U.S. patent application number 13/065157 was filed with the patent office on 2011-09-15 for metallic components for use in corrosive environments and method of manufacturing.
Invention is credited to Douglas J. Hornbach, Paul S. Prevey, III, Jeremy E. Scheel.
Application Number | 20110223443 13/065157 |
Document ID | / |
Family ID | 44560289 |
Filed Date | 2011-09-15 |
United States Patent
Application |
20110223443 |
Kind Code |
A1 |
Scheel; Jeremy E. ; et
al. |
September 15, 2011 |
Metallic components for use in corrosive environments and method of
manufacturing
Abstract
The present invention relates to a metallic component and method
of manufacture of the component for use in a corrosive environment,
such as components used in fossil fuel recovery or used in chemical
facilities. The components comprise at least one metallic portion
having a deep, stable layer of compressive stress for providing
life extension and mitigation of fatigue and corrosion related
failures. Preferably, the layer of compressive stress has a depth
that exceeds the depth of any surface irregularities.
Inventors: |
Scheel; Jeremy E.;
(Cincinnati, OH) ; Hornbach; Douglas J.;
(Guilford, IN) ; Prevey, III; Paul S.;
(Cincinnati, OH) |
Family ID: |
44560289 |
Appl. No.: |
13/065157 |
Filed: |
March 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61340282 |
Mar 15, 2010 |
|
|
|
Current U.S.
Class: |
428/687 ;
72/379.2 |
Current CPC
Class: |
Y10T 428/12993 20150115;
B23P 9/02 20130101; C21D 7/06 20130101; C21D 7/04 20130101; C21D
9/08 20130101; C21D 10/005 20130101 |
Class at
Publication: |
428/687 ;
72/379.2 |
International
Class: |
B22D 25/00 20060101
B22D025/00; B21D 31/00 20060101 B21D031/00 |
Claims
1. A method of improving the properties of a metallic component for
use in a corrosive environment comprising the steps of: identifying
at least one portion of the component that is expected to be
exposed to a corrosive environment; determining a desired
compressive stress distribution for said at least one portion of
the component that is expected to be exposed to a corrosive
environment; inducing said desired compressive stress distribution
within at least a portion of the surface of a component.
2. The method of claim 1 wherein said desired compressive stress
distribution having a magnitude, depth, and cold work effective to
mitigate SCC and H.sub.2S cracking of the component during use in a
corrosive environment.
3. The method of claim 1 wherein the corrosive environment is
identified and wherein the desired compressive stress distribution
is determined for the identified corrosive environment.
4. The method of claim 1, wherein said compressive stress
distribution is induced using a CNC or robotically controlled
burnishing and effective to impart the desired compressive stresses
in a controlled manner.
5. The method of claim 1 wherein the step of inducing the desired
compressive stress distribution includes using a hydraulically
supported burnishing apparatus.
6. The method of claim 1 wherein the step of inducing the desired
compressive stress distribution includes the steps of determining
burnishing parameters to provide a smooth surface along the at
least one portion of the surface of the component such that surface
irregularities that can become crack or corrosion pit initiation
sites are placed in compression
7. The method of claim 6 wherein said burnishing parameters include
the smoothness of the burnishing member that will be used for
inducing compressive stress within the at least one portion of the
surface of the component, the diameter of the of the burnishing
member, the force with which the burnishing member will be pressed
against the at least one portion of the surface of the component,
and the pattern of burnishing.
8. The method of claim 1 further comprises the step of identifying
the material properties of the component, the applied loads
expected to be applied to the component, the environment in which
the component is expected to operate, and the known causes Of
failure for similar components.
9. The method of claim 1 further comprising the step of enhancing
the smoothness of the surface along the at least a portion of the
surface of a component such that surface irregularities that can
become crack or corrosion pit initiation sites are reduced or
eliminated.
