U.S. patent application number 10/152538 was filed with the patent office on 2002-11-28 for apparatus for providing a residual stress distribution in the surface of a part.
This patent application is currently assigned to Surface Technology Holdings, Ltd.. Invention is credited to Prevey, Paul S. III.
Application Number | 20020174528 10/152538 |
Document ID | / |
Family ID | 24055085 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020174528 |
Kind Code |
A1 |
Prevey, Paul S. III |
November 28, 2002 |
Apparatus for providing a residual stress distribution in the
surface of a part
Abstract
The present invention is a novel method and an apparatus for
implementing the method of inducing a layer of compressive residual
stress along the surface of a part comprising the steps of
selecting a region of the part to be treated; selecting the
magnitude of compression and the residual stress distribution to be
induced in the surface of the selected region of the part; exerting
pressure against the surface of the selected region, the pressure
being applied in a selected pattern along the surface to form zones
of deformation having a deep layer of compressive stress; and
varying the pressure being exerted against the surface to produce
the desired residual stress distribution and magnitude of
compression within the surface.
Inventors: |
Prevey, Paul S. III;
(Cincinnati, OH) |
Correspondence
Address: |
Mark F. Smith
SBTECHNOLOGY
7577 Central Park Boulevard
Suite 102
Mason,
OH
45040
US
|
Assignee: |
Surface Technology Holdings,
Ltd.
|
Family ID: |
24055085 |
Appl. No.: |
10/152538 |
Filed: |
May 21, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10152538 |
May 21, 2002 |
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09516328 |
Mar 1, 2000 |
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6415486 |
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Current U.S.
Class: |
29/90.01 |
Current CPC
Class: |
B24B 39/06 20130101;
B23P 9/02 20130101; C21D 7/04 20130101; Y10T 29/47 20150115; B24B
55/02 20130101; C21D 7/08 20130101; B24B 39/00 20130101 |
Class at
Publication: |
29/90.01 |
International
Class: |
B21C 037/30 |
Claims
What is claimed is:
1. A method of inducing a layer of compressive residual stress in
the surface of a part comprising the steps of: selecting a region
of the part to be treated; selecting the magnitude of compression
and the residual stress distribution to be induced in the surface
of the selected region; exerting pressure against the surface of
the selected region, the pressure being applied in a selected
pattern along the surface to form zones of deformation having a
deep layer of compressive stress; and varying the pressure being
exerted against the surface to produce the desired residual stress
distribution and magnitude of compression within the surface.
2. The method of claim 1 whereby the pressure being exerted against
the surface of the part is performed by a burnishing operation.
3. The method of claim 1 wherein said pressure being exerted on the
surface of the part induces a deep layer of compression within the
surface having associated cold working of less than about 5.0
percent.
4. The method of claim 1 wherein said pressure being exerted on the
surface of the part induces a deep layer of compression within the
surface having associated cold working of less than about 3.5
percent.
5. The method of claim 1 further wherein the step of selecting the
magnitude of compression includes the step of programming a control
unit to automatically adjust the magnitude of compression being
induced within the surface of the part.
6. The method of claim 1 wherein the selected pattern along the
surface varies the spacing between the zones of deformation.
7. The method of claim 1 wherein the step of exerting pressure
against the surface of the selected region includes the step of
programming a control unit to control the application of said
pressure.
8. The method of claim 1 wherein the step of varying the pressure
being exerted against the surface includes the step of inducing a
more shallow layer of compressive stress within the surface of the
part.
9. The method of claim 1 further comprising the step of removing a
layer of material along the surface being in low compression or
tension.
10. The method of claim 1 wherein the part is selected from the
group consisting of automotive parts, aircraft parts, marine parts,
engine parts, motor parts, machine parts, drilling parts,
construction parts, pump parts, and parts for use in
turbo-machinery.
11. A method of inducing a layer of compressive stress in the
surface of a part comprising the steps of: selecting a region of
the part to be treated; selecting the magnitude of compression and
the residual stress distribution to be induced in the surface of
the selected region; programming a control unit to pass a
burnishing member of a burnishing apparatus over the selected
region in the selected pattern to produce a zone of deformation
having a deep layer of compression within the surface; and
programming the control unit to increase, decrease or maintain the
pressure being exerted against the surface at selected points along
the selected pattern to obtain the desired residual stress
distribution and magnitude of compression within the surface.
12. The method of claim 11 wherein said pressure being exerted on
the surface of the part induces a deep layer of compression within
the surface having associated cold working of less than about 5.0
percent.
13. The method of claim 11 wherein said pressure being exerted on
the surface of the part induces a deep layer of compression within
the surface having associated cold working of less than about 3.5
percent.
14. The method of claim 11 wherein the burnishing apparatus
comprises means for automatically adjusting the pressure being
exerted against the surface of the selected region to increase on
the high points and decreases on the low points encountered by the
burnishing member along the surface of the part.
15. A method of inducing a layer of compressive stress in the
surface of a part comprising the steps of: selecting a region of
the part to be treated; selecting the magnitude of compression and
the residual stress distribution to be induced in the surface of
the selected region; programming a control unit of a burnishing
apparatus to perform a burnishing operation, the burnishing
operation being performed along the selected region in a selected
pattern to produce a zone of deformation having a deep layer of
compression within the surface having associated cold working of
less than about 5.0 percent; and performing a second operation to
induce a more shallow layer of compressive stress within the
surface of the part to produce the desired stress distribution;
whereby said burnishing apparatus further comprising means for
automatically adjusting the pressure being exerted against the
surface of the selected region that increases on the high points
and decreases on the low points that are encountered along the
surface of the part during the burnishing operation.
