Patent Application
20080032152
To assist those of ordinary skill in the relevant art in making and using the subject matter hereof, reference is made to the appended drawings, wherein:
The present invention relates to the use of laser shock peening and processing in oil and gas exploration, producing, and petrochemical applications to improve the fatigue life, stress corrosion resistance, environmental cracking resistance and other properties of critical regions of steels and corrosion resistant alloys subjected to LSP treatment. The use of LSP in oil and gas exploration, producing, and petrochemical applications is distinguishable over the prior art in providing for conventional and friction stir welds, conventional and friction stir weld repairs, and treatment of critical regions of structures to yield improved properties and performance.
Laser shock peening or processing (LSP) is a mechanical process for treating of metallic materials where a high-energy, pulsed Neodymium-glass laser or yttrium aluminum garnet (YAG) crystal lasing rod, producing a very short pulse (from about 14 to 30 nanoseconds long) and a wavelength of about 1.06 μm with an energy per pulse of about 50 joules or more is directed from the laser through a chain of mirrors and lenses onto the surface of the part being treated. The surface of the metal part to be treated via LSP is first covered with two types of overlays. The first type of overlay on the surface of the part is an opaque overlay which is opaque to the laser beam. The opaque overlay may be, but is not limited to, a black coating, black paint, lead, aluminum, copper, and zinc. The type of opaque overlay may be used to tailor the shape and amplitude of the stress waves generated via LSP. Black paint is a particularly preferred opaque overlay. The second type of overlay is positioned on top of the opaque overlay, and may be any material that is transparent to the laser beam. The transparent overlay may be, but is not limited to, water, quartz and K7 glass. The surface area of the metal part to be treated with LSP is first coated with an opaque overlay, and then coated with the transparent overlay.
The laser beam is then directed onto the surface of the metal part and passes through the transparent overlay and strikes the opaque overlay where it immediately vaporizes a thin surface layer of the opaque overlay. The vapor or plasma generated from the opaque overlay then absorbs the incoming laser energy and rapidly heats and expands against the surface of the metal part to be treated and the transparent overlay. The transparent overlay functions to trap the thermally expanding vapor or plasma against the surface of the metal part to be treated, and results in the pressure rising to a much higher level than if the transparent overlay were not present. The trapped vapor or plasma builds to a pressure of up to 100,000 atmospheres. The sudden, high pressure against the surface of the metal part to be treated causes a shock wave to propagate into the metal part to be treated, and if the peak stress of the shock wave is above the dynamic yield strength of the material, the metal part yields and plastically deforms. As the stress wave propagates deeper into the metal, the peak stress of the wave decreases, but deformation of the metal continues until the peak stress falls below the dynamic yield strength of the metal. The shock wave generated by the laser and the coated metal part gives rise to compressive residual stresses at the surface of the metal part to be treated. The peak pressure generated during LSP treatment may be controlled by changing the power density of the laser beam. The peak pressure generated is proportional to the square root of the peak power density. The power density may range from 0.1 to 1×104 depending upon the laser type, the treated material type and the depth of residual compressive stresses desired. LSP variable parameters include, but are not limited to, laser power density, spot size, and pulse width.
The shape of the spot treated on the metal part with the laser is generally round, but other shapes may be used to provide more efficient and effective processing conditions. The size of the area of the metal part to be treated with LSP with one pulse depends on number of material, laser and processing factors. The spot size may range from about 0.1 inch to about 1 inch in diameter. A typical spot size is generally from about 0.24 to about 0.35 inches in diameter. For metal parts that are about 0.5 inch in thickness or more, a single laser beam is directed onto the area of the metal part to be treated. For metal parts that are less than about 0.5 inch in thickness, in order to minimize the distortion of the part, the laser beam may be split into two beams of equal intensity, and these beams are used to strike opposite sides of the part simultaneously. Alternatively, thin sections of metal parts to be treated may be treated from one side only by using a back-up support for the side of the metal part not being treated.
The size of the area of the metal part to be treated depends on the part design and the service conditions. A metal part may require that only a small area be LSP treated and a single treated spot may be sufficient, for example around small oil, pin, or bolt holes, or at the root of a notch in the side of a thin section. In other cases, the metal part may require that a large area be LSP treated, for example around the circumference of a weld line joining two pipes or a weld line joining two shafts of deep water oil drill bit. In these cases, successive spots are overlapped until the circumference has been completely treated with LSP. Generally in treating areas larger than 1 centimeter (0.39 inches) in diameter, overlapping spots are needed.