10. The method of claim 1 wherein the desired compressive stress
has a magnitude and depth of compression that extends to a depth of
at least nominally of about 0.5 mm such that the sum of residual
and applied stress never exceeds the threshold for SCC in the
corrosive environment of the application or the fatigue endurance
limit of the material.
11. The method of claim 1 wherein the induced desired compressive
stress has a depth of compression that incorporates a majority of
surface irregularities along the at least one portion of the
surface of the component.
12. The method of claim 1 wherein the desired compressive stress
distribution has a depth of penetration that penetrates entirely
through the at least a portion of the surface of a component.
13. The method of claim 1 wherein the compressive stress
distribution within at least a portion of the surface of the
component has a depth that exceeds the depth of any surface
irregularities.
14. A component for use in a corrosive environment comprising: at
least one portion of the component having a metallic surface; a
compressive stress distribution within said surface; wherein the
depth of said compressive stress distribution is such that it
exceeds a majority of surface irregularities along said surface and
having an amount of cold work induced within said surface that is
less than the amount necessary to damage the crystalline structure
along said surface and to create slip bands, dislocations, and
twinning such that said surface is more susceptible to stress
corrosion.
15. The metallic component of claim 14 wherein the said surface has
a depth of compression is at least about 1 mm.
16. The metallic component of claim 12 wherein said stress
distribution has a magnitude and depth of compression at least as
great as the sum of any residual and applied stress anticipated
within said at least one portion.
17. The metallic component of claim 12 wherein said stress
distribution has a depth of compression that does not exceed the
threshold for SCC in the expected corrosive environment of the
application of the component.
18. The metallic component of claim 12 wherein said at least one
portion has a depth and said compressive stress distribution
penetrates through the entire depth of the at least one portion.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/340,282, filed Mar. 15, 2010.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to metallic
components used in a corrosive environment, such as components used
for fossil fuel recovery that have improved properties and methods
of manufacturing such components. More specifically, the present
invention are new and novel metallic components for use in
corrosive environments such as components used for fossil fuel
recovery, and methods of manufacture, whereby the components have
improved properties for mitigating or preventing the deleterious
effects of stress corrosion cracking (SCC) and fatigue on the
useful life of the metallic components. Such components are
typically used in recovery and distribution of fossil fuels or used
in chemical plant applications.
[0003] In the recovery and distribution of fossil fuels and the
operation of petrochemical refineries and other types of chemical
plants, failure of metallic components is often a result of the
combination of stress as well as one or more corrosive elements,
such as hydrogen sulfide, H.sub.2S, ammonia, or chlorine, to which
the component is exposed in service. The elevated temperatures,
pressures, and applied stresses, either static or alternating, to
which such components are exposed, contribute to their degradation
and the rate at which corrosive related failure processes occur,
especially SCC and corrosion fatigue. The life of metallic
components in these environments and applications is often limited,
and premature component failures restrict production and increase
operational costs.
[0004] In oil and natural gas well drilling to recover fossil
fuels, the depth to which drilling can be performed is limited by
the materials available for the drill pipe, tubing, and casing,
generally referred to as Oil Country Tubular Goods (OCTG). In
offshore drilling, the material strength of the OCTG and drill
components limits the depth of water and thus the distance from
shore that is accessible. The pipe used for distribution of the
fossil fuel is generally known as Line Pipe Tubular Products
(LPTP). As used herein, OCTG, LPTP and drill and drill rig products
and components will collectively be referred to as Fuel Recovery
Components. Line Pipe Tubular Products may be fabricated as either
seamless or welded, with the seamless generally used for the most
demanding applications. It is well known that the strength of Fuel
Recovery Components formed from metallic materials, such as steel
alloys, can be increased by various heat treatment procedures, and
a variety of heat treatable Fuel Recovery Components are available
having a range of strengths (including the yield, ultimate, and
fatigue strengths). The toughness of the alloy in stress corrosion
cracking, given by the parameter K.sub.ISCC, is a measure of the
resistance to cracking, and generally is reduced as the yield and
ultimate strength are increased by heat treatment. Therefore, the
strength of metallic material that can be used in a SCC prone
environment, such as environments that Fuel Recovery Components
often operate in, is limited.
