress for a number of commercial alloys. Curve 1 of FIG. 2 represents commercial alloy A.

As may be further seen from FIG. 2, commercial alloy C (curve 2) also lacks adequate elevated temperature strength. Commercial alloys D and E (curves 3 and 4, respectively, of FIG. 2) possess better high temperature strength characteristics, but not as high as desired at temperatures above about 1600 °F. Although the strength of commercial alloy B is excellent through about 2200° F. , the total lack of environmental corrosion resistance severely restricts its use.

Commercial alloys C, D and E possess exceptional resistance to environmental deterioration. However, the thermal expansion properties of all of these alloys are high, typical of nickel-base alloys falling within the shaded area 1 of FIG. 1. The high thermal expansion properties of these elements is a major drawback with respect to their use for fabrication of metallic dendrites. The continuity of the metallic phase on a microstructural ι i scale can be controlled by varying the alloy composition, but the network of particulate carbides and borides remains fairly continuous .

It has further been found that in addition to an improvement in room temperature hardness, the elevated temperature- creep rupture s trength of alloys in accordance with the present invention which contain only 0. 5% to 1. 0% carbon approaches the strength of several commercial cast cobalt base ' superalloys. The simultaneous addition of carbon and boron results in j creep-rupture strength comparable to several widely used commercial cobalt -! base cast alloys. The maximum creep rupture strength observed in alloys ' i in accordance with the present invention containing both carbon and boron is j ! 42, 000 psi for rupture in 100 hours at 1500 °F. This value is approximately j 10% higher than the strongest known cast cobalt-base superalloy. j A number of example alloy compositions in accordance with the j i present invention were studied, using material melted and cast in air in j standard shell test bar and weld rod molds. Thirty to 50 lb. heats were | i produced for each composition studied. Response to heat treatment 1 i was determined by subjecting the test materials to a 24-hour aging expos ure I at 1600 °F. Alloys that demonstrated an aging respons e were given the j (1) Balance TABLE IV Example Mean Coefficient Of Linear Thermal Alloy Expansion from 80 °F to Indicated Temp 400 °F. 800 °F. 1200 °F. 1 6. 79 6. 94 7. 56 2 5. 87 6. 53 7. 02 4 5. 56 5. 83 6. 40 5 6. 17 6. 39 6. 84 6 6. 17 6. 39 6. 76 7 5. 87 5. 97 6. 76 8 5. 87 6. 1 1 6. 84 I 1 1 6. 17 6. 53 7. 02 12 5. 71 6. 31 7. 06 14 6. 31 6. 44 7. 10 15 6. 17 6. 39 7. 20 TABLE V (cont ' d) 1600 20, 000 48. 6 5. 5 6. 4 -- 1600 20, 000 60. 1 6. 5 7. 2 -- 2000 5, 000 9. 6 2. 6 3. 8 2000 5, 000 1 1. 8 5. 0 ' 6. 8 -- 25, 000 1600 20, 000 30. 9 13. 5 22. 6 1600 20, 000 42. 0 10. 9 17. 1 -- 2000 5, 000 5. 2 17. 8 18. 1 2000 5, 000 5. 0 1 1. 8 14. 3 -- 24, 000 f? 1 G00 20, 000 42. 7 20. 1 27. 6 _ _ 1600 20, 000 43. 8 19. 1 20. 9 -- 2000 5, 000 4. 5 23. 6 52. 5 -- 2000 5, 000 9. 1 20. 7 46. 8 -- 24, 000 1 1 1600 20, 000 21. 2 24. 2 30. 9 1600 20, 000 18. 6 20. 1 35. 0 -- 2000 5, 000 3. 5 18. 6 29. 9 2000 5, 000 4. 0 21. 2 20. 0 -- -- 22, 500 12 1600 20, 000 24. 8 17. 8 28. 7 1600 20, 000 35. 4 11. 6 14. 9 -- 2000 5, 000 9. 0 6. 6 11. 3 2000 5, 000 11. 7 4. 6 11. 2 -- 23, 000 13 1600 20, 000 20. 8 13. 5 32. 4 _ _ 1600 20, 000 24. 2 18. 0 33. 1 -- 2000 5, 000 4. 9 12. 6 18. 1 2000 5, 000 4. 8 7. 4 25. 0 -- -- 23, 000 14 1600 20, 000 96. 2 8. 4 1 7. 3 _ _ 1600 20, 000 107. 0 7. 5 6. 9 -- 2000 5, 000 40. 5 5. 6 7. 5 2000 5, 000 37. 5 5. 0 6. 7 -- -- 27, 500 ί 15 1600 20, 000 69. 9 19. 8 38. 3 1600 20, 000 44. 1 17. 1 27. 0 -- 2000 5, 000 9. 4 11. 3 24. 0 -- 2000 5, 000 1 1. 9 16. 2 27. 1 -- 24, 500 TABLE VI Notch-Rupture Properties Example Test Conditions Notch Facto Alloy Temp. °F. Stress, psi Kt 7 1600 20, 000 3. 5 1600 20, 000 3. 5 8 1600 20, 000 3. 5 1600 20, 000 3. 5 11 1600 20, 000 3. 5 1600 20, 000 3. 5 12 1600 20, 000 3. 5 1600 20, 000 3. 5 13 1600 20, 000 3. 5 1600 " 20, 000 3. 5 14 (2) 1600 20, 000 3. 5 1600 20, 000 3. 5 14 (3) 1600 20, 000 3. 5 1600 20, 000 3. 5 that would show, absent the relatively large amount of carbon, micro- ' structural instability. Structurally, these alloys consist of primary metallic dendrites and primary "herring bone" eutectic chromium-molybdenum carbides. Example alloys 1 and 2 exhibited Rockwell hardness numbers C-scale, (Rc) of 33 and 42 respectively. Example alloy 2 showed a slight softening to Rc 38 upon aging. Rupture strength of both alloys is relatively low, but approaches that of cast cobalt-base superalloys.

