U.S. patent application number 12/659098 was filed with the patent office on 2010-08-26 for mitigation of stress corrosion and fatigue by surface conditioning.
This patent application is currently assigned to General Electric Company. Invention is credited to Henry Peter Offer, Hsueh-Wen Pao, David Wesley Sandusky.
Application Number | 20100216374 12/659098 |
Document ID | / |
Family ID | 40295818 |
Filed Date | 2010-08-26 |
United States Patent
Application |
20100216374 |
Kind Code |
A1 |
Offer; Henry Peter ; et
al. |
August 26, 2010 |
Mitigation of stress corrosion and fatigue by surface
conditioning
Abstract
Method and apparatus for surface conditioning a metal surface
typically having irregular surface contours, by rubbing the metal
surface with a surface conditioning device having a plurality of
bristles which contact the metal surface during the rubbing and
effect tensile stress reduction or degraded layer removal in the
metal surface.
Inventors: |
Offer; Henry Peter; (Los
Gatos, CA) ; Sandusky; David Wesley; (Los Gatos,
CA) ; Pao; Hsueh-Wen; (Saratoga, CA) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
40295818 |
Appl. No.: |
12/659098 |
Filed: |
February 25, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12149156 |
Apr 28, 2008 |
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12659098 |
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11233031 |
Sep 23, 2005 |
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12149156 |
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Current U.S.
Class: |
451/28 ; 451/162;
451/526 |
Current CPC
Class: |
B24D 13/20 20130101;
B24D 13/145 20130101; B24D 13/10 20130101 |
Class at
Publication: |
451/28 ; 451/162;
451/526 |
International
Class: |
B24B 1/00 20060101
B24B001/00; B24B 19/00 20060101 B24B019/00; B24D 11/00 20060101
B24D011/00 |
Claims
1. A method of conditioning a metal surface, comprising rubbing the
metal surface with a surface conditioning means comprising a
plurality of bristles which contact said metal surface during said
rubbing and effect tensile stress reduction.
2. Method as in claim 1 wherein said tensile stress is reduced
below the metal surface.
3. Method as in claim 1 wherein said tensile stress is reduced to
compressive stress.
4. Method as in claim 1 wherein said tensile stress is a residual
stress not loaded externally.
5. Method as in claim 1 wherein said plurality of bristles are in
the form of a cylindrical brush.
6. Method as in claim 5 wherein said cylindrical brush is helical
or circular.
7. Method as in claim 1 wherein said plurality of bristles are
mounted on a plate member.
8. Method as in claim 1 wherein said plurality of bristles are
mounted on a radiused member and have a contoured brush surface
such that a contour of the metal surface essentially corresponds to
tshe contoured brush surface.
9. Method an in claim 1 wherein an abrasive material is embedded in
said bristles.
10. Method an in claim 1 wherein an abrasive material is coated on
said bristles.
11. Method as in claim 10 wherein said abrasive material is adhered
to the bristles with an adhesive.
12. Method as in claim 10 wherein said abrasive material is applied
as a wet slurry to the bristles.
13. Method as in claim 9 wherein said abrasive material is selected
from the group consisting of aluminum oxide, silicon carbide,
silicon nitride, zirconium, and synthetic diamond.
14. Method as in claim 10 wherein said abrasive material is
selected from the group consisting of aluminum oxide, silicon
carbide, silicon nitride, zirconium, and synthetic diamond.
15. Method as in claim 1 wherein said surface to be conditioned is
wetted by a coolant fluid.
16. Method as in claim 1 wherein said surface to be conditioned is
submerged in a coolant fluid.
17. Method as in claim 15 wherein said coolant fluid is water.
18. Method as in claim 16 wherein said coolant fluid is water.
19. Method as in claim 1 wherein said surface conditioning means
includes an electric, hydraulic, or pneumatic motor to effect
movement of said bristles.
20. Method as in claim 1 wherein said metal surface is first rubbed
with bristles coated or embedded with a coarser abrasive, followed
by one or more further rubbing steps with bristles coated or
embedded with a finer abrasive.
21. Method as in claim 1 wherein said surface conditioning improves
surface micro-geometry smoothness, eliminates degraded surface
composition or microstructure, removes micro-cracked or corroded
surface layer, removes cold-worked surface layer, removes
fatigue-damaged surface layer, and improves surface residual
stress.
22. A method of conditioning a metal surface, comprising rubbing
the metal surface with a surface conditioning means comprising a
plurality of bristles which contact said metal surface during said
rubbing and effect degraded layer removal in the metal surface.
23. Method as in claim 22 wherein said degraded layer removal is
cold work or environmental degradation.
24. A surface treatment tool for conditioning a surface,
comprising: a first motor operatively connected to an abrasive
element having an external surface, for driving said abrasive
element in frictional engagement with a surface to be treated; a
clamping element for mounting the tool to an anchor; a second motor
operatively connected to a extension-retraction system for
effecting movement of said abrasive element towards and away from
said surface to be treated.
25. Tool as in claim 24 wherein said abrasive element is a
brush.
26. Tool as in claim 25 wherein said brush comprises bristles in a
helical configuration.