[0005] The weight of various Fuel Recovery Components, such as pipe
hanging from a drill platform in well drilling operations, is often
a primary source of applied stress. In horizontal drilling, now
used for both oil and gas machinery with minimal environmental
impact, a significant bending stress is applied to the Fuel
Recovery Components (such as a pipe, tubing, casing, or coupling)
in the transition from vertical to horizontal drilling. Higher
strength steel drill pipes, distribution pipes, and casing allow
deeper wells and drilling in deeper water, providing a major
economic advantage in tapping deep oil and gas fields both on land
and off-shore. However, when the oil or gas wells are "sour" with
H.sub.2S present, or when the stressed pipe is exposed to seawater
during offshore drilling or subsurface salt-water deposits
containing dissolved chlorine, the pipe and casing are subject to
SCC. It is then a common practice to limit the strength of steel
pipe, tubing, casing and other Fuel Recovery Components because the
softer, weaker material is not susceptible to SCC. Expensive SCC
"down hole" failures are avoided at the cost of limiting the
possible range of drilling.
[0006] The failure of Fuel Recovery Components as well as
components used in a wide variety of chemical plant applications
generally results in major economic loss, if not catastrophic
damage impacting public safety. It is well known that failure of
such metallic components is most commonly caused by the mechanisms
of SCC or fatigue. SCC occurs from the surface being exposed to a
corrosive media to which the alloy is susceptible under a primarily
constant steady state applied tensile stress exceeding a tensile
stress threshold specific to the alloy and the environment. Fatigue
failures occur under the influence of alternating applied stress,
generally accompanied by a steady mean stress, and often originate
from a surface flaw such as a corrosion pit, SCC, or scratch.
Corrosion fatigue is a combination of fatigue failure in the
presence of a corrosive environment, in effect adding a SCC
component to failure under cyclic loading.
[0007] There are several current chemically-based practices that
are used to attempt to prevent or reduce SCC and corrosion fatigue
failure in metallic components that are used in fossil fuel and
chemical applications: [0008] 1) use alloys with enhanced corrosion
resistance such as stainless steels; [0009] 2) use sacrificial
anodes to cathodically protect the metallic components; [0010] 3)
chemically alter the environment of the component with alkaline
substances or other protective fluids; [0011] 4) paint, plate, or
coat components to shield the metallic surfaces from the corrosive
environment; and [0012] 5) limit the strength of the steels used
and applied stress levels.
[0013] All of these chemistry or coating-based methods have
limitations, or have shown limited improvement in performance at
relatively high implementation costs to the end-user. Components
formed from corrosion resistant alloys are relatively expensive and
often are not cost effective. Cathodic protection offers only
temporary benefit by redirecting the corrosion process to the
sacrificial material until it is consumed. Adding chemicals to
neutralize corrosive elements, often known as `down-hole`
injection, also offers only a temporary solution because the
chemicals will eventually diffuse away or be consumed in reaction.
Paint and coatings will peal, wear away, or be scraped off
eventually exposing the surface to the corrosive environment, and
generally cannot be renewed on the casing and components installed
down in the well. Reducing the strength of the material and
designing for lower applied stresses provides a long-term solution
but limits performance, as noted above.
[0014] Mechanical methods have been used or proposed that are
designed to place the surface layer that will be in contact with
the corrosive media in a state of residual compression in an
attempt to mitigate either SCC or fatigue. Such methods include
shot peening, laser shock processing (LSP), low plasticity
burnishing (LPB), and deep rolling.
[0015] Shot peening has been widely used in many industries for
decades to introduce a relatively shallow surface layer (<0.5
mm) of residual compression in metallic components, and has been
used to reduce the susceptibility of such components to SCC.