16. A burnishing apparatus for inducing a compressive stress in the
surface of a part comprising: a burnishing member; a socket having
a inner chamber and a seat for receiving said burnishing member;
means for applying a force against said burnishing member for
exerting pressure against the surface of the part; and means for
providing a constant volume of fluid to said inner chamber; wherein
said socket provides a clearance between said seat and said
burnishing member for permitting the fluid to pass.
17. The burnishing apparatus of claim 16 further comprising: a
pressure sensor for monitoring the fluid pressure; and means for
adjusting the force being applied against the burnishing member and
the corresponding pressure being exerted by said burnishing member
against the surface in response to the fluid pressure.
18. The burnishing apparatus of claim 16 further comprising: a
programmable control means configured to continuously track the
position of the burnishing member and for automatically adjusting
the force being applied against the burnishing member and the
corresponding pressure being applied against the surface by said
burnishing member.
19. The burnishing apparatus of claim 16 further comprising: a
programmable control means configured to direct the motion of said
burnishing member in a selected pattern across the surface of a
part.
20. A blade for use in turbo-machinery comprising: a generally
rectangular platform; an elongated airfoil having a leading edge
and a trailing edge, the airfoil being attached to and extending
radially outwardly from said platform; and a root attached to said
platform and extending radially inwardly from said platform;
wherein said blade having been treated by the method of claim
1.
21. The blade of claim 20 further comprising the step of inducing a
more shallow layer of compressive stress within the surface of the
selected region.
22. The blade of claim 20 further comprising the step of removing a
layer of material along the surface being in low compression or
tension.
23. A rotor disk for use in turbo-machinery comprising means for
supporting a plurality of blades, wherein said rotor disk having
been treated by the method of claim 1.
24. The rotor disk of claim 23 further comprising the step of
inducing a more shallow layer of compressive stress within the
surface of the selected region.
25. The rotor disk of claim 23 further comprising the step of
removing a layer of material along the surface being in low
compression or tension.
Description
RELATED PATENT APPLICATIONS
[0001] The present Application deals with related subject matter in
co-pending U.S. patent application entitled METHOD FOR REDUCING
TENSILE STRESS ZONES IN THE SURFACE A PART, filed on the same day
as the present application and having the same inventor in
common.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a method and an apparatus for
imparting residual stress in the surface of a part and, more
particularly, to a method of inducing a selected compressive
residual stress distribution within the surface of a part to
improve fatigue and stress corrosion performance of the part and an
apparatus for implementing the method.
[0003] Surface residual stresses are known to have a major effect
upon the fatigue and stress corrosion performance of component
parts. Tensile residual stresses, which can develop during
manufacturing processes such as grinding, turning, or welding are
well known to reduce both fatigue life and increase sensitivity to
corrosion-fatigue and stress corrosion cracking of the part.
Further, many parts that are subjected to high dynamic stresses or
have areas where stress concentrations occur, such as blades and
the rotor disks of turbo machinery, are prone to crack initiation
and relatively rapid crack growth. The blades typically comprise an
airfoil portion, a platform for partially defining a surface for
fluid flow there over when the blade is mounted to the rotor disk,
and a root portion having retention grooves which engage in
corresponding axially extending complementary grooves of the disk.
During engine operation, the rotor disk and the blade are subjected
to large centrifugal loads that produce high dynamic stresses that
may cause high cycle fatigue along portions of the rotor disk and
the blade causing cracking and possible failure of the part.
Further, the leading edge of the airfoil is often subjected to
damage caused by the impact of foreign objects in the fluid stream.
Such impact often results in cracks forming along the leading edge
that may result in failure of the blade.
[0004] It is well known that compressive residual stresses induced
in the surface of a part can increase fatigue life and reduce
susceptibility to corrosion-fatigue and stress corrosion cracking.
There are currently several methods used in industry for inducing
compressive stress in the surface of a metal part and the
particular method selected has been dependent on factors such as
the dimensions and shape of the part, its strength and stiffness,
the desired quality of the finished surface, the desired physical
properties of the finished part, and the expense of performing the
operation.
[0005] One method commonly used in industry to induce compressive
stress in the surface of a part is shot peening, whereby a
plurality of metallic or ceramic pellets are projected mechanically
or through air pressure to impinge the surface of the part. While
such a method is relatively inexpensive and is preferred for many
applications, shot peering is unacceptable for parts requiring a
superior finish or requiring a greater depth of compressive stress
penetration and has also been found to be unacceptable for parts
requiring localized or well defined compressive stress regions.
Further, for parts such as a rotor disk for use in turbo machinery,
the bore surfaces of the rotor disk are subjected to low levels of
plastic strain (typically between about 0.2% to about 0.5%) when
the rotor disk is accelerated to full speed. If the surfaces have
been highly cold worked, such as during shot peening, the cold
worked compressive surface material will not yield in tension while
the lower yield strength interior material will yield during engine
operation. On unloading, such as when the rotor speed is reduced,
the surface is driven into tension and will remain in tension,
reducing its fatigue life, for the remaining life of the
component.
[0006] Another method commonly used in industry to induce
compressive stress in the surface of a part is laser shock peening,
whereby multiple radiation pulses from high power pulsed lasers
produce shock waves on the surface of the part to produce a high
magnitude localized compressive stress within a particular region.
Unfortunately, however, laser shock peening is relatively expensive
and time consuming making it unacceptable for many
applications.
