Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/12—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure
  • B23K26/123—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure in an atmosphere of particular gases
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/12—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure
  • B23K26/123—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure in an atmosphere of particular gases
  • B23K26/125—Working by laser beam, e.g. welding, cutting or boring in a special atmosphere, e.g. in an enclosure in an atmosphere of particular gases of mixed gases
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/16—Removal of by-products, e.g. particles or vapours produced during treatment of a workpiece
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/20—Bonding
  • B23K26/21—Bonding by welding
  • B23K26/24—Seam welding
  • B23K26/28—Seam welding of curved planar seams
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/20—Bonding
  • B23K26/32—Bonding taking account of the properties of the material involved
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/34—Laser welding for purposes other than joining
  • B23K26/342—Build-up welding
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23P—METAL-WORKING NOT OTHERWISE PROVIDED FOR; COMBINED OPERATIONS; UNIVERSAL MACHINE TOOLS
  • B23P6/00—Restoring or reconditioning objects
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F11/00—Arrangements for sealing leaky tubes and conduits
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2103/00—Materials to be soldered, welded or cut
  • B23K2103/50—Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F2275/00—Fastening; Joining
  • F28F2275/06—Fastening; Joining by welding
  • F28F2275/067—Fastening; Joining by welding by laser welding

Definitions

• the invention relates to reforming degraded areas in ductile materials, in particular by melting a localized area to a predetermined depth, re-forming the localized area by cooling it, and advancing the localized melting and cooling through the degraded area to restore it to an integrally continuous form.
• the invention is particularly applicable to fusing service-induced stress and corrosion defects in coolant circuit tubes of pressurized water nuclear reactors.
• FIGURE 1 is a partial section view showing application of the invention to the repair of heat exchang ⁇ er tubes in a nuclear steam generator plant;
• FIGURE 2 is a schematic illustration of a welding means for directing laser emissions against the inner walls of a tube to be repaired by localized melting of the tube along a scanned progressive pattern;
• FIGURE 3 is a schematic illustration of a method for relative displacement of the tube and welding means
• FIGURE 4 is an elevation view, partly in section, showing application of an alloying agent in connection with the welding
• FIGURE 5 is plan view of a tube inside surface following a direct tube repair as described
• FIGURE 6 is a longitudinal section view through a weld line according to FIGURE 1;
• FIGURE 7 is a lateral cross section through an alternative form of repair using a consumable insert alloying material.
• a welding head 32 is placed into the tube 24 at the deterio- rated zone.
• the welding head is activated and moved progressively relative to the tube so as to melt a local ⁇ ized point along a line 42 on a surface of the wall.
• a welding line is formed, with the tube material behind the point of application of the weld head cooling and solidifying.
• the welding process melts and fuses the degraded area over a welding line having a width equal to the localized point of melting, and to a depth in the wall 22 defined by the dimensions over which the welding head applies energy, the amplitude of the energy applied and the time the energy is applied to a given location.
• the welding head is operated at a suffi ⁇ cient power level and is advanced at a sufficiently slow speed that the localized point is melted to a depth such that after solidifying the tube is restored to serviceable condition for its intended use.
• Solid material surrounds the localized area that is melted at any one time, and supports the melted material. After passage of the welding head, the surrounding solid material cools the material quickly by carrying away the thermal energy applied by the welding head.
• the welding head 32 is displaced laterally of the first line. Localized melting is continued along a line which is adjacent or overlapping the first line to melt and cool, thus to reconstitute the degraded area over a further width adjacent the first weld line.
• the weld head is advance linearly and laterally in this manner, successively melting linear sections of the wall and fusing the wall over the entire degraded area in a raster-like series of passes.
• the weld line is preferably advanced laterally by an amount less than the width of the weld line 42, such that the first weld line and the further weld line partly overlap, and a part of the first weld line is remelted in the process of forming the next.
• the lateral advance can be stepwise or continuous and can involve any pattern of adjacent, preferably-over ⁇ lapping passes which encompass the whole area of the repair.
• One alternative is to rotate the welding head relative to the axis of the tube to form the welding line and axially to advance the welding head relative to the tube to form the further width. When advancing the line of welding continuously, this motion produces a helical pattern of weld lines as shown in FIGURES 1 and 2.