However, because it is a random impact process, shot peening
severely cold works the surface in order to cover the surface with
impact dimples and produce the compressive layer. The beneficial
compressive residual stresses in the highly cold worked surface are
then known to be susceptible to rapid thermal relaxation at
relatively lower service temperatures and is therefore unacceptable
for certain components. Further, the relatively shallow cold worked
residual compression layer is also susceptible to loss of
compression by tensile overload in work hardening materials, again
making the method unacceptable for certain components. Shot peening
also produces a roughened dimpled surface that makes it more
difficult to detect a crack or flaw using nondestructive inspection
(NDI) methods such as ultrasonic and eddy current means, again
making it unacceptable for certain applications.
[0016] LSP has been proposed for oil, gas, and petrochemical weld
applications. LSP can produce relatively deep (.about.1 mm)
compression, but is prohibitively expensive for components having
large surface areas needing treatment. Further, LSP requires an
ablative coating to be applied along the surface of the component
being treated that generates post-processing debris that must be
removed, thereby adding additional cost. In addition, LSP requires
repeated shocking cycles to achieve a 1 mm deep compressive layer,
thereby adding additional cost and process time. Also, LSP has been
known to damage the surface in three ways that have been shown to
contribute to component failure. First, internal cracking can occur
due to superposition of echoing shock waves. Second, LSP shock
waves are known to cause twinning in some crystals, like in
titanium alloys, that are associated with subsequent fatigue crack
initiation. Third, LSP is known to produce laser burns and local
areas of residual tension that occur if the ablative coating is
breached so that the laser strikes the bare metal surface; surface
tension from such burns will exacerbate SCC. Accordingly,
components treated using LSP often require post-processing
inspection that can significantly increase cost and processing
time. Components that have been damaged by the LSP process often
require additional processing or must be scrapped, thereby further
increasing cost and processing time. Further, denting of the
surface at each shock point by LSP may also require refinishing
operations. Like shot peening, the dented surface reduces the
effectiveness of eddy current and ultrasonic NDI techniques that
are vital to monitor the integrity of critical components.
[0017] Deep rolling, a form of roller or ball burnishing, has also
been proposed to introduce a layer of compression and cold work
deeper than that provided by shot peening and improves the surface
finish of the treated area. Deep rolling creates a highly cold
worked surface in the area being treated in order to mechanically
strengthen the surface material while introducing compression. The
depth and magnitude of cold work along the surface typically
exceeds that produced by shot peening. Cold working, however, is
well known to increase the susceptibility of metals to SCC.
Annealing or tempering to reduce or eliminate cold work and reduce
hardness is a common remedy used to reduce such susceptibility to
stress corrosion cracking. However, such processes increase
processing time and cost and may be difficult to perform on certain
components. Accordingly, deep rolling suffers from the conflicting
influences of the detrimentally increased cold working of the
surface as the beneficial residual compression is introduced.
[0018] Both SCC and fatigue failures are well known to initiate
from very small surface irregularities such as surface cracks,
small crevices, flaws, scratches, persistent slip bands, even
crystal twin boundaries created by deformation, and the like. Such
surface irregularities are known to serve as sites of increased ion
concentration, exacerbating SCC. The surface irregularities are
also points of stress concentration that are well known to serve as
fatigue crack initiation sites. Cold working, such as by shot
peening and deep rolling, is known to damage the crystalline
structure, creating slip bands, dislocations, and twinning that
make the surface more susceptible to chemical attack. Work
hardening, like hardening by heat treatment, makes metals more
susceptible to SCC. Further, deforming the surface to introduce
residual compression in ways that increase the surface
irregularities, such as by shot peening and LSP, is also known to
create local sites for SCC and fatigue initiation.
