US2381459A

US2381459A - Turbine bucket for exhaust turbine superchargers

Info

Publication number: US2381459A
Application number: US422421A
Authority: US
Inventors: Albert W Merrick
Original assignee: AUSTENAL LAB Inc
Current assignee: AUSTENAL LAB Inc; AUSTENAL LABORATORIES Inc
Priority date: 1941-12-10
Filing date: 1941-12-10
Publication date: 1945-08-07
Anticipated expiration: 1962-08-07

Description

Alli 7 .1945- A, w.. MERR|cK 2,381,459

TURBINE BUCK-EIT FOR EXHAUST TURBINE SUPERCHARGERS Y Filed Dec. 1o, 1941 INVENTOR Patented Aug. 7, 1945 v TURBINEBUCKET FOR EXHAUST TURBINE SUPERCHARGERS Albert W. Merrick, Ossining, N. Y., assignor to Austenal Laboratories, Inc., New York, N. Y., a corporation of New York Application December 10, 1 941, SerialNo. 422,421 (Cl. 253-77) 5 Claims. This invention relates, generally, to exhaust turbine superchargers forinternal combustion engines, and it has particular relation to an improved turbine bucket for superchargers of the class described.

'I'he invention may be embodied in a wide variety of forms of turbine buckets for use with a wide variety of exhaust turbine superchargers. For purposes of illustration I shall refer more or less generally to a diagrammatically illustrated form of exhaust turbine supercharger and to an illustrative form-of turbine bucket, but it is to be understood that the invention is not limited to use with the particular form of supercharger selected for illustration, nor to embodiment in the particular form of turbine bucket shown and described.

superchargers are required for high power output and are desirable for aircraft engines, for example, for take-olf power, and to compensate for the rare atmosphere at high altitudes. 'I'hey are also desirable forautomotive engines at high speeds, and Yfor Diesel engines for increased output.

'I'he rotors of these superchargers run at very high peripheral speeds-at substantially the speed of a riie bullet-and the turbine buckets operate within the path of the exhaust gases where the temperatures are very high-of the order of from about 1400" F. to 1500 F. The turbine buckets, therefore, are subject to very severe conditions, particularly in regard to temperature and the high stresses to which they are subjected as the res'ult of the action of centrifugal force.

Heretofore, forged and machined buckets have usually been employed, and these buckets have necessarily been formed of alloys which would permit the forging and machining operations.

With the improvement of aircraft and their engines, the necessity for better turbine buckets for exhaust turbine superchargers and better materials for such turbine buckets has become urgent.

The improvement of the present invention corn sists in forming the bucket by a casting process and of an alloy providing new and advantageous results not equalled by the alloys previously used in making such bucket, and which alloy it would be hopeless to consider for a forged and machined bucket on account of the hardness and difficulty of machining and impracticability of forging the same.

More specifically, the improvement of the present invention consists in providing a cast turbine bucket for exhaust turbine superchargers,

such bucket being formed of a cobalt-chromium alloy, and, more particularly, formed of a cobalt, chromium, molybdenum alloy With the constituents combined and` proportioned in a. manner better to withstand the severe conditions to which such buckets are subjected.

Further and 'more specific features and advantages of the invention will appear from the following detailed description taken in connection with the accompanying drawing, in which:

Figure 1 is a diagram of one form of an exhaust turbine supercharger with which the turbine' buckets of the present invention are adapted to be used;

Figure 2 is a fragmentary view showing one illustrative manner in which the turbine buckets may be attached to the periphery of the 'rotor or turbine`whee1;

Figure 3 is a back view of one of the turbine buckets;

FigureA is a front view of the turbine bucket The exhaust turbine supercharger comprises a combined turbine and compressor shaft I3 having fixed thereon the rotor or turbine wheel I4 and an air impeller I5. 'I'he turbine buckets I6 are fastened, as will hereinafter appear, to the periphery of the rotor or turbine wheel I4 and operate Within the path of the exhaust gases which serve as the motive uid for turning the turbine wheel and thereby the shaft I3 and air impeller I5. In aircraft engines, the exhaust gases will drive the turbine wheel at very high peripheral speed, commonly from seven to twelve times crankshaft speed, or at substantially the speed of a rifle bullet.