The high-energy, pulsed Neodymium-glass laser for use in LSP may be positioned in close proximity to the work station where the metal parts to be treated are held and manipulated during LSP. In this case, a metal part is placed in a work station by loading it into a fixture, and then the part and fixture are moved into the proper position relative to the laser for LSP. After the metal part is positioned, the laser beam is directed into the work station, treating the desired spot on the metal part. The metal part is then either moved to the next position for the following spot to be treated or is removed from the work station and replaced with the next part to be treated. For high production rates, the steps of part pick-up, positioning, LSP, and part removal may be performed automatically, for example, via a robotic means. Alternatively, the laser may be transported to a field area, for example to pipeline or drilling platform areas where it may be used to treat the weld area after joining two pieces of pipeline or two pieces of drill shafts. In this case, the laser as opposed to the part to be treated may be repositioned following the LSP of a spot on the metal part. The circumference of a weld line in a pipeline or drill shaft may be treated via LSP by rotating the laser around the circumference of the circular part. Spot overlap is again utilized to provide for complete treatment of the weld area.
Laser peening treatment decreases the grain size thickness of the steel or corrosion resistant alloy near the surface region which induces a plastic strain relative to the bulk of the structure. The relative deformation of the surface region relative to the bulk region of the structure results in the generation of residual compressive stresses near the surface of the metal part. These residual stresses may be measured using x-ray diffraction techniques by measuring the spacing of the crystallographic lattice planes at the surface of the metal part relative to the unstressed crystal lattice of the same materials not subjected to LSP. Tension increases the spacing between the lattice planes and compression decreases the spacing between the lattice planes. The distribution of the residual stress below the metal surface is determined by successively removing a thin layer from the surface by electropolishing and then making x-ray measurements of the new surface. This incremental process is continued down to the maximum depth of interest, generally from about 0.020 to about 0.050 inches in depth. The actual depths of the LSP-induced compressive stresses will vary depending on the type and intensity of the laser processing conditions and the properties of the metal to be treated. With LSP, the residual compressive stresses are generally highest at the surface and decrease gradually with increasing depth below the surface.
The distribution of the residual stresses below the surface is generally much deeper for LSP than it is for shot peening. For comparison, the depth of compressive stresses induced with shot peening is generally less than 0.010 inches as opposed to 0.10 inches with LSP or an order of magnitude deeper with LSP.
Among the properties improved by the introduction of residual compressive stresses induced by LSP treatment include, but are not limited to, surface strength, fatigue life, fretting fatigue resistance, stress corrosion resistance, fatigue cracking resistance, environmental/corrosion cracking resistance, and surface hardness. In particular, the compressive residual stresses imparted by LSP prevent cracks from growing in metal structures, and hence improve the part's fatigue life. The compressive residual stresses are effective for reducing both fatigue cracks, environmental/corrosion cracks.
In one aspect of the present invention, LSP is useful in treating critical regions of ferrous materials, preferably for treating the critical regions of steels, and more preferably for treating high carbon steels having a CE equal to or greater than 0.48. Exemplary, but not limiting, plain carbon and alloy steels include, AISI 1010, 1020, 1040, 1080, 1095, A36, A516, A440, A633, A656, 4063, 4340, and 6150. Exemplary, but not limiting, high carbon steels include, AISI WI, SI, O1, A2, D2, M1, and API L80. In another aspect of the present invention, LSP is useful in treating ferrous corrosion resistant alloys, including but not limited to, stainless steel. Exemplary, but not limiting, stainless steels include, AISI 409, 446, 304, 316L, 410, 440A, 17-7PH and duplex s.s. In a further aspect of the present invention, LSP is useful in treating non-ferrous alloys, including but not limited to, titanium alloys and nickel alloys.
The critical regions of ferrous or non-ferrous material components include, but are not limited to, notch areas, areas surrounding bolt and pin holes, and at the root of a notch in the side of a thin sections.
In one embodiment of the present invention, LSP is used following conventional fusion welding methods in the weldment area to improve the aforementioned properties in the surface region of the weld, and hence improve the integrity and fatigue properties of the fusion weld.