[0019] Accordingly, a need exists for corrosive resistant
components, such as Fuel Recovery Components as well as components
for use in a wide variety of chemical plant applications where SCC
failures occur, that have improved properties for mitigating or
preventing the deleterious effects of SCC and fatigue on useful
life. A practical, inexpensive method is needed for introducing a
relatively deep, stable layer of beneficial compressive stress
along and into the surface of such components that protects against
or reduces SCC, fatigue, corrosion fatigue and related failure
modes, and provides an improved surface finish with low cold
working, so that the metallic materials forming the components can
be used at their full available strength.
SUMMARY OF THE INVENTION
[0020] The present invention relates generally to corrosive
resistant components, such as Fuel Recovery Components as well as
components used in a wide variety of chemical plant applications,
and their method of manufacture. Such corrosive resistant
components have improved stress corrosion and fatigue properties
for mitigating or preventing the deleterious effects of SCC and
fatigue on useful life of the metallic components.
[0021] The preferred method of the invention disclosed herein
dramatically improves the SCC, corrosion fatigue, and general
fatigue performance of metallic components used in a wide variety
of applications, such as fossil fuel recovery and chemical plant
applications, manufactured from traditional low-cost alloys, such
as carbon steel, without altering either the alloy chemical
composition or the geometry of the component. The invention puts
the surface of the metallic component that is in contact with the
corrosive environment, and the layer of material immediately below
the surface, into a state of high residual compression with
controlled low cold working to a sufficient depth to encompass the
surface irregularities. In a preferred embodiment, the components
include tubular products such as pipe, tubing, casing, and
couplings, having the outside, or inside, or both surfaces
processed using various machine tools or robots commonly available
that can be used to position and move the burnishing tools to cover
all or a portion of the surface being treated. In a preferred
embodiment of the invention, a preferred method includes a surface
treatment which is performed in a single automated operation during
initial manufacture or during repair and overhaul of existing
components.
[0022] In a preferred embodiment of the invention, a layer of
compression is created using one or more ball or roller burnishing
tools and normal forces and tool positioning that produce a
relatively uniform layer of compression extending to a depth of
about 1 mm or more, so that the surface being treated when in
contact with a corrosive media and any surface irregularities, such
as discussed above, are confined in a layer of compressive residual
stress. The magnitude of the residual compression is generally on
the order of the yield strength of the alloy so that the surface
layer remains in compression under any applied tensile stresses
experienced by the component during service, and the stress at the
surface in contact with the corrosive environment never exceeds the
critical tensile threshold for SCC. In a preferred embodiment,
scratches and other surface irregularities are maintained in
compression, even under external applied tensile loading during
service, thereby fatigue initiation is prevented or significantly
reduced.
[0023] In another preferred embodiment of the invention, the
position and force applied to one or more tools during the
burnishing process is controlled to develop a specified level of
low cold work while introducing a layer having a desired magnitude
of compression. The layer of compression is of a magnitude and
depth such that the sum of residual and applied stress at the
surface and to a depth of at least nominally about 0.5 mm never
exceeds the threshold for SCC in the specific corrosive environment
of the application or the fatigue endurance limit of the material.
In a preferred embodiment, the depth of compression is chosen so
that the all or a majority of surface irregularities that may
operate as sites of crack initiation are confined within the depth
of the compressive layer.
[0024] Processing by the method of this invention allows
inexpensive steel or alloy to be used for components that operate
in a corrosive environment, such as Fuel Recovery Components as
well as components used in a wide variety of chemical plant
applications (such as piping, casing, couplings and related
components), that are normally restricted to use only in
applications not subject to SCC, to then be placed in service in
corrosive environments, such as in "sour" wells, and applications
previously requiring more costly alloys, such as stainless steels.
Inexpensive steels processed by the method of this invention can
then be used at their optimum temper and strength to allow higher
applied stresses in service allowing drilling of deeper wells at
lower cost.
[0025] In a preferred embodiment, the surface finish is improved by
burnishing with a finely finished tool to both reduce surface
irregularities while enhancing the detection limits of NDI.