'I'he air impeller I5 operates Within the impeller housing I8 which has an air compressor inlet I9 and an air discharge 20 to the carburetor induction pipe 2 I. The air impeller I 5'forces air through the carburetor shown diagrammatically at 22, and the explosive mixture is delivered from the carburetor through the intake II into the cylinders of the engine, one of which cylinders is shown'. The carburetor may, of course, be located before the supercharger instead of after the same, as shown in the drawing. The intake into the cylinder of the engine is controlled by the usual or any suitable intake valve 23, and

the exhaust from the cylinder of the engine is controlled by the usual or any suitable exhaust lvalve 24.

' preferably cast from an alloy whose essential or principal ingredients are cobalt and chromium. In the broader aspects of the invention, the cobalt is present as the principal ingredient, and more specifically, in amount more than 50%, and the chromium is present to the extent of from approximately to approximately 40%.

For a full understanding of the various alloys from which I contemplate, within the broader aspects of the invention, casting or forming the turbine buckets I6, attention is directed to the alloys more fully described in Charles H. Prange Reissue Patent No. 20,877, reissued October 4, 1938; also to Charles H. Prange Patent No. 2,135,- 600, patented November 8, 1938, and to Charles H. Prange Patent No, 2,180,549, patented November 21, 1939.

One preferred form of alloy from which highly satisfactory turbine buckets have been made is substantially as follows:

Per cent Cobalt 63.0 Chromium 30.0 Molybdenum 6.0 Silicon 0.25 Manganese 0.50 Carbon 0.25

In addition, there are likely to be small quantities of iron and nickel, but these are simply impurities and are not introduced purposely.

As a commercial specification, substantially the following range of compositions would cover this latter alloy as it could be produced on a commercial basis:

Percent Cobalt 60.0 to 65.0 Chromium 28.0 to 32.0 Molybdenum 5.0 to 7.0 Silicon .10to 1.0 Manganese .20 to 1.0 Carbon .00to .50

The physical properties of the preferred form i of turbine bucket alloy above set forth as exemplified by actual tensile tests are as follows:

Ultimate strength lbs. per sq. inch 110,000 114,000

Yield point 0- 71,000 Elongation per cent" 12.0 14.0 Reduction of area do 14.0 12.0

The above tests are at room temperature. At

These figures show the excellent properties of the composition in the cast condition, the one set at room temperature and the other at 1500 F. This is illustrative of the excellence of the alloy for the high temperature conditions to which turbine buckets for exhaust turbine superchargers are subjected.

The mold and method for its production disclosed in Arthur B, Ray Patent No. 2,027,932, patented January 14, 1936, as well as the casting procedure disclosed in Charles H. Prange reissue patent Reissue No. 20,877, reissued October 4, 1938, and the casting investment material and process disclosed in Charles H. Prange Patent No. 2,180,549, patented November'21, 1939, are highly suitable and advantageous in the casting of the turbine buckets of the present invention, and reference is hereby incorporated herein for the further details 'of these casting investment materials and processes as casting investment materials and processes suitable for casting the turbinel buckets of the present invention.

The founding apparatus and method disclosed in the present applicants prior Patent No. 2,125,- 080, patented July 26, 1938, are also highly suitable and advantageous in the casting of the turbine buckets of the present invention, and reference to that patent is hereby incorporated herein for the further details of the founding apparatus and method as the same may be used in connection with the present invention.

I find that the characteristics of the alloy, when applied to turbine buckets for exhaust 'turbine superchargers for internal combustion engines, impart greatly desired properties not provided in the turbine buckets of the prior art.

Most alloys suffer a very marked reduction in strength at elevated temperatures, but a turbine bucket made from the alloy described herein retains a proportionately greater strength at such temperatures.

A further aspect is the matter of creep" strength. `This refers to the gradual stretching of a metal under stress at elevated temperatures which ultimately results in failure. It differs from the matter of hot strength" in that the time element is involved. Turbine buckets formed of an alloy as herein disclosed have excellent resistance to creen This should not be confused with red hardness which is not the same as hot strength.

Turbine buckets formed of an alloy as herein disclosed also have great resistance to oxidation and corrosion. The exhaust gases of an airplane engine are of high temperature and contain corrosive products such as sulphur compounds and possibly also anti-knock compounds. At any rate, turbine buckets of the class described are subjected to severe oxidizing and corroding conditions, and it has been found that buckets formed of an alloy as herein disclosed stand up exceln lently in this type of service. On tests, they have shown no deterioration other than a slight surface discoloration, whereas buckets made out of other alloysfor instance, nickel alloyshave shown definite deterioration.

