US3592681A - Metal surface treating process by use of lanthanum compounds - Google Patents

Abstract

METHOD FOR IMPROVING THE OXIDATION RESISTANCE OF IRON, NICKEL AND COBALT BASE ALLOYS CONTAINING CHROMIUM BY CONTACTING AN ALLOY OF THIS TYPE WITH A HEAT DECOMPOSABLE LANTHANUM COMPOUND AT ELEVATED TEMPERATURE.

Description

United States Patent O 3,592,681 METAL SURFACE TREATING PROCESS BY USE OF LANTHANUM COMPOUNDS Henri Hatwell and Willy Robert De Sutter, Brussels, Belgium, assignors to Cabot Corporation N Drawing. Filed May 15, 1968, Ser. No. 729,400 lint. Cl. C230 3/ 04, 9/00 US. Cl. 117-50 13 Claims ABSTRACT OF THE DISCLOSURE Method for improving the oxidation resistance of iron, nickel and cobalt base alloys containing chromium by contacting an alloy of this type with a heat decomposable lanthanum compound at elevated temperature.

BACKGROUND OF THE INVENTION This invention relates to the treatment of metal surfaces to improve the oxidation resistance thereof. More particularly, the present invention is directed to a method for treating the surfaces of chromium-containing cobalt, nickel and iron base alloys with lanthanum compounds to improve the oxidation resistance of such alloys without significantly adversely affecting the mechanical properties of the alloys.

Cobalt, nickel and iron base alloys have been widely used in the past in high temperature applications, such as for example, turbine blades which require high strength at temperature, and these alloys have been found to be, in general, highly effective. However, it is very important, in many high temperature situations, such as that mentioned above, that the materials employed be also highly resistant to oxidation.

To meet this requirement, it has been proposed to incorporate lanthanum in certain molten alloys whereby the resulting cast or wrought material is improved with respect to oxidation resistance. However, depending on the alloy composition, alloyed lanthanum can lead to the formation of intermetallic phases which can adversely affect the forgeability of the material.

It is therefore an object of the present invention to provide a method for improving the oxidation resistance of cobalt, nickel and iron base alloys Without significantly adversely aifecting the mechanical properties of the alloys.

Other objects will be apparent from the following description and claims.

A method in accordance with the present invention for improving the oxidation resistance of cobalt, nickel and iron base alloys comprises contacting a substrate of cobalt, nickel or iron base alloy containing from about 15 to 25% chromium with lanthanum oxide or a heat decomposable lanthanum salt at a temperature sufiicient to cause lanthanum to be incorporated in the alloy substrate.

By the practice of the present invention, a wide range of cobalt, nickel and iron base alloys can be improved with respect to oxidation resistance. Such alloys contain, in addition to Co, Ni and Fe, and from about 1525% Cr up to 15% W, and may contain minor proportions of elements ccustomarily present in Co, Ni and Fe base alloys. The following Table I shows specific alloys which can be treated by the practice of the present invention.

Table I 22% Cr, up to 1% W, 18% Fe, up to 1% Si, 2% Co,

9% Mo, up to 1% Mn, up to 0.15% Cr, bal. Ni.

20% Cr, 15% W, up to 3% Fe, up to 1% Si, 10% Ni,

1.5% Mn, up to 0.15% C, bal. Co.

21% Cr, 14% W, 2% Fe, 22% Ni, 0.1% C, 0.3% Si,

bal. Co.

In addition to lanthanum oxide, lanthanum salts which are decomposable by heating at temperatures below the melting point of the alloy to be treated, to provide a gaseous reaction product can also be used in the practice of the present invention. A preferred salt is lanthanum nitrate, La(NO .6H O which can be employed in the practice of the present invention in the form of a sprayquench solution as hereafter more particularly illustrated. Other useable soluble lanthanum salts which can be employed in the form of a spray or quench solution are lanthanum acetate, lanthanum bromate, lanthanum bromide, lanthanum chloride, lanthanum iodate, lanthanum iodide and lanthanum sulfate. Insoluble lanthanum salts such as lanthanum carbonate, lanthanum molybdate and lanthanum oxalate can be used in the form of slurries in the practice of the present invention as hereinafter described.