Improved NDI detection limits reduce inspection costs and allow
more reliable detection of flaws. Elimination or the reduction of
surface irregularities improves both SCC and fatigue resistance, as
noted above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The foregoing and other features of the invention will be
best understood with reference to the following detailed
description of a specific embodiment of the invention when read in
conjunction with the accompanying drawings, wherein:
[0027] FIG. 1 is a plot of the subsurface residual stress and
diffraction peak width distributions produced by the burnishing
method of the subject invention in low cost P110 casing coupling
material. The residual stress shown is additive to the applied
stress in service;
[0028] FIG. 2 is a bar chart comparing the surface roughness
measured on the surface of P110 steel coupling stock
as-manufactured versus after-processing showing the improved finish
with the method of the current invention;
[0029] FIG. 3 is a bar chart showing failure by SCC in the National
Association of Corrosion Engineers' (NACE) 1% H.sub.2S solution of
as-manufactured P110 coupling stock samples after only 10 hour
exposure, and showing that exposure for over 420 hours did not
break the same P110 material after processing with the method of
the invention;
[0030] FIG. 4 is a schematic illustration showing the relationship
between the burnishing apparatus and control system for properly
inducing a desired stress distribution along and into the surface
of a component;
[0031] FIG. 5 is a flow diagram illustrating a preferred method of
the subject application;
[0032] FIG. 6 is a schematic illustration of a portion of a
component being manufactured using the method of the subject
invention; and
[0033] FIG. 7 is a schematic illustration of the surface of the
component shown in FIG. 6 illustrating various surface
irregularities.
DETAILED DESCRIPTION OF THE INVENTION
[0034] Referring to FIG. 1, the "as-received" subsurface residual
stress distributions created by the current methods of
manufacturing components used in a corrosive environment, such as
Fuel Recovery Components as well as components used in a wide
variety of chemical plant applications (including, but not limited
to pipe, tubing, casing and OCTG), is shown in comparison to the
beneficial high magnitude deeper compressive residual stress layer
created by a preferred method of the current invention. The
residual stress is shown in both units of the common engineering
usage in the United States, where 1 ksi=1000 psi, and in SI units
of MPa. Prior art manufacturing methods used for such typical
components, such as OCTG products, provide only relatively shallow
compression, generally less than -30.times.10.sup.3 psi (-30 ksi or
-200 MPa) extending to a depth of only about 0.020 in. (0.5 mm),
such as in the example shown. The prior art practice does not
attempt to control or optimize in any way the state of residual
stress on the surface of the products. The deeper and higher
magnitude compressive residual stress distribution produced by the
method of the present invention is shown as "LPB" in FIG. 1. The
method of the invention introduces compression of much higher
magnitude, nominally -100 ksi (-700 MPa), approaching the yield
strength of the material and extending to a depth greater than
about 0.040 in. (1 mm). In a preferred embodiment of the invention,
the depth of compression produced by the method of the subject
invention exceeds the depth of any surface irregularities.
[0035] Referring to FIG. 3, SCC of P110 casing material loaded as
U-bend samples in tension in a "sour" H.sub.2S solution is shown as
having been eliminated after processing by the method of the
subject invention. As-manufactured casing material failed in only
10 hours. In a preferred embodiment of the present invention, SCC
and failure from fatigue or corrosion fatigue damage is mitigated
by introducing a layer of compressive residual stress using a
process of LPB. Inducing a compressive stress distribution along a
surface by LPB is shown and described in U.S. Pat. Nos. 5,826,543
and 6,415,486, which are incorporated herein by reference.
[0036] Referring to the bottom of FIG. 1, the method of the
invention creates a desired compressive stress distribution by
deforming the material a minimal amount to achieve the required
compression. LPB produces less than approximately 5% cold work
while creating a depth and magnitude of compression comparable to
LSP or deep rolling. It has been found that using LPB for
components used in a corrosive environment, such as Fuel Recovery
Components as well as components used in a wide variety of chemical
plant applications, provides the desired compressive stress
distribution along and into the surface being treated without the
increased susceptibility to SCC and corrosion fatigue caused by
cold working of the surface such as by shot peening or deep rolling
and without the detrimental effects often caused by laser shock
peening.