As a further aspect, there is the matter of resistance to erosion by hot gases. Hot gases and vaporsas for instance, steam, have a tendency to wear away metal by their mechanical action. In other words, they cause erosion. Where this is combined with the effect of corrosion and oxidation, as in turbine buckets of the class described, the effect may be quite severe. Buckets formed of an alloy as herein disclosed have excellent resistance to4 erosive influences.

There are also other important aspects-for instance, the matter of slight shrinkage upon solidifying-Which enter into the making of sound turbine bucket castings.

Moreover, the alloy bucket as herein disclosed h'asa very high degree of resistance to repeated ance to fatigue breakage is related to the favorable damping characteristics of the alloy in its cast form. In this connection it will be noted that, in general, cast metals have a greater damping effect than forgedor machined metals.

Referring to the bucket as a oast product, it is important to note that cast buckets which have been made h'ave proved to be sound, in general, whereas the forged buckets have given a great deal of trouble due to internal defects, such as forging cracks which are hard to discover by X-ray. The Y alloys which are suitable for turbine buckets on account of their strength and hardness at high' temperatures are generally hard to handle by forging. Consequently, there is a great likelihood of cracks in forged and machined buckets.

Buckets made in accordance with the present invention have been tested by X-ray and have been found to be uniformly sound and reliable. Forged buckets, on the oth'er hand, sometimes have concealed defects which are difllcult to vdiscover, even by X-ray, such as internal cracks resulting from the forging operations. In an article, such as a turbine bucket for exhaust turbine superchargers, this, of course, is a very serious objection. As a result of the casting method, fthe accuracy of the bucket is suchI that machining and grindying are reduced to an extremely small amount.

With the low carbon content as set forth in connection with the preferred forms of alloy, tough cast buckets are produced. With this low carbon content the molybdenum content is believed to makev up the necessary strength and stiffness. Toughness and cold ductility are highly advantageous properties in buckets of th'e class described. y

With the low carbon content as set forth-in connection with the preferred forms of alloy, the resulting buckets appear to be better on account of there being less likelihood of internal structural changes under the conditions of service to which such buckets are put.

I am unable to state with certanty all considerations in connection with the present invention, and therefore I reserve the right to supplement and correct any considerations herein set forth. For example, it is conceivable that with higher carbon content, carbide preci'plation may occur through the action of high temperature and time which would result in embrittlement of the bucket.

As will appear from Charles H.y Prange Patent No. 2,135,600, molybdenum may be substituted for carbon, and a` substantially carbonless alloy of high strength and corrosion resistance may be made by increasing the percentage of molybdenum. As indicated in the last mentioned patent, molybdenum, to excess, will cause both brittleness and high melting points.

There are other considerations in connection with carbon about which I am not fully aware. For instance, it is conceivable that a composition having a substantially zero carbon content might not be stable when exposed to exhaust gases which contain carbon monoxide and carbon dioxide at highl temperatures. In' short, a carburizing effect may occur, so that th'e carbonless alloy would take up carbon gradually and become brittle. It is possible in Athis connection that the preferred forms of alloy-and particularly the alloy containing about .25% carbonis an exceptionally favorable one forthe purposes of the present invention in that having some carbon in it, it would be less likely to take up more carbon and thereby become brittle.

It is to be understood that tungsten may operate in a, similar role to molybdenum as replacing carbon, increasing amounts adding strength and stiffness, and, of course, there is the limitation as expressed in the patents h'ereinbefore identified that excesses of tungsten will produce brittleness and diillcult melting.

As between molybdenum and tungsten, it is felt that molybdenum is preferable. With it, it is possible to retain a greater degree of toughness or ductility while at the same 'time attaining strength and stiffness. Further-incre, it is found that the molybdenum'alloy has somewhat great corrosion resistance than th'e tungsten alloy.

'Ihe resulting buckets have not only accuracy and the other properties herein set forth, but they are smooth of surface. This results mainly from the character of the alloy and the method of spraying on a thin coat of investment over the Wax pattern before investing it. The investment herein referred to is inert so that no carburization or decarburization or sulphidization occurs.