27. Tool as in claim 24 further comprising a motor operatively
connected to said abrasive element for causing lateral movement of
said element across and in contact with said surface to be
treated.
28. A surface treatment tool for conditioning a surface,
comprising: a first motor operatively connected to an abrasive
element for driving said abrasive element in frictional engagement
with a surface to be treated; a clamping element for mounting the
tool to an anchor; a second motor operatively connected to said
abrasive element for causing lateral movement of said abrasive
element across said surface to be treated; a tilting system for
causing angular tilting the abrasive element from a non-tilted to a
tilted orientation relative to the anchor to permit said abrasive
element to follow changes in orientation of the surface to be
treated.
29. Tool as in claim 28 wherein said abrasive element comprises a
circular bristle brush plate member mounted on a turntable.
30. A surface treatment tool for conditioning a surface,
comprising: a first motor operatively connected to an abrasive
element for driving said abrasive element in frictional engagement
with a surface to be treated; a clamping element for mounting the
tool to an anchor; a second motor operatively connected to a
gearing arrangement for effecting translational movement of said
abrasive element across the surface to be treated; a third motor
operatively connected to said abrasive element for effecting
movement of said abrasive element towards and away from said
surface to be treated.
31. Tool as in claim 30 wherein said abrasive element comprises a
plurality of bristles.
32. Tool as in claim 31 wherein said bristles are fabricated from a
plastics or rubber material.
33. Tool as in claim 31 wherein an abrasive material is embedded in
said bristles.
34. Tool as in claim 31 wherein an abrasive material is coated on
said bristles.
35. Tool as in claim 33 wherein said abrasive material is adhered
to the bristles with an adhesive.
36. Tool as in claim 34 wherein said abrasive material is selected
from the group consisting of aluminum oxide, silicon carbide,
silicon nitride, zirconium, and synthetic diamond.
37. Tool as in claim 34 wherein said abrasive material is selected
from the group consisting of aluminum oxide, silicon carbide,
silicon nitride, zirconium, and synthetic diamond.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
12/149,156 filed Apr. 28, 2008, which is a continuation of Ser. No.
11/233,031 filed Sep. 23, 2005, the entire contents of each of
which are hereby incorporated by reference.
[0002] The present invention relates to a method and apparatus for
providing mitigation of stress-corrosion cracking (SCC) and fatigue
initiation in metallic components, including for example base
metals and welds in austenitic stainless steels and nickel-base
alloys. In particular, the invention provides a method and
apparatus for mechanical surface conditioning of metals to mitigate
susceptibility to crack initiation or growth of small cracks due to
tensile surface stresses.
BACKGROUND OF THE INVENTION
[0003] Stress-corrosion cracking (SCC) in metals is generally known
to be caused by the simultaneous presence of a susceptible
material, an aggressive environment, and tensile stresses. In the
past, mitigation of SCC has focused on reducing or eliminating one
or more of these causes, either in the bulk state or locally at the
exposed work surface.
[0004] Conventional peening and burnishing processes, including
water-jet peening, laser shock peening, shot peening, hammer
peening, and roller or ball burnishing, are known to locally reduce
surface stresses, but do not reduce the micro-structural,
micro-chemical, or micro-geometrical susceptibility of the material
surface, and do not refresh degraded surfaces due to abusive
fabrication practices or to service in an aggressive
environment.
[0005] Conventional peening methods do nothing to remove existing
degraded surface conditions, such as from frequently abusive
fabrication processes, including forming methods such as rolling or
bending, and surfacing methods such as machining or grinding, or
from exposure to normally aggressive in-service environments,
including high-temperature oxygenated water or from exposure to
contaminated water. In fact, these methods add to the undesirable
cold work that is already present. Peening is subject to excessive
build-up of near-surface cold work, since surface material is not
removed as application of the process continues, and the thickness
and severity of the cold-worked layer is not inherently
self-limiting. Likewise, since peening does not remove the existing
surface layer, any pre-existing surface micro-cracking, or grain
boundary corrosion, also remains to collectively leave a
potentially worsened condition with respect to SCC initiation than
existed before peening, considered from a microstructural
susceptibility viewpoint. This is especially true if the intended
stress improvement shakes down when the treated component is
returned to service.
[0006] Moreover, existing noble metal coating processes provide a
means of catalytically controlling the aggressive local excess of
oxygen or hydrogen peroxide when added hydrogen is present. Weld
cladding covers the susceptible surface, sealing it from the
aggressive environment. However, none of these or other known SCC
surface mitigation methods provides both stress-reduction and
surface-conditioning redundancy in reducing susceptibility to SCC,
especially in the critical crack-initiation phase.
[0007] A need exists, therefore, for an improved method for
mitigation of SCC. The present invention meets that need.
BRIEF DESCRIPTION OF THE INVENTION
[0008] It has now been discovered, surprisingly, according to the
present invention, that it is possible to effect surface
conditioning of a metal surface by rubbing the metal surface with a
surface conditioning means comprising a plurality of abrasive
bristles. The stretching action of the abrasive in the bristles on
the metal surface during the rubbing causes tensile stress
reduction in the metal surface.
[0009] In one aspect there is provided a method of conditioning a
metal surface, comprising rubbing the metal surface with a surface
conditioning element comprising a plurality of bristles which
contact the metal surface during the rubbing and effect tensile
stress reduction in the metal surface.