In the case of lanthanum oxide, this material is used in the form of a slurry of La(OH) In a particular embodiment of the present invention, an aqueous solution of lanthanum salt is prepared, for example, by dissolving La(NO .6H O in water. The alloy material to be treated, suitably in strip form, is heated in an inert environment to a temperature sufficient to decompose the lanthanum salt, e.g. about 1100 C. and then plunged into and quenched in the lanthanum salt solution.

The resulting material, in which lanthanum is incorporated as a result of the aforedescribed treatment, exhibits a remarkably increased resistance to oxidation.

In the embodiment as aforedescribed involving the quenching of an alloy substrate in a lanthanum salt solution, the preferred temperature range for the heated metal substrate is from about 300 C. to 1200 C. and the lanthanum concentration in the solution is suitably at least about 2%. Treatment of alloy substrates in the foregoing range provides for the incorporation of at least about 0.4 mg./cm. of lanthanum, and the lanthanum content can range up to 2 mg./cm. This lanthanum concentration ensures improved resistance to oxidation.

In a further embodiment of the present invention, a solution of lanthanum salt in the form of fine droplets is sprayed onto a heated cobalt base alloy, in an inert gas environment, whereby the lanthanum salt is decomposed and lanthanum is incorporated in the alloy substrate.

In this embodiment, salt solutions having the concentrations previously noted can be used and similar substrate temperatures are employed to produce as oxidation resistant material. Atomizer type apparatus can be used to apply the lanthanum salt solution to the heated alloy substrate and it has been found that increased amounts of lanthanum can be incorporated in the alloy substrate by spraying in this manner as compared to the quench technique previously described.

In the spray embodiment of this invention by spraying between about 0.4 to 2 milligrams of lanthanum per square cm. of substrate surface, a very highly oxidation resistant material is obtained.

In a still further embodiment of the present invention, a slurry containing heat decomposable lanthanum salt is applied, e.g. painted on the cobalt base alloy substrate to be heated and the thus prepared alloy is heated to an elevated temperature sufficient to decompose the lanthanum salt and cause lanthanum to be incorporated in the cobalt base alloy substrate.

By way of example, the slurry can be in the form of lanthanum hydroxide or a lanthanum nitrate solution with a suitable wetting agent added and the temperature range in which the slurry coated substrate is heated is suitably in the range of about 300* to 1200 C.

The following examples will further illustrate the present invention.

3 EXAMPLE I Wrought sheet samples 20 x 15 X 1.5 mm. were prepared from the following composition:

Cr Ni W Fe Si M La 00 A 20 20 Bal B 20 20 01 Bal A solution was prepared by dissolving 20 grams of La(NO .6H O in 50 grams of water. The samples were mechanically polished with 600 grit abrasive paper and then electrolytically polished. Each sample was then heated to 1100 C., and quenched; sample A was quenched in the prepared lanthanum salt solution and sample B was quenched in water. The heating and quenching 'was performed under argon atmosphere. After quenching the samples were rubbed lightly with filter paper and dried.

The weight gain of the samples quenched in the lanthanum solution was in the order of 0.4 milligram per cm.

The samples were then subjected to isothermal oxidation tests by exposure to dry air at 1000 C. for fifty hours. A sample A having the same composition as sample A, but which was not quenched in the lanthanum salt Was also tested for comparative purposes. The results are shown below:

Sample Reacted Spelled weight oxygen, oxide, change,

Sample lug/em. lug/cm. 1ng./em.

A: No La treatment 2. 5 0.4 2. 2 A: La quenched 0.14 0.11 0.08 B: No La treatment 0. 6 0.03 0. 56

The results of Example I show the improved oxidation resistance of alloys treated in accordance with the present invention as compared to untreated alloys and alloys containing lanthanum as an alloying constituent.

EXAMPLE II Samples were prepared as in Example I and a sample having the same composition A was placed in a chamber filled with argon and induction-heated to 900 C. Using an atomizer assembly, the lanthanum salt solution of Example I was sprayed on the samples. The temperature of the sample was increased to 1000-1050 C. and maintained in this range for two minutes after which the samles were permitted to cool to room temperature.