Referring again to the drawing, the cast buckets I8 selected for illustration have concave fron-t or leading surfaces 28 arcuate in, transverse section as shown in Figure 5 and extending radially from th'e periphery of the rotor or turbine Wheel I4 when the buckets are applied th'ereto. The back surface 29 of the bucket follows generally the contour of the concave fron't surface 2,3. The inner end of the bucket has an enlarged flattened neck or tongue 30 adapted to enter a transverse slot 3l in the periphery ofthe turbine wheel I4. Extending transversely along th'e inner end of the neck or tongue 30 is an integral rounded head or enlargement 32 which enters a corresponding enlargement 33 at the inner end of the slot 3| for fastening the blade or bucket in place on the periphery of the wh'eel I4. Alternate buckets I6 preferably have long and short necks 3|) as shown in Fig-ure 2, in order to avoid weakening of the wheel Il, a continuous circle of the buckets I6 being mounted around the periphery of the wheel as shown.

The outer ends of the buckets I 6 have generally rectangular end walls 3l which cooperate, as shown in Figure 2, when the buckets are in place on the wheel. Transverse ribs 35, one integral with the back of each bucket, seat in and interlock with transverse grooves 36, one in the front of each bucket when the buckets are in place in the periphery of the wheel.

The circle shown in dotted lines at 38 in Figure 3 indicates the position of small circular projections which are the remains of the sprues where they have been cut off. These small circular projections are preferably ground oif or removed, and, therefore, are not shown in Figure 2.

The embodiment of the invention shown in the drawing is for illustrative purposes only, and it is to be expressly understood that said drawing and the accompanying specification are not to be construed as a definition of the limits or scope of the invention, reference being had to Attempts have been made to make turbine buckets by powder-metallurgy-that is. by pressing and sintering. So far, these attempts have not been successful because they have not been able to get the required properties. 'I'he alloy herein disclosed in cast form is strongerv and does not have the minute porosity found in articles molded from powder. One disadvantage of this porosity is a decreased thermal conductivity which may be important in an air-cooled bucketthat is, one having an internal cooling passage.

I also contemplate,wlthin the scope oi' the appended claims, casting the Whole turbine wheel in one piece, that is, casting the wheel and the bucket as an integral or unitary construction.

I claim:

1. As a new article of manufacture, a turbine bucket for a turbine wheel in which the bucket is directly exposed to a high temperature high velocity stream of combustion gases for driving the wheel at high peripheral speed, said bucket being cast of a cobalt-chromium alloy containing cobalt 50% to 70%, chromium 20% to 40%,

molybdenum 3% to 7%, and carbon up to 0.5%, said alloy bucket being practically'incapable of being machined and Worked and having high tensile strength and high resistance to corrosion and erosion by the combustion gases at temperatures on the order of 1500 F.

2. As a new article of manufacture, a turbine bucket `for a turbine wheel in which the bucket is directly exposed to a high temperature high velocity stream of combustion gases for driving the wheel at high peripheral speed, said bucket being cast of a cobalt-chromium alloy containing cobalt 50% to 70%, chromium 20% to 40%, metal from the group consisting of molybdenum and tungsten 3% to 7%, and carbon up to 0.5%, said alloy bucket being practically incapable ot being machined and worked and having high tensile strength and high resistance to corrosion and erosion by the combustion gases at temperatures on the order of 1500 F.

3. As a new article of manufacture, a turbine bucket for a turbine wheel in which the bucket is directly exposed to a high temperature high velocity stream of combustion gases for driving the wheel at high peripheral speed, said bucket being composed of an alloy containing cobalt to 70%, chromium 23% to 32%, molybdenum up to 7%, and carbon up to 0.5%, said alloy bucket being resistant to the high temperatures and accompanying high stresses at the periphery of the turbine wheel.

4. As a new article of manufacture, a turbine bucket for a turbine wheel in which the bucket is directly exposed to a high temperature high velocity stream of combustion gases for driving the wheel at high peripheral speed, said bucket being cast of an alloy containing cobalt 60% to '70%, chromium 23% to 32%, molybdenum up to 7%, and carbon up to 0.5%, said alloy bucket being resistant to the high temperatures and accompanying high stresses at the periphery of the turbine wheel.

5. A turbine bucket for a turbine wheel in which the bucket is directly exposed to a high temperature high velocity stream of combustion gases for'driving the wheel at high peripheral speed, said bucket being composed of an alloy containing cobalt approximately chromium approximately 27.5%, molybdenum 5% to 6%, and carbon approximately 0.25%, said alloy bucket being resistant to the high temperatures and accompanying high stresses at the periphery of the turbine wheel.

` ALBERT W. MERRICK.