HIGH-TEMPERATURE OXIDATION AND CORROSION-RESISTANT COBALT-BASE ALLOYS
United States Patent 3615375
Cobalt-base alloys having improved high-temperature strength and ductility characteristics and corrosion resistance consist essentially of, in percent by weight, carbon 0.1 to 0.7, chromium 24 to 35, tungsten 6 to 9, nickel 8.5 to 11.5, boron in an effective amount of about 0.005 up to 0.05, iron up to 2, zirconium 0.1 to 1,7, yttrium 0.03 to 1, with the remainder essentially cobalt except for impurities.
Inventors:
Beltran, Adrian M. (N/A, NY)
Sims, Chester T. (N/A, NY)
Mcgarrigan, Donald E. (N/A, NJ)
Sims, Chester T. (N/A, NY)
Mcgarrigan, Donald E. (N/A, NJ)
Application Number:
05/001884
Publication Date:
10/26/1971
Filing Date:
01/09/1970
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Export Citation:
Primary Class:
Other Classes:
148/674, 420/440
International Classes:
C22C19/07; C22C19/00
Field of Search:
75/171,170 148/32,32.5,158
Primary Examiner:
Dean, Richard O.
Claims:
What we claim as new and desire to secure by Letters Patent of the United States is
1. A cobalt base alloy characterized by good high-temperature strength, ductility, and corrosion resistance consisting essentially of about, by weight, carbon 0.1 to 0.7%, chromium 24 to 35%, tungsten 6 to 9%, nickel 8.5 to 11.5%, boron an effective amount up to 0.5%, iron, up to 2%, zirconium 0.1 to 1.7%, yttrium 0.03 to 1%, with the remainder essentially cobalt except for impurities.
2. A cobalt base alloy as in claim 1 consisting essentially of about, by weight, carbon 0.5%, chromium 29%, tungsten 7%, nickel 10%, boron 0.10%, iron 1%, zirconium 1.5%, yttrium 0.15%, with the remainder essentially cobalt except for impurities.
3. A cobalt base alloy as in claim 1 consisting essentially of about, by weight, carbon 0.46%, chromium 27.2%, tungsten 6.7%, nickel 10%, boron 0.01%, iron 0.71%, zirconium 1.29%, yttrium 0.21%, with the remainder essentially cobalt except for impurities.
4. A cobalt base alloy as in claim 1 consisting essentially of about, by weight, carbon 0.51%, chromium 28.3%, tungsten 7.23%, nickel 10%, boron 0.10%, iron 0.68%, zirconium 1.23%, yttrium 0.24%, with the remainder essentially cobalt except for impurities.
1. A cobalt base alloy characterized by good high-temperature strength, ductility, and corrosion resistance consisting essentially of about, by weight, carbon 0.1 to 0.7%, chromium 24 to 35%, tungsten 6 to 9%, nickel 8.5 to 11.5%, boron an effective amount up to 0.5%, iron, up to 2%, zirconium 0.1 to 1.7%, yttrium 0.03 to 1%, with the remainder essentially cobalt except for impurities.
2. A cobalt base alloy as in claim 1 consisting essentially of about, by weight, carbon 0.5%, chromium 29%, tungsten 7%, nickel 10%, boron 0.10%, iron 1%, zirconium 1.5%, yttrium 0.15%, with the remainder essentially cobalt except for impurities.
3. A cobalt base alloy as in claim 1 consisting essentially of about, by weight, carbon 0.46%, chromium 27.2%, tungsten 6.7%, nickel 10%, boron 0.01%, iron 0.71%, zirconium 1.29%, yttrium 0.21%, with the remainder essentially cobalt except for impurities.
4. A cobalt base alloy as in claim 1 consisting essentially of about, by weight, carbon 0.51%, chromium 28.3%, tungsten 7.23%, nickel 10%, boron 0.10%, iron 0.68%, zirconium 1.23%, yttrium 0.24%, with the remainder essentially cobalt except for impurities.
Description:
This invention relates to new and useful cobalt base alloys which are characterized by improved high-temperature strength and increased resistance to corrosion at elevated temperatures.
Gas turbines and other equipment which are driven by the forces of combustion gases operate more efficiently as the operating temperature rises. However, at such higher temperatures the strength characteristics of many alloys often decrease rapidly and the alloys become susceptible to corrosion, consisting of both oxidation and hot corrosion, caused by contact with oxidizing and other corrosive constituents of the hot combustion gas streams such as sodium and sulfur. While many alloys aimed at curing such deficiencies have been formulated, there is a constant search for new and better alloys such that only relatively slight improvements often become critical and of unexpected advantage in the higher temperature operating ranges. For example, in gas turbines operating at temperatures of the order of about 1,600° F. with peak temperatures of about 2,000° F., an improvement of only 100° F. in the resistance of the materials of construction to the environment represents a notable advance. In a typical gas turbine, an increase in operating temperature from about 1,500° F. to 1,600° F., represents an increase in power output of about 14 percent and an increase in efficiency of about 1 to 5 percent.