[0037] Referring to FIG. 2, processing by the method of the
invention has been shown to improve the surface finish from about
204 to about 90 micro-inches, reducing the roughness of the surface
being treated. In the preferred embodiment, the method of the
subject invention improves the surface finish by rolling a hardened
ball or roller having a generally smooth surface along the surface
of the component. The surface produced by the subject invention
depends upon the burnishing parameters selected, such as the
smoothness of the hardened ball or roller, the ball or roller
diameter, and the force with which it is pressed against the
surface of the piping or other component. In a preferred
embodiment, the selected burnishing parameters are selected to
produce a smooth surface effective for improving detection limits
for NDI and elimination or reduction of surface irregularities that
can become fatigue crack or corrosion pit initiation sites.
[0038] In another preferred embodiment, as illustrated in FIG. 4,
the method uses a burnishing apparatus 100 having a constant volume
flow of fluid 102 to support a hydrostatic burnishing member 104
(such as shown or taught in U.S. Pat. No. 6,415,486 which is
incorporated herein by reference) that rolls along a surface
portion 106 of the component 108 being treated with sufficient
force to induce compressive stress 110 having a desired magnitude
and depth of compression and also allows large surface areas of
components, such as Fuel Recovery Components as well as components
used in a wide variety of chemical plant applications, to be
processed rapidly with minimum down time and tool costs.
[0039] Another embodiment of the method of the subject invention,
as shown in FIG. 4, a computer numerically controlled (CNC)
apparatus 112 is used to position one or more burnishing members
104 of a burnishing apparatus 100 to guide the members 104 in a
predetermined pattern along the surface portion 106 of the
component 108 being treated with sufficient, but not necessarily
constant pressure, to create a desired distribution of compressive
residual stress 110 on and into the surface portion 106 of the
component 108.
[0040] A further embodiment of the method utilizes a means of
rotating the component 114 being processed in the manner of a lathe
or similar means, while one or more burnishing apparatuses are held
at fixed angular positions and are positioned down the length of
the rotating component in a helical pattern to cover at least a
portion of the outside or inside surface.
[0041] Referring to FIG. 5, a preferred embodiment of the method of
the present invention is shown whereby components expected to
operate in a corrosive environment are identified (step 200). One
or more surface portions of one or more of the identified
components are identified and selected for receiving a surface
treatment (step 202), such as burnishing. The environment that the
surface portions will be exposed to, as well as various operating
applied, static, and alternating stresses expected to be
encountered, are identified (step 204). A stress distribution for
each surface portion is then determined based upon the geometry of
the component; the material forming the component along the surface
portion being treated; the environment to which the component will
be exposed; the use of the component; and the temperatures,
pressures, and applied, static, and alternating stresses to which
the component is expected to be exposed during service (step 206).
Burnishing parameters, such as the smoothness of the burnishing
member, the diameter of the burnishing member, the force with which
the burnishing member is pressed against the surface being treated,
and the pattern of burnishing are then determined based on the
desired stress distribution (step 208). In a preferred embodiment
of the invention, the burnishing parameters are selected such that
a relatively uniform layer of compression is induced along the
surface portion and extending to a depth sufficient such that the
surface being treated when in contact with a corrosive media and
any surface irregularities are confined in a layer of compressive
residual stress. In another preferred embodiment of the invention,
the burnishing parameters are also selected such that the magnitude
of the residual compression induced along and into the surface
portion is generally on the order of the yield strength of the
alloy forming the surface portion of the component so that the
surface layer remains in compression under any applied tensile
stresses expected to be experienced by the component during
service. In another preferred embodiment of the invention, the
burnishing parameters are selected such that in operation of the
component, the stress at the surface in contact with the corrosive
environment never exceeds the critical tensile threshold for SCC.