Increasing chromium content of nickel base alloys generally results in a lowering of elevated temperature strength. However, as shown by the data in Table V with respect to example alloys 3, 4, 5, and 6, increasing chromium content while simultaneously adding relatively large percentages of carbon and boron results in sharp increases in strength. In the case of example alloy 6, the stress to produce rupture in 100 hours at 1500°F is more than doubled in comparison to example alloys 1 and 2. Of course, this is an unusually large and unexpected increase in strength. By comparing FIGS. 2 and 4 it may be seen that the level of strength of example alloy 6 is approximately 10% above that of commercial alloy E.

Alloy E is one of the strongest cobalt base alloys which has been developed.

Example alloy 4 not only has good strength, it possesses a lower mean coefficient of thermal expansion from 80°F. to 1600 °F. than any other known nickel-base alloy. The surprisingly low mean coefficient of thermal expansion of example alloy 4 from 80°F to 1600 °F. is shown in Table IV.

A comparison of this data with the curves of FIG. 1 illustrates the low degree of thermal expansion of example alloy 4 compared to various commercially available superalloys.

Example alloys 4 and 6, respectively, show a weight loss of 50. 4 . 2 and 48. 1 mg/ cm and surface recession rates of 0. 0035 and 0. 002 inches in 300 hours in the sulfidization test. This represents excellent resistance to the severe test conditions employed and demonstrates that these alloys may be categorized as hot corrosion resistant.

Despite the fact that example alloy 6 showed a remarkable increase in strength, example alloy 4 may be the more attractive material for certain types of use. The very low expansivity combined with excellent hot corrosion resistance and moderate strength makes example alloy 4 very attractive for fabricating components which require a very low degree of thermal expansion at elevated temperatures. Compositional modifications around example alloys} 4 and 6 resulted in some strength improvement over alloy 4, in example alloy 14, but at some sacrifice in expansion properties.

In producing the alloys of the present invention, and objects prepared from the alloys of the present invention, no special skills or techniques are required other than normal conventional foundry practice. The alloys may be readily cast in sand, shell , or investment molds and melted and cast in air or under vacuum. Although the alloys were developed for use in the cast condition, several specific compositions within the ambit of the present invention may be employed in wrought form if produced by powder metallurgy techniques.

The alloys of the present invention may generally be described as a class of nickel-base alloys possessing a duplex structure consisting of a nickel-chromium -molybdenum matrix and a semi-continuous network of refractory carbides and borides. The alloy compositions possess a combination of physical and mechanical characteristics which have generally been considered mutually exclusive.

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. Such modifications are considered to be within the purview and scope of the invention and appended claims.

Claims (14)

45853 -2 WHAT IS CLAIMED IS:

1. A nickel base alloy having elevated temperature strength, resistance to oxidation and hot corrosion, and a low coefficient of thermal expansion, consisting essentially of the following elements in the weight percent ranges set forth: Elements Percent Chromium 24-42 Molybdenum 8-22 Carbon 0. 1-1. 4 Boron 0. 04-0. 7 the balance of the alloy being essentially nickel and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the alloy.

2. The nickel base alloy of claim 1 wherein the carbon content is about 0. 5% to about 1. 2% by weight.

3. A component for use in a gas turbine engine formed of the alloy of claim 1.

4. A hard facing welding rod formed of the alloy of claim 1.

5. The alloy of claim 1 which contains, on a weight basis, about 28% to about 42% chromium, about 12% to about 20% molybdenum, about 0. 15% to about 1. 2% carbon, and about 0. 04% to about 0. 7% boron.

6. The alloy of claim 5 which contains, on a weight basis, about 16% to about 20% molybdenum and about 0. 2% to about 0. 7% boron.

7. A component for use in a gas turbine engine formed of the alloy of claim 5 .

8. A component for use in a gas turbine engine formed of the alloy of claim 6.

9. A hard facing welding rod formed of the alloy of claim 5.

10. A nickel base alloy having elevated temperature strength, resistance to oxidation and hot corrosion, and a low coefficient of thermal expansion consisting essentially of the following elements and the weight percentage ranges set forth: Elements Percent Chromium 38 42 Molybdenum 12 16 Carbon 0. 5 1. 2 Boron 0. 2 0. 7 10 the balance of the alloy being essentially nickel and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the alloy.

11. A component for use in a gas turbine engine formed of the alloy of claim 10.

12. A nickel base alloy having elevated temperature strength, resistance to oxidation and hot corrosion, and a low coefficient of thermal expansion consisting essentially of the following elements in the weight percentage ranges set forth: Elements Percent Chromium 33 - 37 Molybdenum 16 - 20 Carbon 0. 3 - 1. 2 Boron 0. 15- 0. 5 10 the balance of the allo bein essentiall nickel and minor amounts of ( )

13. A component for use in a gas turbine engine formed of the alloy of claim 12.

14. The alloy of claim 1 which contains not more than 0. 2% titanium.