The use of cobalt-base alloys containing relatively large amounts of chromium for high-temperature operation under corrosive conditions is well known. However, it has been the teaching of the prior art that increases in the chromium content of such alloys over about 25 percent by weight actually result in an increase in scaling or surface deterioration. This teaching is set forth, for example, in Journal of the Electrochemical Society, Vol. 103, in a paper by Pfalnikar et al. entitled "High Temperature Scaling of Cobalt-Chromium Alloys." Typical prior art alloys which proceed in the face of this teaching by providing relatively large amounts of chromium, which in critical balance with other alloying constituents provide good high-temperature strength and increased resistance to corrosion, are those set forth in U.S. Pat. No. 3,383,205, assigned to the same assignee as the present invention.
As pointed out above, there is a constant search toward the improvement of such high-temperature alloys so that they may operate at higher temperatures for longer periods of time under highly corrosive conditions, and it is a primary purpose of the present invention to provide such new and useful improved alloys. Another object is to provide such improved alloys which are useful in other equipment and devices subjected to elevated temperatures and oxidative and corrosive atmospheres, such as furnaces and the like.
Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be better understood and further objects thereof appreciated from a consideration of the following description.
Briefly, there are provided by the present invention high-temperature oxidation and corrosion-resistant alloys having a percent by weight content of carbon 0.1 to 0.7, chromium 24 to 35, tungsten 6 to 9, nickel 8.5 to 11.5, boron an effective amount of about 0.005 up to 0.05, zirconium 0.1 to 1.7, yttrium 0.03 to 1, iron up to 2 maximum as added with other alloying constituents, with the remainder cobalt except for incidental impurities such as sulfur and phosphorous both less than 0.04 percent, along with silicon, manganese, copper, titanium and the like which should be held to less than about 0.1 percent. It has been found that alloys having the above balanced composition are characterized by substantial increases in corrosion resistance at elevated temperatures, at the same time having improved physical characteristics such as high tensile and rupture strength and ductility.
As pointed out above, the present compositions represent carefully balanced combinations of constituents each of which contributes to the desirable characteristics obtained. Departures from the specified proportions of materials destroy this balance and result in materials which have been found to be lacking in one or more desired characteristic. Carbon, for example, is added for strengthening purposes and when lower amounts than those prescribed are used, there is an undesirable loss in strength. In the prescribed range, carbon combines with zirconium and chromium to form, respectively, zirconium carbide and Cr 23 C 6 , which precipitation-harden the alloy. Zirconium carbide, in particular, remains thermally stable to near the melting point of the alloy; hence, it contributes greatly to strength at very high temperatures. On the other hand, carbon in amounts above those prescribed results in embrittlement and loss of strength through excessive precipitation of these two carbides, particularly at grain boundaries. Chromium contributes to the oxidation and corrosion resistance of the material at elevated temperatures and lesser amounts than those set forth inhibit these characteristics. Larger amounts than those specified result in embrittlement through precipitation of the electron compound sigma, composed in major part of cobalt and chromium, in the alloy matrix and at the grain boundaries. Larger amounts of tungsten than those set forth also enhance formation of sigma phase due to substitution of tungsten in the sigma lattice structure, thus embrittling the alloy. In the prescribed range, tungsten increases strength by substitutional solid solution hardening of the alloy matrix lattice structure. Lesser amounts of tungsten detract from the strength.
Nickel serves as a matrix stabilizer to maintain the FCC alloy matrix structure, and decreases the tendency toward formation of embrittling stacking faults. Nickel fails in this role in lesser amounts than those set forth. Higher than prescribed amounts of nickel reduce the high-temperature strength and hot corrosion and oxidation resistance. Boron, which serves as a strengthener, results in embrittlement when used in excess due to the precipitation of metal borides at the alloy grain boundaries, and in poor high-temperature strength when used in too small amounts. Iron, as pointed out above, is added with other alloying compounds and is not harmful up to the amount allowed, but when present in greater amounts than those prescribed reduces the strength and hot corrosion resistance of the final alloy. Zirconium serves as a strong carbide former, contributing to the high-temperature strength. In amounts either less or greater than that prescribed, high-temperature strength is reduced due to an imbalance of the two major carbide-strengthening precipitates, ZrC and Cr 23 C 6 . Yttrium is particularly critical to the hot corrosion resistance including both oxidation and corrosion caused by sodium, sulfur and the like. It is known that yttrium interacts during the oxidation process with the predominant Cr 2 O 3 oxide scale, enhancing its adherence to the underlying substrate as well as reducing Cr diffusion, therefore reducing oxidation kinetics. These features arise as a result of preferential oxidation of yttrium at key sites, such as grain boundaries near the oxidizing surface, thereby blocking subsequent diffusion. Amounts above those prescribed result in undesirable embrittling due to the precipitation of cobalt/yttrium-rich intermetallic compounds, such as Co 5 Y. The cobalt base, of course, is well known for its contribution to corrosion resistance.