• Another alternative is to relatively displace the point of application of energy via the welding head and the tube axially in an oscillating motion to form the welding line.
• the welding head is also relatively rotated with respect to the tube to form the further width.
• the pattern produced by this motion is represented by FIGURE 3.
• the rotation can be stepwise, continuous or oscillating.
• the welding process uses laser welding, although other means for isolated local melting of a point on the tube are also possible.
• the welding head comprises an optical system 62, directing laser emissions onto the degraded area 26.
• Mirrors 64, lenses 66 and fiber optic light conduits 68 can be em ⁇ ployed.
• a drive means 72 is operable to rotate and axially translate a stem 74 compris ⁇ ing the welding head 32.
• the fiber optic cable 68 couples the welding head to a high powered laser 76, for example a ND:YAG laser.
• the distal end 82 of the fiber optic cable is spaced from mirror 64.
• a first lens 66 collimates the light diverging from the end of the fiber optic cable and a second lens 67 focuses the light at the point of applica ⁇ tion to the tube wall.
• Lens 67 has a focal length substan ⁇ tially equal to the sum of the distances between lens 67 and the center of mirror 64, and between mirror and the point of welding.
• the light emitted from the fiber optic cable is thereby focused at a spot on the area 26 of tube 24 that is being repaired.
• the drive means 72 can rotate the stem relative to the fiber optic cable. Whereas the light is collimated between lenses 66 and 67, the axial position between end 82 and lens 66 is held constant, i.e., at the focal distance of the lens.
• the distance between lenses 66 and 67 can be varied, e.g., with axial displace ⁇ ment due to operation of the drive means 72. However, it is preferred in connection with axial displacement to move the welding head or stem axially as a unit to effect axial displacement . o
• FIGURES 1 and 2 illustrate an embodiment arranged to produce a helical pattern 48 of weld lines.
• an axial pattern is produced, using an axially oscillat ⁇ ing drive means that moves the weld head up and down in the tube.
• a motor 96 can be provided for this purpose as shown.
• lenses focus the light emitted at the end 82 of the fiber optic cable 68.
• the welding head is advanced axially and rotation- ally to cover the entire deteriorated area 26, in a series of passes.
• Parallel axial weld lines as shown in FIGURE 3 can be made by rotationally indexing the weld head.
• Slanting or helical lines can be made by rotating the weld head continuously during scanning of the laser beam.
• each weld line is tracked relative to the position of a previous weld line. This can be accomplished by providing a guide on the welding head, operable to rest against a ridge or other dimensional variation at the edge of the last weld line.
• FIGURE 4 shows the surface appearance of the inside wall of a tube following a direct surface repair according to the invention.
• Each weld line in this case is placed adjacent the previous line, with a slight overlap, e.g., 50 to 80% of the width of the weld line.
• the specif ⁇ ic power level of the laser can be varied as needed to accommodate a desired area over which the laser is to be focused, and a desired rate of advance.
• An average power of at least 200 watts can be used for welding, and an average power of 200-800 watts can be used advantageously.
• the depth of the weld can be varied as a function of power level, focusing and rate of advance, in order to melt the tube material to the required depth.
• the tempera- ture of melting of course varies with the material of the tube.
• the melting temperature is about 1,350 to 1,410°C (or 2,470 to 2,575°F) .
• the typical thickness of the tube wall of a nuclear steam generator is about 0.050 to 0.055 inches (1.3 to 1.4mm).
• the weld depth extends through 80 to 100% of the wall thickness.
• FIGURE 5 shows an elevation view of an actual tube weld, including the partly overlapping weld lines.
• the surface of the inner surface of the tube is rendered somewhat less smooth due to the welds, however the inside diameter of the tube is only minimally reduced.
• FIGURE 6 via a longitudinal cross section through a line of welding, a shallow penetration surface repair by welding melts the tube through about 40% of its thickness. With the use of a narrow bead, the weld can extend through 100% of the tube thickness.
• the bead tends to taper in cross section, having a typically conical shape as shown in FIGURE 7.
• the melted material extends through the wall, the lateral dimensions of the bead at the outer wall surface are relatively small. Accordingly, the unmelted portion of the tube mechanically supports the melted bead. The area which is melted at any one time is relatively small and does not tend to flow, making it possible using this technique to weld quite deeply into the tube. Additionally, the heat energy applied at the welding point is quickly carried away and the melted portion cools promptly after the welding head passes.
• An alloying material 54 can be diffused into the material of the tube during the welding process, and consumed.
• the alloying material can be applied as a powder that is sprayed or painted onto the tube surface, either before or during welding, for example together with appli ⁇ cation of a welding cover gas.
• the alloying material may also be applied as a sleeve shaped insert that is consumed in the process and fused with the melted and reformed material of the tube.