[0010] In another aspect, there is provided a method of
conditioning a metal surface, comprising rubbing the metal surface
with a surface conditioning element comprising a plurality of
bristles which contact the metal surface during the rubbing and
effect degraded layer removal, such as for example cold work or
environmental degradation, in the metal surface.
[0011] In a further aspect there is provided a surface treatment
tool for conditioning a surface, comprising a first motor
operatively connected to an abrasive element having an external
curved surface, for driving the abrasive element in frictional
engagement with an inside curved surface to be treated, a clamping
element for mounting the tool to an anchor, and a second motor
operatively connected to a extension-retraction system for
effecting movement of the abrasive element along the inside curved
surface to be treated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention will now be further described with reference
to the accompanying drawings, in which:
[0013] FIG. 1 is a longitudinal cross-section of an inside diameter
(ID) polishing tool for a bottom mounted instrumentation (BMI) in a
pressurized water reactor (PWR);
[0014] FIG. 2 is a more detailed view of the upper end of the tool
of FIG. 1 showing the extend/retract motor assembly;
[0015] FIG. 3 is a perspective view of the tool of FIG. 1 with the
brush in the extended position;
[0016] FIG. 4a is a detailed perspective view of a tool of FIG. 1
having a helical bristles brush;
[0017] FIGS. 4b and 4c are perspective views of the tool of FIG. 4a
in the retracted position and the extended position,
respectively;
[0018] FIGS. 5a and 5b are perspective views of the tool of FIG. 1
in a work position in a pressurized water reactor (PWR);
[0019] FIG. 6 is a perspective view from the underside of an
outside diameter (OD) surface improvement tool of the
invention;
[0020] FIG. 7 is a more detailed view of the underside of the tool
of FIG. 6 showing the bristles;
[0021] FIGS. 8a and 8b are side views of the tool of FIG. 6 showing
the normal and tilted orientations;
[0022] FIG. 9 is a side view of the tool of FIG. 6 in a tilted
orientation abutting an inner surface of a vessel;
[0023] FIG. 10 is an upper perspective view of a tool of the FIG. 6
in the tilted orientation;
[0024] FIGS. 11a and 11b are perspective views from above and
below, respectively, of a further embodiment of an OD surface
improvement tool which can use bristle hone, bristles or abrasive
brush, or combinations thereof;
[0025] FIG. 12 shows a cross-sectional side elevation of another
example of an outside diameter surface improvement brush tool for a
BMI;
[0026] FIG. 13 is an upper elevational view of the improvement tool
of FIG. 12;
[0027] FIG. 14 is an upper elevational view of the improvement tool
of FIG. 13 showing linear guides for the tool;
[0028] FIGS. 15a and b are further elevational views of the tool of
FIG. 12;
[0029] FIGS. 16a and b are side elevations of the tool of FIG. 12
showing upper and lower vertical displacement features;
[0030] FIG. 17 is a plan view of the tool of FIG. 12;
[0031] FIGS. 18a and b are side elevational views of a further
embodiment of an outside diameter surface improvement tool to back
side weld showing gear features;
[0032] FIG. 19 is a side elevational view of a outside surface
improvement tool for up-hill side CRC flexible shaft coupling
arrangement;
[0033] FIGS. 20a and b are side elevational views of the tool of
FIG. 19;
[0034] FIG. 21 shows a perspective view of a vessel BMI-penetration
mockup block for surface improvement evaluation;
[0035] FIG. 22 shows residual stress depth profile measurements
before surface improvement;
[0036] FIG. 23 shows residual stress depth profile measurements
after surface improvement using a flexible hone abrasive;
[0037] FIG. 24 shows residual stress depth profile measurements
after surface improvement using a flexible abrasive brush;
[0038] FIG. 25 shows a perspective view of a full size mockup of a
PWR vessel bottom head instrument housing penetration attachment
weld;
[0039] FIGS. 26 and 27 show residual stress depth profile before
surface improvement using flexible brush abrasive;
[0040] FIGS. 28 and 29 show residual stress depth profile after
surface improvement using flexible brush abrasive.
DETAILED DESCRIPTION OF THE INVENTION
[0041] The present invention resides in the discovery of a surface
conditioning method suitable for both generally smooth or locally
irregular surface contours including as-deposited or roughly
ground/machined weld surfaces using a brush surface conditioning
means having bristles or tines which are able to penetrate small
cracks in the surface. During the surface conditioning process, the
bristles are pressurized against and readily deform into recessed
areas not reached by less flexible surface conditioning apparatus,
such as wheels made from 3-dimensionally bonded matrix products.
Since the bristles can conform to fully and more evenly contact an
existing irregular contour work surface, the degree of surface
conditioning is highly uniform compared to more rigid surface
conditioning wheels or pads.
[0042] The surface conditioning method of the invention is
effective with any rubbing action that is abrasive enough to remove
material and thereby deform the remaining substrate surface
plastically, which will effect tensile stress reduction and
advantageously leave the surface in compression if it is not
overheated to cause local thermal expansion strains sufficient to
approach or exceed the corresponding mechanical elongation
strains.