[0007] A method which have been developed and is widely used in
industry to improve surface finish, fatigue life, and corrosion
resistance by deforming the surface of a part is burnishing whereby
a rotary or sliding burnishing member is pressed against the
surface of the part in order to compress the microscopic peaks in
the surface into adjacent hollows. Burnishing operates to develop
compressive stresses within the part by yielding the surface in
tension so that it returns to a state of compression following
deformation.
[0008] The burnishing apparatus utilized for working the surface of
a part typically comprise a plurality of cylindrical rollers or
balls which contact the surface of the part with sufficient
pressure to induce a compressive stress therein. Unfortunately,
sharp surface demarcation typically exists along the boundaries of
the burnished area often resulting in tensile residual stresses
being formed along such boundaries. As disclosed herein, it has
been found that gradually reducing the pressure being exerted by
the burnishing member to reduce the magnitude of compression at the
boundaries will reduce the build up of tensile residual stress.
Further, it has been found that by controlling the compressive
residual stress distribution and the magnitude of compression, the
tensile stress distributions within a part may be offset or
distributed in such a manner as to optimize the fatigue and/or
stress corrosion performance of the part. Until now, however, a
method and apparatus have not been developed that permitted the
residual stress distributions and the magnitude of compression to
be controlled in such a manner as to optimize fatigue performance
for a specific applied stress distribution.
[0009] Consequently, a need exists for a relatively inexpensive,
relatively time efficient method and apparatus for implementing the
method for improving the physical properties of a part by inducing
a layer of compressive stress in the surface of the part, which is
effective for use with complex shaped surfaces, and which permits
the magnitude of compression and the residual stress distributions
to be produced on a surface to achieve optimum fatigue performance
and stress corrosion performance of the part.
SUMMARY OF THE INVENTION
[0010] The novel method of the present invention for inducing a
layer of compressive residual stress along the surface of a part
comprises the steps of selecting a region of the part to be
treated; selecting the magnitude of compression and the residual
stress distribution to be induced in the surface of the selected
region of the part; exerting pressure against the surface of the
selected region, the pressure being applied in a selected pattern
along the surface to form zones of deformation having a deep layer
of compressive stress; and varying the pressure being exerted
against the surface to produce the desired residual stress
distribution and magnitude of compression within the surface.
[0011] In another preferred embodiment of the invention, the step
of exerting pressure against the surface of the selected region
included performing a burnishing operation using a burnishing
apparatus having a burnishing member for exerting pressure against
the surface of the selected region of the part to produce a zone of
deformation having a deep layer of compression.
[0012] In another preferred embodiment of the invention, the
pressure being exerted on the surface of the part induces a deep
layer of compression within the surface having associated cold
working of less than about 5.0%.
[0013] In another preferred embodiment of the invention, the
pressure being exerted on the surface of the part induces a deep
layer of compression within the surface having associated cold
working of less than about 3.5%.
[0014] In another preferred embodiment of the invention, whereby
the step of exerting pressure on the surface of the part is
performed by a burnishing operation using a burnishing apparatus
having a burnishing member for exerting pressure against the
surface of the selected region to induce a deep layer of
compression within the surface having associated cold working of
less than about 5.0 percent.
[0015] In another preferred embodiment of the invention, whereby
the step of exerting pressure on the surface of the part is
performed by a burnishing operation using a burnishing apparatus
having a burnishing member for exerting pressure against the
surface of the selected region to induce a deep layer of
compression within the surface having associated cold working of
less than about 3.5 percent.
[0016] In another preferred embodiment of the invention, whereby
the selected pattern operates to vary the spacing between the zones
of deformation to produce the desired residual stress
distribution.
[0017] In another preferred embodiment of the invention, the step
of selecting the magnitude of compression includes the step of
programming a control unit to automatically adjust the pressure
being exerted against the surface of the part.
[0018] In another preferred embodiment of the invention, the step
of exerting pressure against the surface of the selected region
includes performing a burnishing operation and the step of
programming a control unit to control the direction of movement of
a burnishing member to produce the desired stress distribution.
[0019] In another preferred embodiment of the present invention the
step of varying the pressure being exerted against the surface of a
part includes the steps of programming a control unit to adjust the
pressure being exerted by a burnishing member against the surface
of the part, and programming the control unit to direct the
burnishing member over the part in a selected pattern to obtain the
desired residual stress distribution.
[0020] In another preferred embodiment of the present invention,
the step of varying the pressure being exerted against the surface
of a part includes the step of gradually varying the magnitude of
compressive stress in the areas immediately adjacent to the
boundaries of the selected region.
[0021] In another preferred embodiment of the present invention, a
method of inducing a layer of compressive stress in the surface of
a part comprises the steps of inducing a deep layer of compression
within the surface and inducing a more shallow layer of compressive
stress within the surface of the selected region.
[0022] In another preferred embodiment of the present invention, a
method of inducing a layer of compressive stress in the surface of
a part comprises the steps of inducing a deep layer of compression
within the surface and removing a layer of material along the
surface being in low compression or tension.
[0023] In another preferred embodiment of the present invention,
the method of inducing a layer of compressive stress in the surface
of a part comprises the steps of programming a control unit to
adjust the pressure being applied by the burnishing member against
the surface of the part; programming the control unit to direct the
burnishing member over the part in a predetermined pattern to
induce a layer of compressive stress in the surface of the part;
and applying a secondary process to impart a relatively shallow
layer of compressive residual stress along the surface of the part
to produce the desired residual stress distribution.
