US3549356A - High temperature corrosive resistant cobalt-base alloys - Google Patents

Description

United States Patent 3,549,356 HIGH TEMPERATURE CORROSIVE RESISTANT COBALT-BASE ALLOYS Chester T. Sims and Adrian M. Beltran, Ballston Lake,

N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed Jan. 6, 1969, Ser. No. 789,390

Int. Cl. C22c 19/00 U.S. Cl. 75--171 2 Claims ABSTRACT OF THE DISCLOSURE Cobalt-base alloys having improved high temperature oxidation and corrosion resistance consist essentially of, in percent by weight, carbon 0.1 to 0.8, nickel 8.5 to 11.5, chromium 24 to 35, tungsten 6 to 9, tantalum 1 to 5, manganese up to 1 max., silicon up to 0.25 max., boron 0.005 to 0.5, yttrium 0.01 to 1.0, iron up to 2.0 max., hafnium 0.05 to 2.0, with the remainder essentially cobalt except for impurities.

This invention relates to new and useful alloys. More particularly, it relates to alloys which are characterized by good high-temperature strength and at the same time are resistant to oxidation and corrosion by combustion gases at elevated temperatures.

Equipment depending upon the driving force of combustion gases, as in gas turbines, operates more efficiently and with greater output as the temperature rises. It is also well known that at elevated temperatures, which enhance this increased efliciency and output, the strength of many alloys often decreases rapidly and they become more and more susceptible to oxidation and corrosion caused by constituents in the combustion gas stream. Among such corrosive constituents are sulfur, sodium and vanadium.

As the operating temperature of such equipment rises, relatively small improvements in corrosion resistance and strength of the structural materials become important. For example, in gas turbines operating at average temperatures of the order of about 1600 F., with peak temperatures of about 2000 R, an improvement of only about 100 F. in the oxidation and corrosion resistance of the structural materials represents a notable advance. To illustrate, an increase in operating temperature of a typical gas turbine from about 1500 F. to 1600 F. produces an increase in power output of about 14% and an increase in efiiciency of from about 1 to It is therefore a principal object of this invention to provide new and useful alloys which will permit substantial increases in the operating temperatures of such equipment as gas turbines, while at the same time providing suitable strength characteristics at such higher temperatures which can range up to 1900 F., 2000 F. or higher. Another object of the invention is to provide improved materials of construction for high temperature equipment in general, which are subjected to corrosive atmospheres such as furnaces and the like.

Briefly, there are provided by the present invention strong high-temperature corrosion-resistant cobalt base alloys having a percent by weight content of carbon 0.1 to 0.8, nickel 8.5 to 11.5, chromium 24 to 35, tungsten 6 to 9, tantalum 1 to 5, manganese up to 1 max., silicon up to 0.25 max., boron 0.005 to 0.5, yttrium 0.01 to 1.0, iron up to 2.0 max., hafnium 0.05 to 2.0, with the remainder essentially cobalt, except for incidental impurities such as phosphorus and sulfur which are preferably held below a maximum of about 0.015%.

It has been found that alloys of the above precisely balanced metallic composition are characterized by substantial improvement in corrosion resistance at elevated temperature, while at the same time retaining suitable 3,549,356 Patented Dec. 22, 1970 strength, ductility, and other physical characteristics for elevated temperature operation. The materials are also particularly useful in that they are particularly adapted to precision investment casting techniques permitting the formation of various shaped structures suitable for high-temgerature apparatus such as in the hot stages of gas turmes.

Those features of the invention which are believed to be patentable are set forth with particularity in the claims appended hereto. The invention will, however, be better understood and further advantages and objects thereof appreciated from a consideration of the following descrip tion.

With regard to the present compositions, the relatively high range of chromium, specifically from about 24 to 35 weight percent, is directly contrary to prior art teaching which states that cobalt-base alloys having a chromium content of over about 25% by weight exhibit increased scaling or deterioration due to corrosive influences at elevated temperatures. This prior art teaching is set forth, for example, in Journal of the Electrochemical Society, volume 103, No. 8, by Pfalnikar et al., entitled, High Temperature Scaling of Cobalt-Chromium Alloys.

As pointed out above, the present compositions comprise a carefully formulated combination of constituents, each of which contributes to the desirable characteristics obtained. Deviations in the proportions of the materials destroy this critical balance, and result in materials which have been found to be wanting in essential characteristics. For example, when the carbon content is lowered beyond that prescribed, an undesirable loss of strength results. On the other hand, increasing the carbon content above that stated would detract from the rupture ductility of the material and may also detract from weldability. Reduction of the chromium content below that set forth results in detrimental loss of oxidation resistance. The nickel,

iron and tungsten additives do not appear to be particularly critical for oxidation and corrosion resistance but it has been found desirable to have them present in the stated ranges for suitable structural mechanical properties. Boron imparts ductility to the alloy but increasing the boron beyond the content set forth causes detrimental low-melting phases to form in the alloy. Yttrium, when used in lower amounts than that set forth above, results in decreased oxidation and corrosion resistance. As a practical matter, amounts of yttrium greater than the upper prescribed limit are diflicult to retain effectively in the product by normal melting procedures. Within the prescribed limits, yttrium greatly improves the scale spalling resistance of the alloy under the thermal cycling conditions encountered in gas turbines, for instance. Hafnium contributes strength to the alloy in the amounts indicated by modifying the type of carbides present in favor of carbides with greater thermal stability. Lower amounts detract from this favorable characteristic while larger amounts than those indicated do not contribute any additional benefit.