[0043] The expression "tensile stress reduction" as used herein
means that the tensile stress of the metal is reduced from an
initial level to a lower level of tensile stress, advantageously to
a zero tensile stress level or to a negative tensile stress (i.e.
compressive stress) level. Typically, an initial tensile stress
level might be in the region of 25 to 150 ksi, for example 75-120
ksi, and a reduced tensile stress level might be in the region of
0-5 ksi, optimally in a negative tensile stress region (i.e.
compressive tensile stress) of minus 25 to minus 150 ksi, for
example minus 50 to minus 120 ksi.
[0044] The redundancy of the collective bristle abrasive action
against the work surface during surface conditioning provides a
high degree of uniformity of the improvement in the surface
condition, similar to that provided by more rigid wet polishing
media on smooth contour surfaces. The term "redundancy", as used
herein, means the repeated rubbing of a specific small area by a
plurality of bristles, ensuring effectively full surface coverage
of the entire contour as the process progresses in space and
time.
[0045] Referring to FIG. 1, there is shown an inside diameter
surface treatment tool 2, having a first motor 10, which may be
electric or other type, operatively connected to an abrasive
element 4 which in the embodiment shown is a cylindrical shaped
brush 4 with bristles 6 having an external curved or faceted
surface. The motor 10 drives the brush 4 in frictional engagement
with an inside curved surface to be treated. A clamping element 14
which is a clamping cylinder in the embodiment shown, mounts the
tool to an anchor, and a second motor 20 is operatively connected
to a extension-retraction system 22 for effecting movement of the
abrasive element 4 towards and away from the inside curved surface
to be treated.
[0046] The transverse cross-section of the brush 4 may have any
suitable shape and need not be circular. The brush 4 is mounted on
a shaft 8 which is connected to the motor 10 contained in a housing
12 and held in place with respect to the work piece via the clamp
cylinder 14. A yoke 16 is provided for lateral support of the
rotating brush shaft 8.
[0047] At the upper end of the tool 2, there is provided a plate 18
on which is mounted the motor 20. Upon actuation of the motor 20,
the motor 10 inside the housing is caused to move by way of the
extension-retraction mechanism 22 which, in the embodiment shown,
is a ball/screw, lead screw or other linear drive arrangement,
thereby extending or retracting the shaft 8 and the brush 4 mounted
thereon with respect to the housing 12. A handle 24 is provided on
the plate 18 to facilitate maneuvering of the tool.
[0048] FIG. 2 is a more detailed view of the upper end of the tool
of FIG. 1 showing the extend/retract motor assembly. The motor 20
is connected via a pulley and belt or chain or sprocket system
26,28,30 to the ball screw 22 which is mounted in a bearing 32 in
the top of the housing. Actuation of the motor 20 causes rotation
of the ball screw 22 which, in turn, causes the motor 10 to move
within the housing 12, resulting in extension or retraction of the
shaft 8 and the brush 6 with respect to the housing 12.
[0049] FIG. 3 is a perspective view of the tool 2 of FIG. 1. The
brush 6 is shown in the extended position with respect to the
housing 12.
[0050] FIG. 4a is a perspective view of a tool 2 of FIG. 1 with a
detailed perspective view of a helical bristle brush 6. FIGS. 4b
and 4c are perspective views of the tool of FIG. 4a in the
retracted position 34 and the extended position 36,
respectively.
[0051] FIGS. 5a and 5b are perspective views of the tool 2 of FIG.
1 in a work position in a PWR vessel bottom head 38. The tool 2 is
seen mounted on top of a penetration pipe 39 extending upwardly out
of the PWR bottom head 38.
[0052] FIG. 6 is a perspective view from the underside of an
outside diameter (OD) surface improvement tool 40. The tool 40
includes a first motor 50 operatively connected to an abrasive
element 42 shown as a circular bristle brush plate member mounted
on a turntable 44, for driving the brush in frictional engagement
with a surface to be treated. Drive gear 46 meshes with brush drive
gear 48 formed around the circumference of the brush plate member
42 to cause rotation of the brush plate member 42 upon actuation of
the motor 50. A clamping element 61 is provided for mounting the
tool to an anchor such as a BMI, as shown in FIG. 10. A BMI
protector is provided at 63. A second motor 70 is operatively
connected to the abrasive element for causing lateral movement of
the abrasive element across the surface to be treated, as shown by
the arrows 72,74 (FIG. 10). The tool 40 is provided with linear
guides 52 to facilitate oscillation or positioning of the brush
against a surface. The turntable 44 is mounted on suspension
members 54 to support the turntable.
[0053] FIG. 7 is a more detailed view of the underside of the
bristles brush member 42 of the tool 40 of FIG. 6. In this
embodiment, the bristles 56 are shown extending axially and
radially from a central hub area 58.
[0054] FIGS. 8a and 8b are side views of the tool 40 of FIG. 6
showing a tilting mechanism for tilting the tool between a normal
orientation 60 and a tilted orientation 62. A screw member 64
passes through member 65 having a tapped hole. The screw member 64
is provided with a pin 67 towards the lower end thereof which
slideably engages a slot 69 in a bracket 71 mounted on the tool.