[0024] The novel apparatus for implementing the method of the
present invention utilizes a burnishing process for inducing a
layer of compressive residual stress having a preselected magnitude
of compression and a desired stress distribution. In particular,
the burnishing apparatus comprises a burnishing member for applying
pressure against the surface of the selected region of the part to
produce a zone of deformation having a deep layer of compression
and a preselected magnitude within the surface. The burnishing
apparatus further comprises means for moving the burnishing member
in a predetermined pattern across the selected region to produce a
desired residual stress distribution.
[0025] In another preferred embodiment of the invention the
burnishing apparatus for implementing the burnishing method of the
subject invention comprises a burnishing member for applying
pressure against the surface of a part to induce a layer of
compressive stress therein; means for adjusting the pressure being
applied against the surface of the part by the burnishing member;
and means for directing the burnishing member over the surface of
the part in a predetermined pattern to provide the desired residual
stress distribution.
[0026] In another preferred embodiment of the invention, the
burnishing apparatus for implementing the burnishing method of the
subject invention is coupled to a control unit for automatically
controlling the movement, position, and application pressure of the
burnishing member.
[0027] In another preferred embodiment of the invention, the
burnishing apparatus for implementing the burnishing method of the
subject invention comprises means for supplying a constant flow of
fluid to support the burnishing member.
[0028] In another preferred embodiment of the invention, the
burnishing apparatus for implementing the burnishing method of the
subject invention comprises magnetic means for maintaining the
burnishing member within the socket.
[0029] Another preferred embodiment of the invention is a blade for
use in turbo machinery having having a desired stress
distribution.
[0030] Another preferred embodiment of the invention is a rotor
disk for use in turbo machinery comprising selected regions having
desired stress distributions.
[0031] Another preferred embodiment of the invention, a part
selected from the group consisting of automotive parts, aircraft
parts, marine parts, engine parts, motor parts, machine parts,
drilling parts, construction parts, pump parts, and the like
comprises regions of compressive residual stresses having
predetermined stress distributions.
[0032] Another preferred embodiment of the invention, a part
selected from the group consisting of automotive parts, aircraft
parts, marine parts, engine parts, motor parts, machine parts,
drilling parts, construction parts, pump parts, and the like
treated by the method comprising the step, or a combination of
steps, of the present invention.
[0033] A primary object of this invention, therefore, is to provide
a method and an apparatus for implementing the method of providing
a part with an improved finish and with improved physical
properties.
[0034] Another primary object of this invention is to provide a
method and an apparatus for implementing the method of inducing a
compressive stress layer on the surface of a part.
[0035] Another primary object of this invention is to provide a
method and an apparatus for implementing the method of inducing a
compressive stress layer that varies in magnitude of compression
across the part in a predetermined pattern.
[0036] Another primary object of this invention is to provide a
method and an apparatus for implementing the method of inducing a
compressive stress layer having a well defined stress
distribution.
[0037] Another primary object of this invention is to provide a
method and an apparatus for implementing the method of inducing a
compressive stress layer having a predetermined stress
distribution.
[0038] Another primary object of this invention is to provide a
method for forming a part having deep compression with a minimal
amount of cold working and surface hardening.
[0039] Another primary object of this invention is to provide a
method of inducing a relative deep layer of compressive stress and
a relative shallow layer of compressive stress in the surface of
the part.
[0040] Another primary object of the invention is to provide a
burnishing apparatus that permits the pressure being exerted on the
surface of a part to be varied to produce regions having residual
stress distributions of arbitrary shape and magnitude of
compression.
[0041] Another primary object of the invention is to provide a
burnishing apparatus comprising means for automatically adjusting
the burnishing force and the corresponding pressure being exerted
against the surface of a part that increases on the high points
encountered along the surface and decreases on the low points
encountered along the surface of the part.
[0042] Another primary object of this invention is to provide an
apparatus having a burnishing member within a socket and magnetic
means for maintaining the burnishing member within the socket.
[0043] Another primary object of this invention is to provide an
apparatus having a burnishing member within a socket which can be
easily removed and inserted into place within the socket.
[0044] Another primary object of this invention is to provide a
blade for use in turbo machinery having relatively good fatigue and
stress corrosion performance.
[0045] Another primary object of this invention is to provide a
rotor disk for use in turbo machinery having relatively good
fatigue and stress corrosion performance.
[0046] Another primary object of this invention is to provide a
method and an apparatus for implementing the method of inducing a
compressive stress layer on the surface of a part which is
relatively inexpensive.
[0047] Another primary object of this invention is to provide a
blade for use in turbo machinery comprising regions of compressive
residual stresses having predetermined stress distributions.
[0048] Another primary object of this invention is to provide a
blade for use in turbo machinery having a compressive stress layer
that varies in magnitude of compression across the part.
[0049] Another primary object of this invention is to provide a
disk for use in turbo machinery comprising regions of compressive
residual stresses having predetermined patterns of magnitude of
compression and residual stress distribution.
[0050] Another primary object of this invention is to provide a
part selected from the group consisting of automotive parts,
aircraft parts, marine parts, engine parts, motor parts, machine
parts, drilling parts, construction parts, pump parts, and the like
comprising regions of compressive residual stresses having
predetermined patterns of magnitude of compression and residual
stress distributions.
