Patent Application
20090029631
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.
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.
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.
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.
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.
A need exists, therefore, for an improved method for mitigation of SCC. The present invention meets that need.
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.
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.
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.
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.
The invention will now be further described with reference to the accompanying drawings, in which:
a is a detailed perspective view of a tool of
b and 4c are perspective views of the tool of
a and 5b are perspective views of the tool of
a and 8b are side views of the tool of
a 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;
a and b are further elevational views of the tool of
a and b are side elevations of the tool of
a and b are side elevational views of a further embodiment of an outside diameter surface improvement tool to back side weld showing gear features;
a and b are side elevational views of the tool of
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.
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.
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.
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.
Referring to
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.
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.
a is a perspective view of a tool 2 of
a and 5b are perspective views of the tool 2 of
a and 8b are side views of the tool 40 of
a 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
a 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.
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.
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.
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.
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.
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″).
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.
Commercial bristle length (“trim length”) ranges from 1.4″ to 4″, depending primarily on brush diameter. Typically, lengths from ¾″ to 1½″ are employed.
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×6″ diameter brush. Good results have been obtained using 6-17 HP hydraulic motors driving a 2″ wide×4″ diameter brush. Both were 80 grit silicon carbide in 0.06″ diameter Nylon bristles. A 6″ diameter brush having 0.045×0.090 rectangular bristles has been evaluated with similar productivity results as for the 0.060 round bristles.
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 ⅜″ bristle engagement, set statically (less when running due to apparent increase in brush stiffness at high rpm due to centrifugal force).
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.
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.
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.
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.
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.
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.
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”.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Referring to
a, 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.
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.
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.
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).
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.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11233031 | Sep 2005 | US |
| Child | 12149156 | US |