Upon rotation of the screw member 64 through fixed member 65, the
tool is tilted upwardly and the pin slides along slot 69, causing
the tool 40 rotate upwardly in the direction of arrow A towards the
fixed member 65, as shown in FIG. 8b to the tilted orientation
62.
[0055] FIG. 9 is a side view of the tool of FIG. 6 in a tilted
orientation 62. The tool 40 is shown abutting an inner surface 66
of a vessel 68, such as a BWR or PWR instrument penetration.
[0056] FIG. 10 is an upper perspective view of a tool of FIG. 6 in
the tilted orientation 62. An oscillation motor 70 is provided for
effecting oscillation or positioning of the assembly in the
directions of the arrows 72,74. Oscillation motion 72 and 74 of the
brush sub-assembly (turntable 44) is achieved using motor 70 to
drive for example a conventional lead screw and nut assembly (not
shown). The motor is mounted on top of the oscillator assembly in
these figures.
[0057] FIGS. 11a and 11b are perspective views from above and below
respectively of a further embodiment of an OD surface improvement
tool 76. In this embodiment, there are provided multiple brushes 78
(four brushes are shown in FIGS. 11a and 11b, but the tool is not
limited to four brushes). The tool can be tilted employing tilting
means similar to that shown in FIG. 8.
[0058] FIG. 12 is a cross-sectional side elevation of another
embodiment 80 of an outside diameter surface improvement brush tool
for a BMI. The tool 80 comprises a rotatable surface improvement
element 82, typically a brush, which contacts and rubs the surface
84 to be treated. The brush is rotated by a first motor 86, mounted
on a support housing 88, through a gear system 90, shown in more
detail in FIG. 15b. The configuration shown is able to access all
portions of the sloped and radiused weld surface. The tool 80 is
clamped onto a BMI 92 by way of clamp 94. The BMI is protected from
damage by the clamp by a protector sleeve 96. A second motor 98 is
operatively connected to a gearing arrangement for effecting
translational movement of brush across the surface to be
treated.
[0059] FIG. 13 shows the tool 80 provided with the tool rotation
orbiting motor 98 mounted on a platform 100. The motor 98 is
operatively connected to gear mechanism 102, 104 to effect orbiting
of the motor body about the longitudinal X, as shown in FIGS. 17
and 18a. The orbiting motor 98 is typically synchronized with an
oscillation or translation motor(s) (such as shown in FIG. 10) to
keep the brush in contact with the work surface. A vertical
displacement motor 106 is provided for effecting vertical
displacement of the tool in the direction of arrows 108 (see FIGS.
14, 15a and 16a and b). Actuation of the motor 106 causes vertical
displacement along linear guides 110, thereby bringing the brush 82
into and out of contact with the surface being treated.
[0060] FIGS. 18a and b are side elevational views of a further
embodiment of an outside diameter surface improvement tool to back
side weld showing gear features as an example of a compact brush
rotation drive assembly.
[0061] FIGS. 19, 20a and 20b are elevational views of a further
embodiment of an outside surface improvement tool having an angled
support arm 112 for a brush head 114. In the embodiment shown the
brush head has a partial conical configuration, and is particularly
adapted for surface treatment of an inner surface of a reactor
having a curved, e.g. semi-spherical, bottom, where the brush
contacts the limited-access up-hill side of the inner curved
surface, as well as the downhill side. In a typical embodiment, the
inner surface of a reactor is rubbed with a flexible, conforming
abrasive carrier, such as an abrasive-filled bristle or
abrasive-coated bristle or strand carrier.
[0062] The brush or brushes employed in the apparatus of the
invention may be a rotating (circular) or sliding (linear)
configuration to effectively surface condition surfaces of varying
contour. Either configuration may provide surface conditioning by
continuous unidirectional motion, or by vibratory motion in a
periodically fixed position. Combinations of brush configuration,
grit sizes and types of motion may be used to provide optimum
surface microstructure, surface finish, and stress improvement.
[0063] The bristles are typically fabricated from a Nylon such as
Nylon 6 or Nylon 621, which has good water absorption, resistance
and toughness. Other plastics or elastomers may be employed.
Lubrication is important as otherwise the Nylon may heat
sufficiently to melt onto the work if sufficient pressure/speed is
applied. Water is an acceptable lubricant coolant, and may contain
a slurry of abrasive particles.
[0064] Alternatively, the bristles may be, for example, extruded or
molded Nylon with abrasive particles embedded in or adhered to the
plastic. A highly flexible brush or flexible hone fitted with
bristles or strands containing selected abrasive or other hard
particles (relative to the hardness of the metal) exposed on the
bristle surface may be employed. Examples of abrasives include
aluminum oxide, silicon carbide or boron carbide, zirconium,
synthetic diamond, etc., according to the hardness of the material
to be conditioned. The high flexibility and corresponding
conformability of the bristle-based abrasive carrier allows the
abrasive to be easily applied to asymmetric shapes and irregular
surface contours. The cross-section of the bristles may be round,
rectangular, or other extrudable or moldable shape. Relative
movement of the brush against a work surface removes a degraded
surface layer by abrasion, and imparts a residual stress reduction
by rubbing and tensile micro-deformation of the surface grains.