[0051] These and other objects and advantages of the invention will
be apparent from the following description, the accompanying
drawings and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] FIG. 1 is a schematic block diagram illustrating the method
of the present invention;
[0053] FIG. 2 is a graph illustrating the predicted longitudinal
residual stress distribution, following 2.1% plastic strain, of a
part having been treated by the method of shot peening and a part
having been treated by the method of burnishing;
[0054] FIG. 3 is a graph illustrating the percent of cold work and
yield strength distribution of a part having been treated by the
method of shot peening and a part having been treated by the method
of burnishing;
[0055] FIG. 4 is a schematic view of a generally rectangular region
being treated by the method and apparatus of the invention for
inducing a desired residual stress distribution and magnitude of
compression whereby the pressure being exerted (force normal to the
surface) against the surface is varied in two directions;
[0056] FIG. 5 is a schematic view of another region being treated
by the method and apparatus of the invention for inducing a desired
residual stress distribution and magnitude of compression whereby
the density of the burnishing pattern is varied in the one
direction;
[0057] FIG. 6 is a schematic view of another region being treated
by the method and apparatus of the invention for inducing a desired
residual stress distribution and magnitude of compression whereby
the density of the burnishing pattern is varied in the two
directions.;
[0058] FIG. 7 is a schematic view of another region being treated
by the method and apparatus of the invention for inducing a desired
residual stress distribution and magnitude of compression, such as
around a bolt hole, whereby the pattern is a symmetrical
pattern;
[0059] FIG. 8 is a graph illustrating the residual stress
distribution induced in the surface of a part in the direction of
burnishing (parallel) and in the transverse direction
(perpendicular);
[0060] FIG. 9 is a graph illustrating the percent cold work
distribution for the burnishing operation shown in FIG. 8;
[0061] FIG. 10 is a diagrammatic view of the burnishing apparatus
for implementing the method of the present invention;
[0062] FIG. 11 is a diagrammatic view of the socket of a preferred
embodiment of the burnishing apparatus of the present invention
showing magnetic means for maintaining the burnishing member within
the socket;
[0063] FIG. 12 is a bottom diagrammatic view of the socket of FIG.
11 with the burnishing member removed;
[0064] FIG. 13 is a diagrammatic view of the socket of FIG. 11
showing the magnetic field lines for maintaining the burnishing
member within the socket; and
[0065] FIG. 14 is a partial perspective view of a blade and a rotor
disk of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0066] The present invention relates to a method and an apparatus
for implementing the method of inducing a layer of compressive
residual stress along the surface of a part. In a preferred
embodiment of the invention, as shown in FIG. 1, the method of the
present invention comprises the steps of selecting a region of the
part to be treated; selecting the magnitude of compression and the
residual stress distribution to be induced along the surface of the
selected region, such as for example by finite element analysis;
and inducing a layer of compressive residual stress along the
surface of the selected region having the desired magnitude of
compression and stress distribution.
[0067] It has been found that for parts having a surface that has
been substantially cold worked, such as a rotor disk for turbo
machinery that has been treated by the process of shot peening, the
cold worked compressive surface material will typically not yield
in tension, such as during high speed operation, while the lower
yield strength interior material will yield. On unloading of the
part, such as when the speed of revolution of the rotor disk slows,
the surface of the part is driven into tension and will remain in
tension, reducing the fatigue life, throughout the parts remaining
life. Referring to FIG. 2, the inversion into tension of a surface
of a part having been treated by the method of shot peening is
shown compared to a surface of a part having been treated by the
method of burnishing, each having a 2.1% plastic strain single
cycle. Referring to FIG. 3, the corresponding percent of cold work
and yield strength distribution are shown. As illustrated, upon
unloading, the part that underwent the method of shot peening may
actually invert from compression into a relative high level of
tension, if a yield strength gradient exists, thereby significantly
reducing the fatigue life of the part.
[0068] Accordingly, it has been found that the preferred method of
the present invention for improving the surface finish, fatigue
life, and stress corrosion resistance of a part is burnishing,
whereby a rotary or sliding member is pressed against the surface
of the part in order to compress the microscopic peaks in the
surface into adjacent hollows. Such compression develops
compressive stresses within the part by yielding the surface in
tension so that it returns to a state of compression following
deformation. As shown in U.S. Pat. No. 5,826,435, by the same
inventor and incorporated herein by reference, by cold working the
surface less than about 3.5%, and preferably less than about 2.0%,
results in layer retention of compressive residual stress at
elevated temperature, less rapid relaxation under cyclic loading,
and minimizes the alteration of the residual stress field during
tensile or compressive overload than conventional cold working and
surface hardening processes. Accordingly, the method of the present
invention is shown in FIG. 1 and preferably utilizes the process of
burnishing to provide deep compression with a minimal amount of
cold working and surface hardening. In particular, the region to be
burnished along the surface of the part is first defined and a
burnishing apparatus having a single-point of contact burnishing
member is pressed against the surface of the part to create a zone
of deformation producing a relatively deep layer of compression
within the surface. The burnishing member is then passed in a
predetermined pattern across the region. Preferably, the pattern of
burnishing is such that the zones of deformation formed by each
pass of the burnishing member do not overlap. As disclosed in U.S.
Pat. No. 5,826,435, applying a single-pass, or multiple passes
having a reduced compressive pressure, produces compressive
residual stresses following tensile deformation of the surface
having deep compression with minimal cold working.
[0069] In another preferred embodiment of the invention, the method
further comprises the steps of determining the optimum magnitude of
compression to be induced at particular points along the surface of
the selected region by controlling the pressure being exerted by
the burnishing member. In another preferred embodiment of the
invention, the method comprises the steps of varying the pattern of
burnishing to produce a desired residual stress distribution. In
another preferred embodiment of the invention, the method further
comprises the steps of programming a control unit, such as a
computer or numerical controller, to automatically regulate the
burnishing force being applied to the burnishing member thereby
controlling the pressure being exerted against the surface of the
part and the corresponding magnitude of compression being induced
by the burnishing apparatus. The control unit may also be
programmed to control the direction of movement of the burnishing
apparatus to produce the desired residual stress distribution.
