US2398678A - High strength alloy for use at elevated temperatures - Google Patents

Description

Patented Apr. 16,1946

ELEVATED TEMPERA TUBES Rudolf H. Thielemann, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York No Drawing. Application March 1, 1941, Serial No. 381,378

8 Claims. (Cl. 75-170) The present invention relates to alloy which have high strength at temperatures up to and above 1500 F.

In the operation of supercharger turbines and similar equipment temperatures above 1500 F. are commonly encountered. These turbines operate at speeds up to about 30,000 revolutions per minute. Because of the high stresses produced at those speeds it is essential that the alloys employed in the construction of such turbines. have high strength at the operating temperatures.

It is one of the objects of the present invention to provide-an alloy which can be readily fabricated and which is particularly suitable for use as a bucket material in supercharger turbines. Because of its high strength and metallurgical stability at elevated temperatures, the alloy may also be employed to advantage in such applica tions as gas turbine buckets, turbine packing fer to employ an alloy containing, about 4 to 6% tungsten, about 2.5 to 3.5% titanium, about 13 to 18% molybdenum, about to 15% cobalt, a

. nickel content in the neighborhood of 55% with the balance substantially all iron. Carbon, mansanese and silicon may be present in small quantitles as hereinbefore pointed out. However, the total quantity of such elements preferably is less than 3%. The cobalt content of the alloy may be quite high, However, for economical reasons the quantity employed generally should not exceed about 10%. Very satisfactory results have been obtained with a composition containing 15% molybdenum, 5% tungsten, 3% titanium, 5% cobalt, 55% nickel and the balance iron with minor additions of manganese and silicon.

To obtain the desired properties in the alloy, it is precipitation hardened. This treatment comprises two separate operations. The alloy is first springs, engine valves, filament supports and the like. Heretofore commerciall availablealloys have failed to meet the supercharger turbine requrements due to one or more of the following reasons:

(1) They do not have the necessary strength at the high operating temperatures.

(2) They do not have the necessary surface and metallurgical stabilit at the high operating temperatures and as a result are subject to severe oxidationand scaling.

(3) They cannot be fabricated by the usual methods of forging. rolling and machining.

My improved alloy meets all these requirements. It is a nickel base precipitation hardenable, alloy and contains about .30% to 2% manganese} about .20 to 1% silicon, about 2 to 10% tungsten, about 2 to 4% titanium, about 8 to 22% molybdenum, about an, cobalt, about 55% of the alloy.

of nickel with the balance substantially all iron. The manganese and silicon serve to deoxidize the melt and to obtain a clean heat. Manganese also improves the forgeability of the alloy and it is preferable to employ the higher percentages of this ingredient for that purpose. A small quantwo hours is sufiicient.

tlty of carbon may be present, but in general this should not exceed 0.15% and preferably should not exceed 0.10%.

While satisfactory results may be obtained with the above range of alloying elements, I preheated to about 1050 C. to l250 0., preferably about 1100 C.,-for suflicient time to efiect an equilibrium condition in the solid solution state. The time the alloy is held at this temperature will vary with the size of the charge and also with the grain size desired in the alloy. Usually a heating period of about two hours is sufllcient to heat the alloy throughout and establish equilibrium of the solid solution. The alloy is then quenched from the solution temperature preferably in oil or water and then hardened by reheating at a temperature between about 600 C. and 850 C. preferably 750 C. for a period of time which will depend upon the hardness and ductility desired. Generally a heating period of about 7 During the hardening treatment a finely dispersed compound of titanium is precipitated from the solid solution condition. The amount, size and distribution of these precipitated particles determine the properties The alloy may be melted in either the inducti'on or the electric ar furnace. To insure clean,

sound ingots, the nickel, molybdenum, tungsten and cobalt additions preferably should be melted first. The heat is then deoxidized by adding about 1 oz. of aluminum for every pounds of alloy. The manganese andsilicon are now added to the melt and followed by the titanium which is added either as nickel-titanium or ferro-titanimn. As soon as the titanium addition has ferro-titanium metal employed preferably should be low in nitrogen content.