This deformation occurs primarily as superficial plastic shear
strain, with sufficiently low thermal strain component that the net
result is a compressive stress.
[0065] In an abrasive-impregnated or abrasive-coated bristle, the
abrasive particle is generally substantially imbedded in the
bristle matrix (or adhesive, if used) such as a flexible plastic or
rubber, so that deeper scratching of the work surface is minimal.
For bristles that have the abrasive bonded to their surface, the
particles are almost completely covered by the adhesive. This
abrasive structure also allows the deformed layer to be uniform and
limited in depth. The repetitive nature of the abrasive carrier
motion and multiple, repetitive bristle action allows the
deformation to have uniform and complete coverage, even when
applied to uneven surface contours. The primary effect of the
abrasive particles when almost completely embedded in a bristle (as
compared to open-coat type particles being bonded to a substrate
with minimal embedment in the bonding material) is to more gently
rub and repeatedly deform in shear (plastically stretch) the
surface grains, rather than to aggressively groove the grains (as
predominately occurs in conventional grinding or sanding). Surface
grain stretching occurs especially when brushing is applied using
coolant/lubricant, to provide a durable and sufficiently deep
surface stress reduction when the abrasive rubbing forces are
removed.
[0066] Particle sizes available commercially are 46 to 500 grit. It
is also possible to employ 80, 120, and 180 grit. It has been found
that with stainless steel plates, the coarser grit cuts faster and
leaves a somewhat deeper compressive layer, of the order of a few
mills (0.001'').
[0067] Other materials may also be employed for the bristles,
provided a "cold" cutting action is achieved during the process of
rubbing on the metal surface to achieve a highly compressive stress
(rather than a tensile stress as occurs with conventional processes
that cause excessive local surface heating). When an abrasive is
adhered to the bristles, the particles are typically bonded to the
surface along some of the length of the bristle. The adhesive
should have flexibility similar to that of the bristle. As an
alternative, it is possible to employ commercial "Flex Hones"
(supplied by Brush Research Manufacturing Co. Inc.), which have a
ball of abrasive and hard adhesive bonded directly on the end of
each bristle.
[0068] Commercial bristle length ("trim length") ranges from 1.4''
to 4'', depending primarily on brush diameter. Typically, lengths
from 3/4'' to 11/2'' are employed.
[0069] Brushing is usually effective at any practical speed
provided the work surface is kept cool (wet or submerged). However,
surface sliding speed determines cutting productivity. Usually, the
brush speed is maximized to the highest extent possible but is
often limited by horsepower due to the viscous drag of the water
when submerged, which can use as much as half of what is available
for 6 or more horsepower submersible motors driving a 1''
wide.times.6'' diameter brush. Good results have been obtained
using 6-17 HP hydraulic motors driving a 2'' wide.times.4''
diameter brush. Both were 80 grit silicon carbide in 0.06''
diameter Nylon bristles. A 6'' diameter brush having
0.045.times.0.090 rectangular bristles has been evaluated with
similar productivity results as for the 0.060 round bristles.
[0070] Typically, the brush is driven at a rotational speed in the
region of about 800-8000 rpm, for example about 1000-6000 rpm. A
travel speed of about 9-12 inches per minute is usually adopted,
and bristle pressure engagement may range from slight contact
(about 0.050 inch depth) to unlimited, depending on bristle length
and bristle size. The slower the travel speed, the less repetitions
are needed for a desired depth of cut. A typical cutting rate is
about 0.007 inch max. depth in 10 reversed passes with a 4''
diameter brush at about 6000 rpm and 3/8'' bristle engagement, set
statically (less when running due to apparent increase in brush
stiffness at high rpm due to centrifugal force).
[0071] The amount of material removed depends on the depth of a
pre-existing cold worked layer or the aged layer, whichever is
greater (typically the cold work, which is case dependent). The
aged layer is usually about 0.001-0.005 inch thick. The cold work
layer may be as deep as 0.015 inch which can occur during rolling
operations.
[0072] Excessive cold-work of the newly surface conditioned surface
is essentially non-existent, since the depth of the cold-worked
layer and the degree of cold-work are limited by the stiffness and
deformation of the soft bristles 56 which is significantly less
than caused by fabrication. Since the work surface is continuously
removed as the abrasive process is applied, the minor amount of
cold-work produced during surface conditioning is self-limiting. A
multi-step surface conditioning process with graded abrasive sizes
can progressively reduce the cold-work layer even further until it
is negligible, if desired.
[0073] In contrast to peening which may be applied in a very
localized manner, depending on the process which is chosen, surface
conditioning according to the method of the present invention
easily provides uniform coverage and therefore the desired uniform
resistance to SCC. This is to be compared to more localized-area
SCC mitigation processes applied in discrete rastered passes, such
as is required with laser peening.
[0074] In a further aspect of the invention, it has been discovered
that it is the micro-surface condition of a susceptible material
that must also be improved, in addition to the tensile stress, to
fully mitigate against SCC or fatigue initiation. By micro-surface
as used herein, it is meant the surface micro-roughness and
cold-work of the final finishing process, such as is produced by
machining, grinding and polishing.