The following examples will illustrate the practice of the invention, it being realized that they are exemplary only and not to be taken as limiting in any way.
As example 1, there was prepared an alloy consisting of, by weight percent, carbon 0.5, chromium 29, tungsten 7, nickel 10, boron 0.01, iron 1, zirconium 1.5, and yttrium 0.15, the remainder being cobalt except for impurities such as Mn and Si, <0.1, and S and P, <0.04.
As example 2, there was prepared an alloy consisting essentially of, by weight percent, carbon 0.46, chromium 27.2, tungsten 6.7, nickel 10, boron 0.01, iron 0.71, zirconium 1.29, and yttrium 0.21, with the remainder essentially cobalt except for impurities such as titanium, manganese and silicon, which were present in amounts of less than 0.1 each.
As example 3, there was prepared an alloy consisting of, by weight percent, carbon 0.51, chromium 28.3, tungsten 7.23, nickel 10, boron 0.01, iron 0.68, zirconium 1.23, and yttrium 0.24, with the remainder essentially cobalt except for impurities such as silicon, less than 0.15 manganese, titanium and copper, each less than about 0.1, sulfur about 0.009 and phosphorus less than 0.015.
The present alloys can be vacuum cast or cast in an argon atmosphere and subjected to various heat treatments to develop strength characteristics.
Shown in table I below are the tensile properties of the specified examples and heats of the present invention at room temperature and at elevated temperature as compared to a typical prior art alloy of the above mentioned U.S. Pat. No. 3,383,205. ##SPC1##
From table I it will be noted that between room temperature and 1,650° F. the prior art alloy has higher ultimate and yield strengths. However, at higher temperatures the present alloy is equal in strength to the prior art alloy but is significantly more ductile.
Shown in table II below are the stress rupture properties of various examples and heats of the present invention, again as compared to a typical alloy of the above cited prior art patent. The Larsen-Miller parameter (constant = 20) is a well known numerical tool which combines time and temperature to allow comparison of the capabilities of alloys on a normalized basis, such that the magnitude of the number at a given temperature yields a direct comparison of capability. ##SPC2##
From table II it will be noted that the present alloy has an improvement in rupture life for a given stress and temperature of at least 15 times that of the prior art alloy. This would allow a gas turbine part such as a partition to operate 15 times as long with the alloy of the present invention as with the prior art alloy. Alternatively, this would enable a part constructed of the present alloy to operate at a temperature more than 125° hotter than the prior art alloy for a given design stress and service life.
Shown in table III below are the results of burner rig tests of a typical alloy of the present invention. In this test, pieces of alloy 1 inch in diameter and 0.060 inch thick were placed edgewise to a combusted gas stream in a small burner apparatus so as to simulate actual gas turbine operating conditions at the temperature indicated. The combusted gas stream was created by burning natural gas with an air to fuel weight ratio of 50 to 1 and also by burning residual oil having 325 parts per million of sodium chloride and about 3 percent sulfur with an air to fuel weight ratio of 70 to 1. The former creates "oxidizing" conditions and the latter oxidizing conditions with a high potential to cause hot corrosion. Thermal cycles to room temperature were applied every 50 hours to simulate turbine operating conditions which accentuates differences in scale adherence. ##SPC3##
From the above table III by comparison at the same temperatures, it will be seen that the present alloys are far and away more resistant over long exposure periods to hot corrosion of the oxidative type experienced with natural gas and about equal in resistance to hot corrosion caused by sodium and sulfur and other agents which have an effect similar to that of sodium.
There are provided, then, by the present invention alloys which are suitable as materials of construction which combine superior high-temperature rupture stress with good resistance to hot corrosive influences.