The following example will illustrate the practice of the invention, it being realized that it is exemplary only and not to be taken as limiting in any way. There was prepared an alloy consisting of by weight percent carbon 0.40, nickel 10.7, chromium 28.7, tungsten 7.4, tantalum 3.1, manganese 0.30, silicon 0.21, boron 0.016, yttrium 0.18, iron 0.26 and hafnium 0.15, with the remainder essentially cobalt except for sulfur and phosphorus within the abovementioned limit. This alloy was investment cast and standard cast-to-size 0.252" diameter rupture bars and corrosion pins A3" diameter x 1%" long prepared.

Shown in Table I are the room temperature and high temperature mechanical properties of the above exemplary alloy as compared with a typical prior art alloy,

specifically WI-52. The Larsen-Miller parameter (constant=2-0) is a well known numerical tool which combines time and temperature to allow comparison of the capabilities of these alloys on a normalized basis.

TABLE II Mils per side Temp, Time, Surface Max. pene- Alloy rs. loss tration TABLE I [A.Room temperature tensile test] Example Ultimate 0.2 7 0.027

tensile yield. yielt l Percent NI-52 i 888 g strength, strength, strength, Percent reduction 1 1 Alloy K s.i. K s.i. K s.i. elongation in area a p e 8 5 0 From the above table it will be quite evident that the ""f-' present alloy has far and away superior corrosion resist- [B.Stress-Rupture tests] Larsen- Percent Miller Temp., Stress, Life, Percent reduction parameter Alloy F. K s.i. hours elongation in area (0:20)

Example 1, 600 20 405. 5 18. 2 48. 3 4 6 1, 700 226. 4 21. 4 57. 0 48. 3 1, 825 10 50. 9 1, 900 6 54. 2

AV WI-52 1, 600 46. 3 g 1, 700 15 48. 7 1, 825 10 51. 2 1, 900 6 53. 7

From the above table it will be seen that the rupture strength is indicative of the present alloy using room temperature tensile strength comparable to the prior art alloy; the present material, however, has about three times the ductility as measured by reduction in areas. The stress-rupture test results in Table I indicate that the stress-rupture strength of the present alloy using the Larsen-Miller parameter indicated has a strength comparable to the prior art alloy at high stresses, with a clear crossover at low stresses in favor of the present alloy. It will thus be seen that the present alloys are particularly adaptable to applications such as stationary nozzle guide vanes or partitions and similar applications which are characterized by high operating temperatures and relatively low stresses.

Shown in Table II is the hot-corrosion resistance of the present exemplary alloy as compared to the above prior art alloy. In this test, pin-shaped test pieces of the material of the above example and the prior art material were placed in the combustion gas stream flow in a simulated gas turbine burner apparatus at the 1900" F. indicated temperature using combustion Diesel oil (MIL-F- 16884) containing 1% by Weight sulfur at an air to fuel weight ratio of :1. Five ppm. sea salt (ASTM, D665-60) was continuously atomized into the chamber to simulate a marine environment, comparable to contamination by salt water during service. This produces a sulfidizing and oxidizing environment through generation of sodium sulphate. The specimens were thermal cycled every fifty hours to simulate gas turbine shutdown, this procedure being properly rigorous as it evaluates adherence properties of the protective scale. After the times indicated, the surface loss and maximum penetration were measured for each sample in terms of mils per side.

ance at elevated temperatures to the prior art alloy. Such corrosion resistance combined with the favorable strength characteristics at elevated temperatures makes these alloys very useful under low stress-high temperature corrosive conditions which are experienced in gas turbines, furnaces and similar apparatus.

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 oxidation and corrosion resistance consisting essentially of about, by weight, carbon 0.1 to 0.8 percent, nickel 8.5 to 11.5 percent, chromium 24 to 35 percent, tungsten 6 to 9 percent, tantalum 1 to 5 percent, manganese up to 1 percent max., silicon up to 0.25 percent max., boron 0.005 to 0.5 percent, yttrium 0.01 to 1.0 percent, iron up to 2.0 percent max., hafnium 0.05 to 2.0 percent, with the remainder essentially cobalt except for impurities.

2. A cobalt base alloy characterized by good high temperature oxidation and corrosion resistance consisting essentially of about, by weight, carbon 0.40 percent, nickel 10.7 percent, chromium 28.7 percent, tungsten 7.4 percent, tantalum 3.1 percent, manganese 0.30 percent, silicon 0.21 percent, boron 0.016 percent, yttrium 0.18 percent, iron 0.26 percent and hafnium 0.15 percent, with the remainder essentially cobalt except for impurities.

References Cited UNITED STATES PATENTS 3,005,705 10/1961 Cochardt --l71 3,202,506 8/1965 Deutsch 7517l 3,346,378 10/1967 Foster et al. 75l7l RICHARD O. DEAN, Primary Examiner