After being cooled, the sample was placed in boiling distilled water for 10 minutes and then rubbed with moist I filter paper and dried. The weight grain of the sample was on the order of 0.4 to 2 milligrams per cm.

The thus treated sample and untreated samples of compositions A and B were subjected to a cyclic oxidation test which involved minutes heating in a furnace to 1000 C. and 10 minutes out of the furnace during which the sample was cooled to about 50 C. in air. The test was continued for 100 hours, or 200 cycles. The results are shown in the table below:

Sample: Metal consumption mg./cm. ANo La treatment 66 BNo La treatment 61 A-La sprayed 9 A-La sprayed 12 200 hour data.

An interesting and important feature of the present invention is that the method of the present invention does not provide a lanthanum coating as such but a penetration of lanthanum into the cobalt base alloy substrate which becomes more pronounced upon exposure to an oxidizing atmosphere at elevated temperature. Thus, by further treating the material prepared as hereinabove described, to air or oxygen at a temperature of 1000 C. or above, the surface of the material becomes essentially free of lanthanum although the amount of lanthanum incorporated in the alloy substrate is not significantly decreased. As a result, exposure of the treated material to erosive environments does not significantly effect the long term oxidation resistance of the material as would be the situation in the case of a lanthanum coating or plating.

EXAMPLE III Alloy samples having the same composition as alloy A of Example I were treated with lanthanum solution following the procedure of Example II and then rolled to reduction at 1000 C. Specimens 20 x 51 x 2 mm. of the rolled alloy were cycled at 1000 C. for hours as described in Example II. The metal loss was 18 mg./ cm. For untreated specimens the metal loss averaged 50 mg./cm.

EXAMPLE IV The procedure of Example III was followed except that the treated alloy was cold rolled 45%. The metal loss of the treated specimens averaged 14 mg./cm. For untreated specimens the metal loss averaged 38 mg./cm.

EXAMPLE V A sample of essentially 100% nickel was treated with lanthanum solution following the procedure of Example II and cycled at 1000 C. for 48 hours. The metal loss averaged 37.5 mg./cm.

What is claimed is:

1. A method for improving the oxidation resistant properties of alloys selected from the group consisting of cobalt, nickel and iron base alloys containing from about 15% to 25% chromium which comprises contacting a heated substrate of the selected alloy with a liquid dispersion of a material selected from the group consisting of lanthanum oxide and heat decomposable lanthanum salts at a temperature sufiicient to cause lanthanum to be incorporated into the alloy substrate.

2. A method in accordance with claim 1 wherein the temperature is between about 300 and 1200 C.

3. A method in accordance with claim 1 wherein lanthanum is incorporated in the alloy substrate in an amount between about 0.4 and 2 mg./cm.

4. A method in accordance with claim 1 wherein the alloy substrate is heated to a temperature sufficient to decompose the selected lanthanum salt and is immersed in a solution of the selected lanthanum salt.

5. A method in accordance with claim 1 wherein the selected alloy substrate at a temperature from about 300 to 1200 C. is immersed in a solution of lanthanum salt.

6. A method in accordance with claim 5 wherein the concentration of lanthaum in the solution is at least about 2%.

7. A method in accordance with claim 1 wherein a solution of lanthanum salt is sprayed onto the selected alloy substrate, said substrate being at an elevated temperature sufficient to decompose the lanthanum salt.,

8. A method in accordance with claim 7 wherein the alloy substrate is at a temperature between about 300 and 1200 C.

9. A method in accordance with claim 7 wherein the lanthanum content of the spray is at least about 2%.

10. A method in accordance with claim 7 wherein between about 0.4 and 2 milligrams per cm. of lanthanum are sprayed onto a cobalt base alloy substrate.

11. A method in accordance with claim 1 wherein the alloy substrate is contacted with a slurry of heat decomposable lanthanum salt.

12. A method in accordance with claim 11 wherein the alloy substrate is heated to a temperature between about 300 and 1200 C.

13. A method in accordance with calim 12 wherein at least about 0.4 milligrams per cm. of lanthanum deposited on the alloy substrate.

FOREIGN PATENTS 12/1957 Great Britian.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.