Gas turbines and other equipment which are driven by the forces of combustion gases operate more efficiently as the operating temperature rises. However, at such higher temperatures the strength characteristics of many alloys often decrease rapidly and the alloys become susceptible to corrosion, consisting of both oxidation and hot corrosion, caused by contact with oxidizing and other corrosive constituents of the hot combustion gas streams such as sodium and sulfur. While many alloys aimed at curing such deficiencies have been formulated, there is a constant search for new and better alloys such that only relatively slight improvements often become critical and of unexpected advantage in the higher temperature operating ranges. For example, in gas turbines operating at temperatures of the order of about 1,600° F. with peak temperatures of about 2,000° F., an improvement of only 100° F. in the resistance of the materials of construction to the environment represents a notable advance. In a typical gas turbine, an increase in operating temperature from about 1,500° F. to 1,600° F., represents an increase in power output of about 14 percent and an increase in efficiency of about 1 to 5 percent.
The use of cobalt-base alloys containing relatively large amounts of chromium for high-temperature operation under corrosive conditions is well known. However, it has been the teaching of the prior art that increases in the chromium content of such alloys over about 25 percent by weight actually result in an increase in scaling or surface deterioration. This teaching is set forth, for example, in Journal of the Electrochemical Society, Vol. 103, in a paper by Pfalnikar et al. entitled "High Temperature Scaling of Cobalt-Chromium Alloys." Typical prior art alloys which proceed in the face of this teaching by providing relatively large amounts of chromium, which in critical balance with other alloying constituents provide good high-temperature strength and increased resistance to corrosion, are those set forth in U.S. Pat. No. 3,383,205, assigned to the same assignee as the present invention.
As pointed out above, there is a constant search toward the improvement of such high-temperature alloys so that they may operate at higher temperatures for longer periods of time under highly corrosive conditions, and it is a primary purpose of the present invention to provide such new and useful improved alloys. Another object is to provide such improved alloys which are useful in other equipment and devices subjected to elevated temperatures and oxidative and corrosive atmospheres, such as furnaces and the like.
Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be better understood and further objects thereof appreciated from a consideration of the following description.
Briefly, there are provided by the present invention high-temperature oxidation and corrosion-resistant alloys having a percent by weight content of carbon 0.1 to 0.7, chromium 24 to 35, tungsten 6 to 9, nickel 8.5 to 11.5, boron an effective amount of about 0.005 up to 0.05, zirconium 0.1 to 1.7, yttrium 0.03 to 1, iron up to 2 maximum as added with other alloying constituents, with the remainder cobalt except for incidental impurities such as sulfur and phosphorous both less than 0.04 percent, along with silicon, manganese, copper, titanium and the like which should be held to less than about 0.1 percent. It has been found that alloys having the above balanced composition are characterized by substantial increases in corrosion resistance at elevated temperatures, at the same time having improved physical characteristics such as high tensile and rupture strength and ductility.
As pointed out above, the present compositions represent carefully balanced combinations of constituents each of which contributes to the desirable characteristics obtained. Departures from the specified proportions of materials destroy this balance and result in materials which have been found to be lacking in one or more desired characteristic. Carbon, for example, is added for strengthening purposes and when lower amounts than those prescribed are used, there is an undesirable loss in strength. In the prescribed range, carbon combines with zirconium and chromium to form, respectively, zirconium carbide and Cr 23 C 6 , which precipitation-harden the alloy. Zirconium carbide, in particular, remains thermally stable to near the melting point of the alloy; hence, it contributes greatly to strength at very high temperatures. On the other hand, carbon in amounts above those prescribed results in embrittlement and loss of strength through excessive precipitation of these two carbides, particularly at grain boundaries. Chromium contributes to the oxidation and corrosion resistance of the material at elevated temperatures and lesser amounts than those set forth inhibit these characteristics. Larger amounts than those specified result in embrittlement through precipitation of the electron compound sigma, composed in major part of cobalt and chromium, in the alloy matrix and at the grain boundaries. Larger amounts of tungsten than those set forth also enhance formation of sigma phase due to substitution of tungsten in the sigma lattice structure, thus embrittling the alloy. In the prescribed range, tungsten increases strength by substitutional solid solution hardening of the alloy matrix lattice structure. Lesser amounts of tungsten detract from the strength.