• the results of welding over an alloying material 54 are shown in a lateral cross section through a series of weld lines in FIGURE 6. In order to obtain good control of the depth of penetration of the weld repair, it has been found that a relatively slow laser pulse frequency should be maintained, with a relatively long pulse duration.
• a pulse frequency of less than about 20 Hz and a pulse duration of above about .005 seconds are preferable, using a laser at about 300-325 watts of average power.
• Acceptable welds having 80-100% wall thickness penetration have been obtained using a pulsed YAG laser set for a pulse frequency of 14 Hz and a pulse duration of .0076 seconds at these power levels.
• the dominant cooling mechanism for the weld pool is radia ⁇ tion, and a portion of the weld pool returns to the solid condition between the energy pulses.
• conduction cooling is less dominant than radiation cooling under these conditions, the weld repair is less sensitive to heat sink conditions outside of the tube wall, such as the presence or absence of a tube support plate, tube sheet or moisture. It has been found that these parameters can provide crack- free weld repairs for Inconel tubing which has sulfur levels in the usual range of .002-.003 percent by weight. However, when the sulfur levels increase to about .004 percent, the resulting weld repairs remain subject to cracking.
• Inconel tubing having sulfur content above about .004 percent it is desirable to utilize laser energy parameters which result in conduction being the predominant cooling mechanism and which will maintain the weld pool as a liquid between energy pulses, in order to maximize the dispersion of the contaminants in the weld pool.
• pulse frequencies of at least about 100 Hz and pulse durations of no more than about .001 seconds have been found to provide crack free weld repairs in such material .
• the average power output of the laser must be higher due to the relatively lower power peak.
• a disadvan ⁇ tage of such parameters is that the repaired surface tends to be rippled, probably due to instabilities in the weld pool resulting from the higher power level, and this makes it difficult to perform nondestructive testing on the repaired tube wall areas. Furthermore, at these parameters the weld is more sensitive to heat sink variations outside of the tube wall because of the increased influence of conduction cooling. It is known in the art to apply an inert cover gas over a weld pool to isolate the weld from external contami ⁇ nants and oxygen. However, it has been found that reduced sensitivity to impurities in steam generator tube wall material may be obtained by utilizing a reactive cover gas which acts to scavenge the impurities from the weld pool.
• a reactive cover gas may be supplied to the weld area via the inside diameter of the weld head stem 74, as illustrated in Figure 3.
• the cover gas can be made to pass over the mirrors 64, lenses 66 or other optical components in order to protect them from -weld splatter and in order to provide cooling.
• Carbon dioxide supplied in this manner will provide adiabatic cooling as it expands within the weld head, thereby providing cooling to the optical compo ⁇ nents. Since the oxygen in the carbon dioxide is in combined form, it does not oxidize the optical components.
• a second cover gas may be supplied to the weld area via the inside diameter of the tube under repair. In this manner, it is possible to provide a combination of two types of cover gases; a first cover gas being selected primarily for its cooling proper ⁇ ties and a second cover gas being selected primarily for its ability to scavenge impurities from the weld pool.
• carbon dioxide may be provided over the optical components via the weld head stem, and air or oxygen may be provided along the inside of the tube.
• the laser energy parameters for the first step may be selected to have a relatively low pulse frequency and a relatively long pulse duration, thereby providing good control of the depth of penetration.
• a first repair having a weld depth of 80 to 100% of the wall thickness can be obtained with relatively low sensitivity to external heat sink conditions.
• a 14 Hz pulse rate and .0076 second pulse duration may be selected at an average power level of approximately 300-325 watts.
• the invention is particularly applicable to correcting degradation of the heat transfer tubes of a nuclear steam generator plant.
• a plurality of individual tubes 24 are arranged parallel to one another and extending between inlet and outlet manifolds, one wall 25 of a manifold being shown in FIGURE 1.
• Access to the tubes can be obtained from inside the manifolds, for example controlling the weld head by remote control and thus avoiding human exposure to the environment of the reactor systems.
• This invention may also be applied to any other type of tubular product, for example a pipe or a reactor vessel head penetration; and further, it may be applied to any part having a wall, for example a valve body, a tank wall, etc.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Mechanical Engineering (AREA)
• Optics & Photonics (AREA)
• Plasma & Fusion (AREA)
• Thermal Sciences (AREA)
• General Engineering & Computer Science (AREA)
• Laser Beam Processing (AREA)

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• 1994
  • 1994-09-29 WO PCT/US1994/009570 patent/WO1996003250A1/en not_active Application Discontinuation
  • 1994-09-29 KR KR1019970700375A patent/KR970704548A/ko not_active Application Discontinuation
  • 1994-09-29 EP EP95903499A patent/EP0771244A1/de not_active Withdrawn

Non-Patent Citations (1)

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