As the alloy has high strength at very hightemperatures, it is somewhat diiiicult to forge. The most satisfactory results are obtained with small ingots having a cross section not greater than about four inches square. .The most desirable' forging temperature range is from about 1150 C. to 1200 0. Care should be taken during the first few passes to use only light blows so as to avoid cracking. when the "as cas ingot structure is broken up subsequent working becomes easier.

When manufactured in accordance with the above process, my improved alloy has good oxidation resistance at temperatures up to about 1650 F. After forging and heat treating, the short time tensile strength with a testing cross head speed of .05 inch per minute will be between 100,000 and 130,000 pounds per square inch at 1500 F. The alloy maintains its metallurgical stability up to a temperature of about 1650 F. In the form of forged buckets, the average short time tensile strength at 1500 F. is about 115,000

poundsper square inch. After 500 hours at this temperature,- the average strength is about l05,-

' 000 pounds per square inch.

The following table indicates the rupture Stresses that will produce 1% of creep for the same periods of time may be determined by dividing the corresponding rupture stress by about 1.5. My improved alloy, as hereinbefore set forth, can

' be forged, swaged, and machined and the above tests indicate the strength and stability of the alloy under, elevated temperature conditions. What I claim as new and desire to secure by Letters Patent of the United States is: Y

1. A precipitation hardened alloy subject to stress at elevated temperatures, said alloy containing up to 0.15% carbon, about .30 to 2.00% manganese, .20 to 1% silicon, 2 to 10% tungsten, 2 to 4% titanium, 8 to 22% molybdenum, 3 to 20% cobalt, about 55% nickel and the balance substantially all iron.

2. A nickel base precipitation hardening alloy containing about 4 to 6% tungsten, about 2.5 to 3.5% titanium, about .13 to- 18% molybdenum, about 5 to 15% cobalt, about 55% nickel with the balance substantially all iron.

3. An alloy which is characterized by its good oxidation resistance, metallurgical stability, and load carrying ability at elevated temperatures up to 1650 F., said alloy containing a plurality of ingredients of which the following in the proportions stated are the only ones necessary to obtain said characteristics:

Per cent Tungsten 2 to 10 Titanium 2 to 4 Molybden 8 to 22 Cobalt 3 to 20 Nickel About 55 Balance substantially all iron.

4. A forgeable and machinable alloy which is characterized by its good oxidation resistance, metallurgical stability, and long time load carry- -ing ability at elevated temperatures up to 1650 F., said alloy containing a plurality of ingredients of which the following, in the proportions stated, are the only ones necessary to obtain said characteristics:

Per cent Tungsten 4 to 6 Titanium 2.5 to 3.5 Molybdenum 13 to 18 Cobalt 5 to 15 Nickel About 55 Balance iron.

Per cent Nickel 55 Molybdenum l5 Tungsten 5 Cobalt 5 Titanium 3 Balance iron.

6. A precipitation hardened alloy which in normal operation is subjected to high stresses at temperatures in excess of 1200 F., said alloy being characterized by its resistance to oxidation, its high tensile strength, its high rupture strength and resistance to creep under such conditions, sai alloy containing up to .10% carbon, up to 2% manganese, up to 1% silicon, 2 to 10% tungsten, 2 to 4% titanium, 8 to 22% molybdenum, 3 to 20% cobalt, about 55% nickel, with the balance substantially all iron.

7. A precipitation hardened alloy containin about 5% tungsten, 3% titanium, 15% molybdenum, 5% cobalt, about 55% nickel with remain- I der substantially all iron.

,to 2% manganese, .20 to 1% silicon, 2 to 10% tungsten, 3% titanium, 8 to 17% molybdenum, 3

to 20% cobalt, about 55% nickel with the balance substantially all iron, which comprises quenching the alloy from 1050 to 1250 C. and thereafter drawing the alloy at a temperature of from 650 to 850 C.

RUDOLF H.