[0075] The surface conditioning method of the present invention
simultaneously removes an existing degraded surface layer (due to
fabrication cold-work or in-service environmental aging), while
also improving the surface finish, and while additionally improving
the surface residual stress. This makes the present one-step,
triple benefit method a significant improvement over the prior art
of non-redundant or multiple-step surface stress improvement
techniques which provide only a single mitigation effect, i.e.
surface stress improvement.
[0076] Advantageously, the method can be applied in a fabrication
shop environment, during field construction, or in an operating
plant during a maintenance outage. This surface conditioning
technique may be directly applied using simple hand-held or
automatic motorized tooling, or can be indirectly applied in
remote, limited-access locations using robotics in
high-temperature, under-water, or high-radiation or other hazardous
applications in operating chemical or power plants.
[0077] In another embodiment, the surface is submerged in water, or
a similar coolant/lubricant, which increases margin against
possible bristle degradation or substrate overheating due to
frictional heat. Resistance to SCC or fatigue is provided to a
processed component by controlling the initiation phase of cracking
such that the time for initiation is increased to be well in excess
of the time for crack initiation of comparable components which
have not been given this conditioning improvement. This surface
renewal benefit is achieved by removal of heavily cold-worked or
environmentally aged layers, and by typically reducing the
roughness of the surface, which are provided in addition to the
benefit of surface stress improvement. In turn, it is expected that
the newly regained service life of the component treated according
to the present method will be renewed to equal or even exceed that
of untreated components. Such renewed components can be said to be
"better than new".
[0078] The work surface is kept cool typically using flowing gas or
liquid coolant and/or friction-reducing lubricant, or otherwise the
final stress state will typically be tension, as occurs during
conventional dry machining or grinding. The residual stress
improvement resulting from the surface abrasive shearing is
generated to a greater depth than the sheared zone, since stress
equilibrium and material continuity is maintained below the depth
of the sheared surface layer.
[0079] The method of the invention may also be applied to new
components or components under construction to further improve
their surface condition, relative to conventional fabrication
methods. Subsequent crack initiation in service is eliminated or
significantly delayed by providing the following additive benefits:
1) improvement in the surface micro-geometry smoothness (on a
grain-size scale), 2) elimination of a degraded e.g., surface
composition or microstructure (if present), removal of
micro-cracked or corroded surface layer, removal of excessively
cold-worked surface layer, removal of a fatigue-damaged surface
layer, and 3) improvement (reduction) of surface and near-surface
tensile residual stress to a compressive stress.
[0080] In contrast, conventional aggressive cutting action
generates sufficient local heating that the surface grains attempt
to expand thermally, but are restrained by surrounding grains such
that they plastically compress while they are at elevated
temperature, due to the collective mechanical constraint of cooler
(and therefore stronger) neighboring grains. After the heated
grains cool and contract thermally during conventional cutting
methods, the grains go into tension, since they are still
metallurgically bound to and constrained by their surrounding
grains.
[0081] The stress reduction benefit of the invention occurs by
plastically elongating only the very near surface of cool substrate
grains, with reduced cold work depth also due to the simultaneous
continuous surface removal. The repeated abrasion and resulting
surface grain elongation must be done gently (relative to the
cooling conditions available) in order to avoid overheating the
surface, which would cause it to thermally expand and plastically
deform in compression (while hot), leaving the compressed zone in
tension upon cooling. Frictional surface heating, when significant,
can be readily overcome with copious external fluid cooling.
Typically, with inadequate cooling, the greater the depth of
scratching, the less compressive (or more tensile) the residual
stresses will become because of the internal heat formed by grain
shearing, as well as the associated higher friction. This adverse
condition results from increased pressure or speed on the abrasive
and/or from the resulting deeper scratches, typically causing
increased surface shear and cumulative micro-heating.
[0082] In a particular embodiment, the method includes continuous
liquid cooling of the surface being conditioned. The high
flexibility of the brush device allows as-deposited weld bead
surfaces, which are generally not smooth and flush with their
substrate, to be fully surface conditioned even in the low-profile
surface areas more quickly than the conventional case where the
high-profile areas must also be first removed for access to the low
areas. This is the situation when a semi-rigid or hard (such as
3-dimensionally bonded) surface-conditioning wheels are used on a
work surface having a local change in contour greater than the
deformation capability of the abrasive matrix, without applying
excessively high pressure of the matrix against the surface.
[0083] When the pre-existing degraded surface layer is relatively
deep, such as heavily machined base metal or aggressively ground
weld metal, a multi-step brush surface conditioning process may be
used to maximize productivity, without inducing excessive cold-work
in the final step. A first brush surface-conditioning step with a
coarser abrasive, followed by one or more steps with finer
abrasive(s) can more quickly remove the degraded layer and still
leave an essentially scratch-free, stress-improved final surface
with very minimal cold-work, as is desired.
[0084] During movement of the brush or similar abrasive-containing
device against an SCC-susceptible work surface, the rubbing actions
of the abrasive in the bristles wears away a controlled depth of
the work surface. The surface micro-finish geometry is typically
improved, reducing the susceptibility of SCC initiation by reducing
the micro-stress intensity at the scratch tip, which can behave as
an incipient crack. Specifically, intergranular attack (such as
from abusive pickling), micro-cracking, machining grooves, and
similar SCC or fatigue crack-initiation sites are removed.