Nickel serves as a matrix stabilizer to maintain the FCC alloy matrix structure, and decreases the tendency toward formation of embrittling stacking faults. Nickel fails in this role in lesser amounts than those set forth. Higher than prescribed amounts of nickel reduce the high-temperature strength and hot corrosion and oxidation resistance. Boron, which serves as a strengthener, results in embrittlement when used in excess due to the precipitation of metal borides at the alloy grain boundaries, and in poor high-temperature strength when used in too small amounts. Iron, as pointed out above, is added with other alloying compounds and is not harmful up to the amount allowed, but when present in greater amounts than those prescribed reduces the strength and hot corrosion resistance of the final alloy. Zirconium serves as a strong carbide former, contributing to the high-temperature strength. In amounts either less or greater than that prescribed, high-temperature strength is reduced due to an imbalance of the two major carbide-strengthening precipitates, ZrC and Cr 23 C 6 . Yttrium is particularly critical to the hot corrosion resistance including both oxidation and corrosion caused by sodium, sulfur and the like. It is known that yttrium interacts during the oxidation process with the predominant Cr 2 O 3 oxide scale, enhancing its adherence to the underlying substrate as well as reducing Cr diffusion, therefore reducing oxidation kinetics. These features arise as a result of preferential oxidation of yttrium at key sites, such as grain boundaries near the oxidizing surface, thereby blocking subsequent diffusion. Amounts above those prescribed result in undesirable embrittling due to the precipitation of cobalt/yttrium-rich intermetallic compounds, such as Co 5 Y. The cobalt base, of course, is well known for its contribution to corrosion resistance.
The following examples will illustrate the practice of the invention, it being realized that they are exemplary only and not to be taken as limiting in any way.
As example 1, there was prepared an alloy consisting of, by weight percent, carbon 0.5, chromium 29, tungsten 7, nickel 10, boron 0.01, iron 1, zirconium 1.5, and yttrium 0.15, the remainder being cobalt except for impurities such as Mn and Si, <0.1, and S and P, <0.04.
As example 2, there was prepared an alloy consisting essentially of, by weight percent, carbon 0.46, chromium 27.2, tungsten 6.7, nickel 10, boron 0.01, iron 0.71, zirconium 1.29, and yttrium 0.21, with the remainder essentially cobalt except for impurities such as titanium, manganese and silicon, which were present in amounts of less than 0.1 each.
As example 3, there was prepared an alloy consisting of, by weight percent, carbon 0.51, chromium 28.3, tungsten 7.23, nickel 10, boron 0.01, iron 0.68, zirconium 1.23, and yttrium 0.24, with the remainder essentially cobalt except for impurities such as silicon, less than 0.15 manganese, titanium and copper, each less than about 0.1, sulfur about 0.009 and phosphorus less than 0.015.
The present alloys can be vacuum cast or cast in an argon atmosphere and subjected to various heat treatments to develop strength characteristics.
Shown in table I below are the tensile properties of the specified examples and heats of the present invention at room temperature and at elevated temperature as compared to a typical prior art alloy of the above mentioned U.S. Pat. No. 3,383,205. ##SPC1##
From table I it will be noted that between room temperature and 1,650° F. the prior art alloy has higher ultimate and yield strengths. However, at higher temperatures the present alloy is equal in strength to the prior art alloy but is significantly more ductile.
Shown in table II below are the stress rupture properties of various examples and heats of the present invention, again as compared to a typical alloy of the above cited prior art patent. The Larsen-Miller parameter (constant = 20) is a well known numerical tool which combines time and temperature to allow comparison of the capabilities of alloys on a normalized basis, such that the magnitude of the number at a given temperature yields a direct comparison of capability. ##SPC2##
From table II it will be noted that the present alloy has an improvement in rupture life for a given stress and temperature of at least 15 times that of the prior art alloy. This would allow a gas turbine part such as a partition to operate 15 times as long with the alloy of the present invention as with the prior art alloy. Alternatively, this would enable a part constructed of the present alloy to operate at a temperature more than 125° hotter than the prior art alloy for a given design stress and service life.
Shown in table III below are the results of burner rig tests of a typical alloy of the present invention. In this test, pieces of alloy 1 inch in diameter and 0.060 inch thick were placed edgewise to a combusted gas stream in a small burner apparatus so as to simulate actual gas turbine operating conditions at the temperature indicated. The combusted gas stream was created by burning natural gas with an air to fuel weight ratio of 50 to 1 and also by burning residual oil having 325 parts per million of sodium chloride and about 3 percent sulfur with an air to fuel weight ratio of 70 to 1. The former creates "oxidizing" conditions and the latter oxidizing conditions with a high potential to cause hot corrosion. Thermal cycles to room temperature were applied every 50 hours to simulate turbine operating conditions which accentuates differences in scale adherence. ##SPC3##
From the above table III by comparison at the same temperatures, it will be seen that the present alloys are far and away more resistant over long exposure periods to hot corrosion of the oxidative type experienced with natural gas and about equal in resistance to hot corrosion caused by sodium and sulfur and other agents which have an effect similar to that of sodium.
There are provided, then, by the present invention alloys which are suitable as materials of construction which combine superior high-temperature rupture stress with good resistance to hot corrosive influences.