In another preferred embodiment, the burnishing parameters are
selected such that surface irregularities along the surface portion
are maintained in compression, even under external applied tensile
loading during service of the component, thereby reducing or
eliminating fatigue initiation. The method further comprises the
step of performing a burnishing operation along the one or more of
the identified surface portions using the determined burnishing
parameters to induce the desired stress distribution along and into
the identified surface portions (step 210). In a preferred
embodiment, the burnishing operation is performed such that the
amount of cold work induced along the surface portions is less than
about 5%. In a preferred embodiment, the burnishing parameters are
fed into a computer control system that cooperates with a
burnishing apparatus (such as a CNC system) for performing the
burnishing operation (step 212). Inspecting the treated component
for surface irregularities (step 214) is performed after
burnishing.
[0042] It should be understood that the method of the subject
invention will improve the SCC and H.sub.2S cracking resistance of
metallic components formed of less expensive alloys to allow them
to be used in chloride or sulfide corrosive environments such as
Fuel Recovery Components and chemical plant applications, where
they cannot currently be used. It should also be understood that
the method of the subject invention may be integrated into any
existing production or repair processing platform/delivery system
to allow for processing of both new production components as well
as repair/life extension of existing components. It should also be
understood that the method of the subject invention may be used on
various components used in corrosive environments such as most or
all metallic components used in a fossil fuel recovery where any
environmentally assisted cracking is expected or may occur.
[0043] Referring to FIGS. 4, 6, and 7, a schematic cross-section of
a surface portion 106 of a metallic component used in a corrosive
environment 108, such as Fuel Recovery Components or components
used in chemical plant applications having an outer surface 116 and
an inner surface 118 is shown (FIG. 6). In a preferred embodiment,
the component is in the form of a pipe, tube, casing, coupling, or
other similar component. The surface portion 106 of the component
108 has a compressive residual stress distribution 110 induced
therein. The depth D of compression is such that it exceeds any
surface irregularities 120. In another preferred embodiment, the
surface portion 106 has less than approximately 5% cold work. In
another preferred embodiment of the invention, the residual stress
distribution extends through the entire thickness D of the surface
portion 106.
[0044] It should be understood that the component 108 can include a
plurality of surface portions 106 or that the entire component can
include one or more surface portions 106. It should also be
understood that each surface portion can have its own unique
compressive stress distribution or a plurality of residual stress
distributions can be utilized.
[0045] It should be understood that one or both of the inside and
outside surface portions can be treated to improve the surface
finish and produce a depth of compression is such that all or a
majority of surface irregularities, such as flaws, corrosion pits,
persistent slip bands, and the like that may operate as sites of
crack initiation are confined within the depth of the compressive
layer. In another preferred embodiment, the depth of the residual
stress distribution in the surface portion extends to a depth of at
least to about 1 mm. In another preferred embodiment of the
invention, the residual stress in the surface portion is of a
magnitude and depth such that the sum of residual and applied
stress at the surface and to a depth never exceeds the threshold
for SCC in the corrosive environment of the application or the
fatigue endurance limit of the material forming the portion of the
component.
[0046] It should now be apparent to one skilled in the art that the
subject invention provides corrosive resistant components that can
be used at their full available strength due to improved properties
that mitigate or prevent the deleterious effects of stress
corrosion cracking and fatigue. Further, that the invention is a
practical, inexpensive method of introducing a relatively deep,
stable layer of beneficial compressive stress along and into the
surface of Fuel Recovery Components, as well as components for use
in a wide variety of chemical plant applications, that protects
against or reduces SCC, fatigue, corrosion fatigue and related
failure modes with an improved surface finish, and low cold
working.
[0047] While the methods and components described herein constitute
preferred embodiments of the invention, it is to be understood that
the invention is not limited to the precise method and components,
and that changes may be made therein without departing from the
scope of the invention which is defined in the appended claims.
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