[0070] The particular pressure and the pattern of burnishing for a
part may be selected whereby the magnitude of compression and the
residual stress distribution optimizes the fatigue performance of
the part. For illustration, as shown in FIG. 4, a rectangular
burnishing region is selected and the burnishing member is pressed
against the surface of the part in a particular (raster) pattern,
as shown by the arrow indicating the path of the burnishing member.
The normal force (Fz) being applied to the burnishing member is
varied to increase or decrease the pressure being exerted against
the surface of the part. While FIG. 4 shows a linear variation in
the normal force and the corresponding pressure being applied
against the surface, parallel (X-direction) and perpendicular
(Y-direction) to the direction of burnishing, it should now be
apparent to those skilled in the art that the pattern of burnishing
and the form and rate of reduction or increase in pressure being
exerted against the surface can be controlled to provide a wide
variety of residual stress distributions and magnitude of
compression.
[0071] Referring to FIG. 5, another illustration of the method of
the present invention is shown whereby variations in residual
stress distribution may also be achieved by varying the pattern of
burnishing, independently or in conjunction with variations in
burnishing pressure. As shown, the spacing along the X-direction,
perpendicular to the direction of travel of the burnishing member,
has been varied to increase and decrease the spacing between each
pass of the burnishing member thereby changing the density (Dx) of
burnishing. As shown, the spacing between each pass of the
burnishing member varies linearly, however, it should now be
apparent to those skilled in the art that other burnishing patterns
may be selected to produce the desired residual stress
distribution.
[0072] Referring to FIG. 6, another pattern of burnishing is shown
whereby the spacing density (Dx) is varied in two dimensions (X and
Y directions) as a function of the length of the burnishing pass,
in order to produce the desired stress distribution for the part
being burnished.
[0073] Referring to FIG. 7, another pattern of burnishing is shown
whereby a region is designated and the magnitude of compression and
the residual stress distribution is selected that optimizes the
fatigue performance of the part. As shown, the residual stress
distribution has a symmetrical pattern such as what would be
preferred for use around bolt holes or for "feathering" in a state
of compressive stress in the fillet area of a rotor disk. It should
now be apparent to those skilled in the art that the burnishing
pressure, the density of burnishing, and the pattern of burnishing
can be varied to produce the desired residual stress distribution
and magnitude of compression for a part for a specific engineering
application.
[0074] In another preferred embodiment of the invention, the method
of inducing a layer of compressive residual stress along the
surface of a part includes the step of using a secondary process,
such as shot peening, grit blasting, tumbling or other similar
abrasive impact processes to induce a shallow layer of compressive
residual stress near the surface of the part following burnishing.
As shown in FIGS. 8 and 9, burnishing of a surface inherently
produces a Hertzian loading of the surface resulting in maximum
compression beneath the surface of the work piece. The residual
stress at the surface can be near zero or even tensile, and is a
function of the direction of the burnishing operation. The surface
residual stress is typically less compressive in the direction of
burnishing (parallel) than in the transverse direction
(perpendicular) due to the effect of displacement of material
laterally during passage of the burnishing member. The presence of
lower compression at the surface has been found to allow the
initiation of fatigue cracks at the surface of the part. Although
these cracks are arrested as they propagate deeper into the more
highly compressive material, the presence of surface cracks and the
stress intensity factor associated with them is highly undesirable.
It has been found that the method of this invention comprising the
steps of burnishing a part in combination with the secondary
process identified herein above provides surface compression as
well as deep compression resulting in a part having superior
resistance to surface crack initiation and propagation. In another
embodiment of the invention, the method of the present invention
comprises the step, in conjunction with the first step of
burnishing, of removing a layer of low compression by
electropolishing, etching or other similar means that will not
induce a state of stress or through mechanical means, such as low
stress grinding, polishing, tumbling, or other such means, which
will induce a state of shallow compressive stress.
[0075] Referring to FIG. 10, a preferred embodiment of the
burnishing apparatus 100 for implementing the burnishing method of
the subject invention is shown comprising a generally cylindrical
socket 102 which conventionally mounts to a support 104 of any
particular description typically used for supporting burnishing
tools which is attached to a conventional machine tool fixture (not
shown). In a preferred embodiment of the invention, the support 104
is coupled to the socket 102 and provides means for imparting a
normal force F to a burnishing member 106 to effect the proper
burnishing pressure sufficient to deform the surface 108 of the
part 110.
[0076] The socket 102 includes a seat 112 adapted to the surface of
the burnishing member 106 which is disposed within the seat 112,
and an inner chamber 114. The size of the seat 112 is determined by
the size and shape of the burnishing member 106 and is selected to
provide a small clearance 116 between the seat 112 and the
burnishing member 106. As shown, the support 104, in cooperation
with the machine tool fixture, is adapted for controlling the
movement of the socket 102 and includes means for forcing the
socket 102 and the burnishing member 106 against the surface 108 of
the part 110 being burnished. Without departing from the invention,
it should now be apparent to those skilled in the art that various
apparatus may be constructed to allow the socket to be moved to
various positions or to allow the part being treated to rotate or
pass in contact with the burnishing member in such a way that the
selected region is burnished using the method of the present
invention.
