AUSTENAL LAB
Publication
Number: US2381459A
Publication
Date: 1945-08-07
Priority
Number: US1941422421A
Application
Date: 1941-12-10
Title:
Turbine bucket for exhaust turbine
superchargers
Inventor
- w/address: MERRICK ALBERT W
Assignee/Applicant:
AUSTENAL LAB INC
TURBINE BUCKET 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, 1941,
Serial No. 422,421 1 5 Claims.(Cl. 253-77) This
invention relates, generally, to exhaust turbine superchargers for internal I
combustion engines, and it has particular relation to an improved turbine
bucket for superchargers of the class, described.
The 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-off power, and to compensate for the rare atmosphere at high
altitudes. They are also desirable for. automotive
engines at high speeds, and for Diesel engines for increased output. Me rotors
of these superchargers run at very high peripheral speeds-at substantially the
speed of a rifle 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 result 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 consists in forming the bucket by a casting process and of an
alloy providing new and advantageous results not equaled 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 wheel;
Figure 3 is a back
view of one of the turbine buckets;
Figure 4 is a front
view of the turbine bucket shown in Figure 3; and Figure 5 is a transverse
section taken on the line 5-5 of Figure 4.--In the diagrammatic showing in
Figure 1, the internal combustion engine, which may be an aircraft engine, comprises
a cylinder 10 having an intake 11 and an exhaust 12.
The exhaust turbine
supercharger comprises a combined turbine and compressor shaft 13 having fixed
thereon the rotor or turbine wheel 14 and an air impeller 15. The turbine
buckets 16 are fastened, as will hereinafter appear, to the periphery of the
rotor or turbine wheel 14 and operate Within the path of the exhaust gases
which serve as the motive fluid for turning the turbine wheel and thereby the
shaft 13 and air impeller 15. 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. The
air impeller 15 operates within the impeller housing 18 which has an air
compressor inlet 19 and an air discharge 20 to the carburetor induction pipe 2
1. The air impeller 15'forces air through the carburetor shown diagrammatically
at 22, and the explosive mixture is delivered from the carburetor through the
intake 11 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 valve 24.
The turbine buckets 16
which, as already pointed out, operate in the exhaust pipe 12 where they are
subjected to very severe conditions are 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 10% 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 16,
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:
Per cent
Cobalt--------------------------60.0 to 65.0
Chromium----------------------28.0 to 32.0
Molybdenum--------------------5.0 to 7.0
Silicon--------------------------.10 to 1.0
Manganese----------------------. 20 to 1.0
Carbon-------------------------.00 to.50
The physical properties of the preferred form of turbine
bucket alloy above set forth as exemplified by actual tensile tests are as
follows: (See PDF)
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 arid 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 turbine
buckets of the present invention.
The founding apparatus
and method disclosed in the present applicant's 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 "creep." 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
excellently in this type of service. On tests, they have shown no deterioration
other than a slight surface discoloration, whereas buckets made out of other
alloys-for instance, nickel alloys-have shown definite deterioration.
As a further aspect,
there is the matter of resistance to erosion by hot gases. Hot gases and
vapors-as, 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 to 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 has a very high degree of resistance to repeated
stresses. In other words, it is highly resistant to fatigue failure. This is
important in a turbine bucket of the class described which is highly stressed
and operates at extremely high rates of speed, and which-may be subject to
vibrations, synchronous or otherwise. Probably the resistance 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 forged or machined metals.
Referring to the
bucket as a cast product, it is important to note that cast buckets which have
been made have 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 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
oth1er hand, sometimes have concealed defects which are difficult to discover,
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, the accuracy of the bucket is such that machining and grinding
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 make up the necessary strength and stiffness.
Toughness and cold
ductility are highly advantageous properties in buckets of the class described.
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 certainty 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 precipitation may occur
through the action of high temperature and time which would result in embrittlement of the bucket.
As will appear from
Charles H. 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 high temperatures. In short, a
carburizing effect may occur, so that the carbonless alloy would take up carbon
gradually and become brittle. It is Possible in this connection that the
preferred forms of alloy-and particularly the alloy containing about 0.25% carbon
is an exceptionally favorable one for the purposes of the present Invention In
that having some carbon in it, it would be less Iikely
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 hereinbefore identified
that excesses of tungsten will produce brittleness and difficult 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. Furthermore,
it is found that the molybdenum alloy has somewhat great corrosion resistance
than the tungsten alloy.
The 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 I s inert so that no carburization
or decarburization or suIphidization occurs.
Referring again to the
drawing, the east buckets 16 selected for illustration have concave front or
leading surfaces 28 arcuate in, transverse section as
shown in Figure 5 and extending radially from the
periphery of the rotor or turbine wheel 14 when the buckets are applied
thereto. The back surface 29 of the bucket follows generally the contour of the
concave front surface 28. The inner end of the bucket has an enlarged hattened neck or tongue 30 adapted to enter a transverse
slot 31 in the periphery of the turbine wheel 14. Extending transversely along
the 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 31 for fastening the blade or bucket in place on the periphery of the
wheel 14. Alternate buckets 16 preferably have long and short necks 30 as shown
in Figure 2, in order to avoid weakening of the wheel 14,
a continuous circle of the buckets IS being mounted around the periphery of the
wheel as shown.
The outer ends of the
buckets 16 have generally rectangular end walls 34 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 cutoff. These small circular projections are preferably ground off 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 the appended claims for that purpose.
For example, different
designs of buckets attached to the wheel by various types of welding, both
fusion and resistance, are contemplated within the
scope of the Present invention. As a matter of fact, the alloys herein
disclosed are favorable for welded Construction because they are not subject to
carbide precipitation and consequent embrittlement
and lowered corrosion resistance, as is the case with certain other alloys, as,
for instance, stainless steel.
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. The alloy herein disclosed in
cast form Is stronger 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 io bucket-that
is, one having an internal cooling passage.
I also contemplate,
within the scope of 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.
ALBERT W. MERRICK
Claims:
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 2,381,459 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.
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 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.
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 65%, 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.