In another embodiment of the present invention, LSP is used following friction stir welding methods in the weldment area to improve the aforementioned properties in the surface region of the weld, and hence improve the integrity and fatigue properties of the friction stir weld. FSW and LSP are used in combination to improve the service life of welded structures used in the oil and gas exploration, production, and refining industries, as well as the petrochemical industry. More particularly, FSW is used to make the weld followed by LSP being used to treat the weld area to reduce residual tensile stresses by creating residual compressive stresses near the surface of the friction stir weld area.
In another embodiment of the present invention, FSW in combination with LSP finds particular application in the welding of duplex stainless steels (duplex s.s. or DSS). Duplex s.s. derives its strength and corrosion resistance from a controlled balance of ferrite and austenite phases. The desired mixture phases in the bulk duplex s.s. is achieved by controlled hot working and/or a combination of cold working and annealing treatments. However, when the duplex s.s. is welded, the steel is heated to a very high temperature in a single phase ferrite region and cools to the duplex phase upon cooling to room temperature. In order to achieve the required balance of phases in the weldment at room temperature, the cooling rate of the weld has to be controlled. In practice, the cooling rate varies considerably affecting the phase balance and thus the resultant properties of the weldment. FSW of duplex s.s. provides a more consistent phase balance since the temperature of the joints may be more precisely controlled, and in particular may be done at a lower temperature in the two phase region, thus consistently yielding an acceptable microstructure and resultant properties. Following FSW of the duplex s.s. joint, the weldment is subjected to LSP to further enhance the aforementioned surface properties of the weld area.
In another embodiment of the present invention, LSP is used following friction stir repair methods of cracks in the repair area to improve the aforementioned properties in the surface region of the repair, and therefore improve the integrity and fatigue properties of the repair area.
In yet another embodiment of the present invention, LSP is used following a combination of friction stir and fusion welding methods in the weldment area to improve the aforementioned properties in the surface region of the weld, and hence improve the integrity and fatigue properties of the friction stir weld. More particularly, the steel is welded first using fusion welding or other conventional welding method known to have a high rate of productivity. Following high throughput fusion welding, the fusion line and HAZ of the welds may be processed by FSW. This reduces and potentially eliminates the HAZ and the tensile residual stresses in the near surface regions. The combination of fusion welding and friction stir welding enhances the integrity of the joint with regard to resistance to hydrogen embrittlement, fatigue, etc. without sacrificing productivity since bulk of the welding is performed by conventional methods and only the critical subsurface regions are processed by FSW. Finally, the weld area is processed by LSP to further enhance surface properties. Following the combination of fusion welding and friction stir welding of the weldment, it is subjected to LSP to further enhance the aforementioned surface properties of the weld area.
In one embodiment of the present invention, LSP is used to treat critical regions of ferrous and non-ferrous material structures used in the oil and gas exploration, production, and transport industries, as well as the petrochemical industry.
Exemplary, but non-limiting, structures in the oil and gas exploration, production, refining industry where LSP treatment is useful with conventional fusion welding or friction stir welding joining and repair techniques, are high strength pipeline weld areas, SCR and TTR weld areas, threaded components, oil drilling equipment weld areas (i.e. two sections of a deep water oil drill string), LNG and PLNG container weld areas, riser/casing joints, and well head equipment. In particular, LSP treatment reduces residual tensile stresses and softening in the HAZ for fusion welded high strength pipelines used to transport oil and gas. LSP improves the integrity of the weld or joint which correspondingly increases the toughness and fatigue resistance of the welded joints.
Applicants have attempted to disclose all embodiments and applications of the disclosed subject matter that could be reasonably foreseen. However, there may be unforeseeable, insubstantial modifications that remain as equivalents. While the present invention has been described in conjunction with specific, exemplary embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description without departing from the spirit or scope of the present disclosure. Accordingly, the present disclosure is intended to embrace all such alterations, modifications, and variations of the above detailed description.
The following examples illustrate the present invention and the advantages thereto without limiting the scope thereof.
ASTM A656 grade 1 steel was treated with LSP to determine the residual compressive stresses generated below the surface.
The A656 grain structure was subsequently measured using electron back scattered diffraction (EBSD).