[0077] The socket 102 is further provided with a fluid passage 118
in flow communication with the seat 112 and extends from the seat
112 through the inner chamber 114 to a fitting (not shown) for
connecting to a positive displacement pump 120 for providing a
constant volumetric flow of fluid from a fluid supply 122 to the
seat 112. The fluid supply 122 may be an external supply (not
shown) or may be in the form of a sump 124, as shown, thereby
forming a closed-loop fluid system. The positive displacement pump
120 is preferably coupled to a direct current (DC) electric motor
126 and a fast acting motor speed control 128. The motor speed
control 128 functions to maintain a constant angular velocity of
the motor 126 to sustain the constant volumetric fluid flow to the
socket 102 regardless of any changes in fluid pressure. A pressure
sensor 130, such as a pressure transducer, is connected to the
fluid passage 118 for monitoring fluid pressure and is coupled to a
control unit 132, such as a computer or a numerical controller,
which is also coupled to either a position regulator 134, such as a
spring, or a pressure regulator 136, such as a hydraulic or
pneumatic system, that operate with the burnishing member 106 to
provide the proper burnishing pressure being exerted against the
surface 108 of the part 110.
[0078] To understand how the elements of this invention described
are interrelated, the operation of the burnishing apparatus 100
will now be described. During operation, fluid, such as a
lubricating fluid, is fed under pressure from the fluid supply 122
by use of the positive displacement pump 120 through the fluid
passage 118 and into the inner chamber 114. The fluid in the inner
chamber 114 is then fed under pressure around the burnishing member
106 through clearance 116 to force the burnishing member 106
outwardly. The lubricating fluid flows around the outer surface of
the burnishing member 106 to permit the burnishing member 106 to
float continuously upon a thin film of fluid. The socket 102 is
then advanced towards the surface 108 of the part 110 by operation
of the support and the machine tool fixture (not shown) until the
forward most portion of the burnishing member 106 makes contact
with the surface 108. By further adjusting the speed of the motor,
a desired amount of lubrication fluid will flow around the
burnishing member 106 and be transferred onto the surface 108 of
the part 110 to provide the desired lubrication and cooling for the
burnishing operation. During burnishing, the further most portion
of the burnishing member 106 contacts the surface 108 of the part
110 causing the burnishing member 106 to move inwardly into the
socket 102 thereby reducing the clearance 116 between the
burnishing member 106 and the socket 102 thereby increasing the
pressure of the fluid in the fluid passage 118. The increase in
fluid pressure is detected by the pressure sensor 130 which is
coupled to the control unit 132 that functions to adjust the force
F being applied to the burnishing member 106 to maintain a constant
or controlled variable burnishing pressure against the surface 108.
It should now be apparent to those skilled in the art that the
constant flow burnishing apparatus 100 of the present invention,
unlike conventional constant pressure burnishing apparatus that
follow the surface topography of the part, automatically increases
the force F being applied to the burnishing member 106, and the
corresponding pressure being exerted against the surface 108, on
high points and decreases on low points along the surface 108.
Accordingly, the pressure or the compressive force exerted on the
surface 108 of the part 110 by the burnishing member 106 can be
precisely regulated to provide optimum surface finish and uniform
burnishing of the part.
[0079] In a preferred embodiment of the invention, the proper
pressure or compressive force to be applied to the surface 108 of
the part 110 during the burnishing operation is provided by using
the position regulator 134 whereby the force F being applied to the
burnishing member 106 is a function of the position of the socket
102. As shown, the position regulator 134 includes a spring means
140, such as a coil spring, deflection members, or Belleville
washers, having a known spring characteristic, which compresses or
expands axially to apply a given normal force F to the burnishing
member 106. Because the burnishing member 106 is coupled through
the spring means 140, the force F being applied to the burnishing
member 106 and the resulting pressure being exerted on the surface
108 of the part 110 can be accurately controlled by positioning
(moving) the socket 102 using the conventional machine tool fixture
(not shown). The control unit 132 operates with a feed back signal
from the pressure sensor 130 to achieve closed loop control of the
force F and the corresponding pressure being exerted on the surface
by the burnishing member 106.
[0080] Preferably the machine tool fixture supporting the socket
102 is a "three-axis" machine that provides for linear motion along
mutually orthogonal axis of a fixed coordinate system.
[0081] It should now be apparent to those skilled in the art that
by using a programmable control unit 132 which is configured to
continuously track the position of the burnishing member 106, the
socket 102 can be accurately positioned and moved in a selected
pattern. Further, in combination with passing the burnishing member
is a selected pattern across the surface of the part, the pressure
being exerted against the surface may be varied to obtain a region
having the desired residual stress distribution and magnitude of
compression.
[0082] In another preferred embodiment of the invention, the proper
pressure or compressive force to be applied to the surface 108 of
the part 110 during the burnishing operation is provided by use of
the pressure regulator 136. As shown in FIG. 10, the pressure
regulator 136 comprises a source of pressurized fluid 141 for
providing pneumatic or hydraulic pressure against a piston 142,
diaphragm or other similar means. The piston 142 is coupled to the
burnishing member 106 in such a manner that movement of the piston
142 operates to increase or decrease the force F being applied to
the burnishing member 106 thereby increasing or decreasing the
corresponding pressure being exerted by the burnishing member 106
on the surface 108 of the part 110. In operation, for constant
pressure burnishing, the machine tool fixture moves the socket 102
in a predetermined pattern along the surface 108 of the part 110.
The control unit 132 functions with a feed back signal from the
pressure sensor 130 to achieve closed loop control of the force F
and the corresponding pressure being exerted on the surface 108 by
the burnishing member 106. It should now be apparent to those
skilled in the art that by using the control unit 132, the
burnishing member 106 can be accurately moved in a selected pattern
while exerting a predetermined pressure against the surface 108 of
the part 110 to obtain a region having the desired residual stress
distribution and magnitude of compression.