[0085] In addition to improving the work surface finish, the
abrasive action of the surface conditioning also removes the
pre-existing, degraded surface layer having susceptibility to SCC
initiation from prior exposure to an SCC-aggressive environment,
such as oxygenated or halogen-contaminated reactor coolant water.
This surface degradation may exist in the form of a micro-cracked
or grain-boundary corroded layer, either of which can reduce the
time to observable SCC initiation. This degraded layer can also
have pre-existing cold work resulting from heavy machining and/or
grinding that was performed before or after welds were made in the
subject material.
[0086] During the present method, rubbing action of the
particle-filled bristles also changes the near-surface residual
stresses from typically high tensile to low tensile, or to
compressive, as an additional means of preventing SCC. This further
benefit results from the known fact that SCC does not occur within
a component's lifetime under conditions of very low tensile or
compressive stress. The generation of a compressive residual stress
generally keeps the net stress sufficiently low, once tensile
operating stresses are applied.
EXAMPLES
[0087] The method of the invention has been tested in wet or
submerged conditions using powered abrasive brushes and flexible
hones to surface condition stainless steel and Inconel weld and
base metal samples having various initial surface roughness
conditions. These samples were surface conditioned with several
grit sizes and bristle sizes for predetermined durations.
Example 1
[0088] Referring to FIG. 21, there is shown a perspective view of a
vessel penetration mockup block 80 for surface improvement
evaluation in 0.6 inch diameter machine bore (Inconel 600 base
material). The results are discussed below.
[0089] FIG. 22 shows residual stress depth profile measurements
before surface improvement (baseline condition) in 0.6 inch
diameter bore (Inconel 600 base material). FIG. 23 shows the
residual stress depth profile measurements after surface
improvement using a flexible hone abrasive in 0.6 inch diameter
bore, and FIG. 24 shows the residual stress depth profile
measurements after surface improvement using a flexible abrasive
brush in 0.6 inch diameter bore.
Example 2
[0090] FIGS. 25a, b, c, d and e show perspective views of a full
size mockup of a PWR vessel bottom head instrument housing
penetration attachment weld 82. The mockup was completed on one
side with SCC-susceptible Inconel 132 (see Figures b and c) and on
the other side with SCC-susceptible Inconel 182 (see Figures d and
e), each joined to an Inconel housing stub. Half of each weld was
measured for residual stress in the as-welded condition and the
other half was measured in the surface improved condition.
[0091] FIG. 26 shows the residual stress depth profile before
surface improvement using flexible brush abrasive (measurements
made on outside diameter of the vessel penetration attachment weld
mockup, Inconel 600 base material, Inconel 182 weld material). FIG.
27 shows the residual stress depth profile before surface
improvement using flexible brush abrasive (measurements made on
outside diameter of the vessel penetration attachment weld mockup,
Inconel 600 base material, Inconel 132 weld material). FIG. 28
shows the residual stress depth profile after surface improvement
using flexible brush abrasive (measurements made on outside
diameter of the vessel penetration attachment weld mockup, Inconel
600 base material, Inconel 182 weld material). FIG. 29 shows the
residual stress depth profile after surface improvement using
flexible brush abrasive (measurements made on outside diameter of
the vessel penetration attachment weld mockup, Inconel 600 base
material, Inconel 132 weld material).
[0092] A particular feature of the conditioning method of the
present invention is the ability to surface condition
difficult-access geometries, such as vessel head penetrations,
which intersect the head at acute angles. Orbiting an articulated
peening tool or other end-effector around an inclined hemispherical
head penetration is typically difficult because of the changing
angle and blend radius between the head and the penetration around
the circumference, and the limited clearance on the acute-angle
side. Both the intersection welds (called J-welds) and the bores of
these penetrations can be more easily surface conditioned by the
present flexible-abrasive method and apparatus than by other
conventional means, such as weld cladding. Therefore, the flexible
brush method solves the problems associated with conditioning
either a rough surface finish such as a weld crown, and/or a
variable surface contour such as a radiused vessel-to-head
penetration, and provides an improved surface condition as
well.
[0093] Another particular feature of the present method is that it
was surprisingly discovered that the residual stress improvement
was always obtained in stress directions both parallel to and
perpendicular to the direction of the brushing, even though the
brushing was applied uni-directionally, and the lay of the final
surface was uni-directional. This fact can be seen from the
residual stress test data in the Examples. This preferred finding,
based on the results of repeated X-ray diffraction measurements at
the surface and sub-surface (after electro-polishing away
predetermined depths), enables the invention method to allow
brushing in any one direction to obtain stress reduction benefits
in any other direction in the plane of the surface being
stress-improved. In addition, the direction of brushing may
therefore be arbitrary with respect to stress improvement
directions, so that other factors can be used to select the
preferred direction for brushing such as optimization of tool
configuration, brush orientation, and brush movement when deployed
in limited access areas.
[0094] The invention can be carried out employing any type of
rubbing action that is sufficiently abrasive to remove material and
thereby deform the remaining substrate plastically, which will
leave the surface in compression if it is not "overheated (where
the local thermal expansion strains approach or exceed the
mechanical elongation strains).
[0095] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *