US3383205A - Cobalt base alloys - Google Patents

Description

United States Patent 3,383,205 COBALT BASE ALLOYS Chester T. Sims, Ballston Lake, and Allan D. Foster, Schenectady, N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed Dec. 14, 1964, Ser. No. 418,263 3 Claims. (Cl. 75-171) ABSTRACT OF THE DISCLOSURE High temperature alloys which are resistant to oxidizing and corrosive influence consist in percent by weight essentially of carbon 0.1 to 0.60, chromium 27.0 to 35.0, nickel 9.3 to 11.5, tungsten 6.0 to 8.0, iron 6.0 maximum, and boron in an effective amount up to 0.050 maximum to impart ductility with the remainder essentially cobalt.

This invention relates to new and useful alloys. More particularly, it relates to alloys which are capable of operaung under elevated temperature conditions and which are resistant to oxidizing, sulfidizing, and other corrosive combustion gases at such temperatures.

It is Well known that equipment depending upon the driving force of combustion gas, such as gas turbines, operate more efiiciently and with greater power output at elevated temperatures. It is also Well known that at such elevated temperatures the physical or mechanical strength of many materials often decreases drastically and that they tend to become subject to excessive oxidation and/ or corrosion caused by oxidizing atmospheres, often complexed with other contaminants which enter from the fuels or atmosphere, including sulfur, vanadium, sodium and others. As the operating temperature of such equipment rises, relatively small improvements in the oxidation and corrosion resistance and strength of such materials become more and more important. For example, in gas turbines operating at temperatures of the order of about 1600 F., an improvement of only one hundred degrees Fahrenheit in the resistance of the materials of construction represents a notable advance. For example, in a typical gas turbine, an increase in operating temperature from about 1500 F. to about 1600 F. represents an increase in power output of about 14% and an increase in efiiciency of about 1 to 5%.

The use of cobalt-base alloys containing relatively large amounts of chromium for high temperature operation under oxidative and 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% by weight actually result in an increase in scaling or deterioration. This teaching is set forth, for example, in Journal of the Electrochemical Society, vol. 103, No. 8, Phalnikar et al., entitled High Temperature Scaling of Cobalt-Chromium Alloys. Thus, according to the prior art teaching, increases in chromium over 25% by weight would result in less suitable elevated temperature operation in terms of oxidation or corrosion resistance, or both.

It is a primary purpose of the present invention to provide new and useful alloys of the cobalt-chromium class which are characterized by their ability to operate at increasingly elevated temperatures under more oxidative and corrosive conditions than heretofore, such as in gas turbines. Another object is to provide such alloys which are useful in other equipment and devices subjected to elevated temperatures and oxidizing or corrosive atmospheres, or both, such as furnaces and the like.

Briefly, there are provided by the present invention high-temperature, corrosion-resistance cobalt-base alloys having a percent by weight content of carbon 0.1 to 0.60, chromium 27.0 to 35.0, nickel 9.3 to 11.5, tungsten 6.0 to 8.0, iron 6.0 maximum, boron 0.050 maximum, manganese 1.0 maximum as an impurity not to be added, with the remainder cobalt except for residual impurities such as phosphorus, sulfur, silicon and the like. It has been found that alloys having balanced metallic compositions within the limits specified are characterized by substantial increases in corrosion and oxidation resistance, while at the same time having a suitably high rupture strength and ductility for such high temperature operation. The materials are also particularly useful in that they are readily weldable, permitting the fabrication of various shaped structures.

The teaching of the present invention proceeds contrary to the prior art teaching as set forth above, i.e., that in cobalt-chromium high temperature alloys, optimum oxidation and corrosion resistance is obtained at about 25% by weight of chromium. However, the present compositions represent carefully balanced combinations of constituents, each of which contributes to the desirable characteristics obtained. Deviations in the proportions of the materials destroy this balance, and result in materials which have been found to be wanting in one or more essential characteristics. For example, when the carbon content is lowered beyond that prescribed, there results an undesirable loss of strength. On the other hand, increasing the carbon content above that stated detracts severely from the weldability of the material, as well as the rupture ductility, and also lowers the oxidation resistance. Reduction of the chromium content below that set forth results in a detrimental loss of oxidation resistance, whereas increases above the prescribed range of chromium result in a loss in ductility, and are accompanied only by a very marginal increase in oxidation or corrosion resistance, which is more than offset by the loss in ductility. The nickel, tungsten and iron contents do not appear to be particularly critical for oxidation and corrosion resistance, but it has been found desirable to hold them within the stated ranges for mechanical and physical property reasons. Boron imparts ductility to the alloy when added, but increasing the boron content beyond that set forth causes detrimental low-melting phases to form in the alloy. The cobalt base, of course, is well known for its contribution to sulfidation and oxidation resistance. Various impurities are present in the alloys. Manganese can be tolerated in amounts up to about 1.00%, but is not purposely added in any case. Other impurities such as phosphorous and sulfur are held to a maximum of about 0.04% each.

The following examples will illustrate the practice of the invention; it being realized that they are examplary only and not to be taken as limiting in any way.

EXAMPLE 1 There was prepared an .alloy consisting of, by weight percent determined from final chemical analysis, carbon 0.17, chromium 31.8, nickel 9.7, tungsten 7.69, iron 0.99, boron 0.0073, manganese 0.54 with about 0.01 of phosphorous and sulfur as impurities with the remainder essentially cobalt. This alloy was cast into pieces having dimensions of 1% inches by 5 inches from which test pieces one inch in diameter and 0.060 inch thick were cut for testing, and compared with a prior art alloy having a percent by weight content of carbon 0.23, chromium 25.5, nickel 10.5, tungsten 6.9, iron 0.82, boron about 0.007 with manganese 0.54, and about 0.02 each of phosphorous and sulfur as impurities and with the remainder essentially cobalt. It will be noted that the present alloy has a higher amount of chromium than the prior art alloy which in its larger present .amounts has been found to impart unexpected and advantageous qualities.

When test pieces of the above example and the above prior art material were evaluated by being placed edgewise to the gas stream direction in a small burner apparatus (which simulates actual gas turbine operating conditions) at a temperature of 2000 F., burning natural gas with an air to fuel Weight ratio of 50 to 1, the oxidative corrosion in mils per side of the prior art material after 600 hours was 14.6. On the other hand, .the material of the above example had corresponding corrosion after 594 hours of only 7.3 mils per side. After 984 hours, the prior art material exhibited corrosion of 20.4 mils per side, but even after 2553 hours the present material exhibited corrosion of only 14.3 mils per side. It will thus be seen that the present alloy provides a decided advantage over the prior art material insofar as oxidation resistance at elevated temperatures is concerned.

The material of the above example was also tested under sulfidizing conditions with the above prior art ma terial in a test burner operated at a temperature of 1600 F. with an airto-fuel weight ratio of 68 to 1, burning .a distillate oil containing 3.8% by weight of sulfur, to which there had been added 325 parts per million of sodium chloride. This produces an extremely corrosive environment. In this test, deposition of molten material can take place on the leading edge of the test specimen, causing a higher penetration rate than where the deposition does not occur. The data presented here are both the minimum attack (no molten deposition) and the maximum attack where molten material resided. After 465 hours under such conditions, a typical prior art material had a corrosion in mils per side of the test specimen ranging from about 3.8 (minimum) to 18.1 (maximum) mils. On the other hand, the present material, even after 570 hours of operation under such rigorous conditions, had only about 1.0 mil per side of corrosion overall.

As pointed out above, the alloys of the present invention are not only characterized by good oxidation and sulfidation resistance, but their fabrication into various shaped structures is facilitated by the fact that they are readily weldable.

The rupture strength of the material of the above example after hours at a temperature of 1600 F. is about 9,500 p.s.i., which compares very favorably with the corresponding value of 10,500 p.s.i. for the above prior art material. The rupture ductility of the above example material is equivalent to that of the prior art material.

EXAMPLE 2 An alloy was prepared consisting of, by weight percent, carbon 0.19, chromium 27.6, nickel 9.3, tungsten 7.5, iron 5.6, boron 0.0069, manganese 0.56, with the remainder essentially cobalt. The alloy so prepared was cast as in Example 1, and similar test pieces prepared.

When such test pieces were tested for oxidative corrosion in natural gas as in Example 1, the corrosion in mils per side after 594 hours was 7.4 mils. After 2431 hours under the above oxidative conditions, the corrosion per side was 18.4 mils. It will be seen that the material of Example 2 provides a decided advantage over the prior art material mentioned above. The rupture strength of the material of this example at 10 hours at a temperature of 1600 F. is about 10,500 p.s.i., which is the same as the corresponding value for the above prior art material. The rupture ductility is also equivalent to that of the prior art material, and the weldability was found to be good.

There are provided, then, by the present invention, new and useful alloys which are particularly characterized by their high strength at elevated temperatures and their ability to withstand at such elevated temperatures corrosive attack caused by oxidizing and sulfidizing combustion gas constituents. They are particularly useful for fabricating gas turbine components such as nozzle partitions, as well as other static or moving parts of such equipment which are exposed to high temperature corrosive conditions. They are also useful in general in furnaces and other equipment which operate under similar rigorous conditions, where they can serve as materials of construction for fans, runners, and the like.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. A high temperature resistant alloy, consisting essentially of by weight carbon 0.1 to 0.60 percent, chromium 27.0 to 35.0 percent, tungsten 6.0 to 8.0 percent, nickel 9.3 to 11.5 percent, boron in an effective amount up to 0.050 percent maximum to impart ductility, iron 6.0 percent maximum, with the remainder essentially cobalt.

2. A high temperature resistant alloy consisting essentially of by weight carbon 0.17 percent, chromium 31.8 percent, tungsten 7.69 percent, nickel 9.7 percent, boron 0.0073 percent, iron 0.99 percent, with the remainder essentially cobalt.

3. A high temperature resistant alloy consisting essentially of by weight carbon 0.19 percent, chromium 27.6 percent, tungsten 7.5 percent, nickel 9.3 percent, boron 0.0069 percent, iron 5.6 percent, with the remainder essentially cobalt.

References Cited UNITED STATES PATENTS 2,744,010 5/ 1956 Calloway -171 2,746,860 5/1956 Binder et al. 75171 2,855,295 10/1958 Hansel 75171 2,996,379 8/1961 Faulkner 75-171 I-IYLAND BIZOT, Primary Examiner.

DAVID L. RECK, RICHARD O. DEAN, Examiners.