[0083] Conventional constant pressure burnishing apparatus require
a containment means, such as end caps, for maintaining the
burnishing member withing the apparatus. The containment means must
be capable of withstanding high pressure and forces, including the
time when the burnishing member is not in contact with the surface
of the part. In the event that the containment means fails, the
burnishing member could be propelled from the burnishing apparatus
at high velocity. In contrast, the constant flow burnishing
apparatus of the subject invention eliminates the need of a
containment means that is capable of withstanding high
pressure.
[0084] Referring to FIGS. 11, 12 and 13, the burnishing member 106
may be selected from various materials having a higher yield
strength than the part 110 being burnished and having a relatively
high elastic modules to allow maximal deformation of the part 110.
In a preferred embodiment of the invention, the burnishing member
106 is formed from a high carbon steel or a sintered tungsten
carbide containing a portion of a cobalt binder. The inner chamber
114 of the socket 102 is shown having a magnetic means 144, such as
a permanent magnet or an electric magnet or the like, which produce
magnetic flux 145 (FIG. 13) that functions to maintain the
burnishing member 106 withing the seat 112. It has been found that
forming the socket 102 from a ferromagnetic alloy, such as a
martensitic stainless steel AISI 440C, the socket 102 functions as
a pole piece thereby increasing the holding power of the magnet
means 144. Because the bearing member 106 is supported by a low
volume of fluid having a constant flow rate, the bearing member 106
will be retained within the socket 102 even while the fluid is
flowing and the socket 102 is being repositioned or moved out of
contact with the surface 108 of the part 110.
[0085] Referring to FIG. 14, a blade of the present invention is
shown, for use in turbo machinery. The blade 146 includes a
generally rectangular platform 148; an elongated airfoil 149 having
a leading edge 150 and a trailing edge 152, the airfoil 149 being
rigidly attached to and extending radially outwardly from the
platform 148; and a root 154 rigidly connected to and extending
radially inwardly from the platform 148 having a dovetail portion
155 for mounting to a rotor disk 156. As used herein, the term
"outwardly" refers to the direction away from the center of
rotation of the blade and rotor disk and the term "inwardly" refers
to the direction towards the center of rotation of the blade and
rotor disk. The rotor disk 156 includes a plurality of
circumferentially spaced axially disposed slots 158 therein. The
blade 146 is attached to the rotor disk 156 by inserting the root
154 into a slot 158. As shown, the root 154 and the slot 158 have
complementing surfaces for securing the blade 146 to the rotor disk
156. During operation, the rotor disk 156 and the attached blades
146 are subjected to high centrifugal loads that produce high
dynamic stresses that may cause high cycle fatigue along portions
of the rotor disk 156 and each blade 146. Further, the leading edge
150 of the blade 146 is often subjected to damage by the impact of
foreign objects in the fluid stream.
[0086] In a preferred embodiment of the invention, the blade 146 is
treated by the method comprising a step or a combination of steps
disclosed herein. In another preferred embodiment of the invention,
the rotor disk 156 is treated by the method comprising a step or a
combination of steps disclosed herein.
[0087] Another preferred embodiment of the invention, a part is
selected from the group comprising automotive parts, aircraft
parts, marine parts, engine parts, motor parts, machine parts,
drilling parts, construction parts, pump parts, and the like
treated by the method comprising the step, or a combination of
steps, of the present invention.
[0088] The method and apparatus for implementing the method of the
subject invention utilizes a burnishing method that produces cold
work and surface work hardening far less than either conventional
shot peening, gravity peening, and conventional burnishing or deep
rolling methods. The increase in residual compressive stress with
minimal cold work developed by the subject invention penetrates to
a greater depth than most conventional methods, such as shot
peening and results in longer retention of compressive residual
stress at elevated temperature, less rapid relaxation under cyclic
loading, and minimizes the alteration of the residual stress field
during tensile or compressive overload than conventional cold
working and surface hardening processes. Further, the method for
inducing a layer of compressive residual stress along the surface
of a part and the apparatus for implementing the method provides
control of the particular stress distribution and magnitude of
compression that optimizes the fatigue performance of the part. By
controlling the pattern of burnishing and by gradually reducing the
magnitude of compression near the boundaries of the regions being
burnished ("feathering"), the tensile zones which occur immediately
adjacent and parallel to the boundaries may be reduced or
eliminated.
[0089] Accordingly, the method and apparatus for implementing the
method of the subject invention provides a relatively inexpensive
and effective means of providing a compression force on a workpiece
to induce compressive residual stress in a well defined localized
region of a simple or complex part surface configuration with a
minimum of cold working and surface hardening. By minimizing the
amount of cold working and surface hardening, the method of the
subject invention produces longer retention of compressive residual
stress at elevated temperature, less relaxation under cyclic
loading, and minimizes the alteration of the residual stress field
during tensile or compressive overload. Further, the method and the
apparatus of the invention for inducing a layer of compressive
residual stress along the surface of the part permits a variety of
burnishing patterns to be designated to produce regions of residual
stress that are appropriate for a specific engineering application.
In addition, a part treated using the method of the invention have
improved stress corrosion cracking resistance.
[0090] While the method and apparatus described constitute
preferred embodiments of the invention, it is to be understood that
the invention is not limited to the precise method and apparatus,
and that changes may be made therein without departing from the
scope of the invention which is defined in the appended claims.
* * * * *