Direct tube repair by laser welding

The wall of a pressure vessel tube having stress-corrosion cracks or the like is repaired by localized melting using laser welding, for fusing over defects by melting and re-solidifying the metal. A high powered laser beam is focused at a limited area or spot on the internal wall of the tube. The welding head is operated at sufficient power and is relatively moved in the tube at a sufficient rate to melt the spot while advancing along a line on the internal surface of the wall. The wall is melted to a depth of part or all of the thickness of the wall. Points at which the tube has been melted by the focused beam cool following passage of the welding head. The welding head is also advanced laterally of the line, either continuously or stepwise. Localized melting and cooling of the tube material continues progressively, line by overlapping and/or adjacent line, to melt and reform the degraded area. By successively melting, and optionally alloying linear sections of the wall using an alloying material, stress-corrosion cracks and similar defects are fused over, repairing the degraded area. The localization of the melting is such that the metal does not substantially flow and the metal cools quickly following passage of the weld head, to below a temperature of sensitization.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
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 melting preferably involves scanning over 
a surface along successive adjacent lines on the inside surface of a tube, 
to progressively melt and fuse over defects at least to a depth sufficient 
to restore the tube to a serviceable condition for effecting heat 
exchange, pressure confinement or the like. The invention is particularly 
applicable to fusing service-induced stress and corrosion defects in 
coolant circuit tubes of pressurized water nuclear reactors. Localized 
melting is preferably accomplished by welding, especially with a laser 
weld head. An alloying material can be included during welding, the 
alloying material being supplied as a powder, gas or self-supporting tube 
and consumed in the process. 
2. Prior Art 
Heat transfer tubes in the steam generator systems of nuclear power plants 
are subject to degradation over time of the primary pressure barrier in 
zones of high mechanical stress. The pressure and temperature of coolant 
in the coolant circuit can be substantial. Static pressure due to the 
coolant may be on the order of 100 bar (2,200 psi) in the coolant circuit 
and the coolant temperature can be 600.degree. F. when the plant is 
operational. The thermal and mechanical stresses applied to the tubes tend 
to degrade the tubes in the regions of highest stress. 
One area of concern is the tubing in heat transfer devices. In a reactor 
system, heated water from a primary coolant circuit is passed through heat 
exchanger tubes having a second flow of water passing outside the tubes. 
Should the barrier defined by a tube deteriorate, the secondary coolant 
may become contaminated with radiation from the primary coolant. It is 
expensive to replace the heat exchangers, and methods have evolved for 
repairing degraded steam generator heat exchanger tubes in various 
situations. However, the inaccessibility of the heat exchanger tubes as 
well as the radioactive environment surrounding the heat exchanger tubes 
present difficult problems. 
A steam generator heat exchanger generally has a number of substantially 
parallel-flow tubes coupled between inlet and outlet manifolds. Whereas 
there are parallel alternative flow paths, one method of dealing with a 
deteriorated steam generator tube is to simply plug lit to prevent 
leakage. Plugs are inserted upstream and downstream of the plugged zone, 
isolating the zone of deterioration from the primary side coolant, and 
thus preventing leakage. 
Plugging an affected tube reduces the active heat exchange surface area and 
results in a reduction in steam generator performance. Designers build an 
extra margin into the design of heat exchangers of this type, in 
recognition that it may become necessary in the future to plug some of the 
heat exchanger tubes without replacing the entire unit. If carried to an 
extreme in which the number of plugs installed exceeds the plugging margin 
provided according to the steam generator design, the steam generator must 
be de-rated because the rated heat generation capacity exceeds the 
available heat transfer capacity of the heat exchanger. 
An alternative repair known as sleeving involves isolating only the surface 
of the tube in the area of deterioration while allowing primary coolant to 
flow in the tube, i.e., without removing the entire tube from service. 
Sleeving involves fitting an undersized length of tubing (the sleeve) into 
the affected tube, and attaching the sleeve upstream and downstream of the 
degraded zone to the inner walls of the tube, preferably sealing the 
sleeve to the tube to prevent leakage between them. The sleeve forms a 
seal and restores the integrity of the pressure boundary. The sleeve may 
be attached to the walls of the tube by mechanical means, such as by 
forming complementary bulges in the sleeve and the tube after positioning 
the sleeve to lap the area to be repaired. For a hermetic leak-tight seal 
the sleeve can be welded to the tubing, for example using gas tungsten arc 
or laser welding. Alternatively, the sleeve and tube can be attached by 
brazing. In either case, the sleeve is joined to the tube along the ends 
of the sleeve, typically along a circular line at each end of the sleeve. 
The sleeve does not completely occlude the tube like a plug, but there are 
penalties. Thermal transfer declines due to flow reduction resulting from 
the reduced internal diameter of the tube and the discontinuity defined by 
the sleeve. In an extensive sleeving program, or in a generator that 
already has a large number of plucks, sleeving may not be practical. The 
thermal transfer characteristics of the heat exchanger tube are adversely 
affected by a sleeve. The sleeve thickens the overall tube wail. Any area 
of relatively lower thermally conductive contact between the sleeve and 
the original tube, such as a gap or corroded zone, forms an insulating 
zone, reducing the heat transfer efficiency of the system. The effect of a 
single sleeve may not be large, but large scale sleeving, especially in 
the support plate regions, can significantly reduce the efficiency of the 
steam generator. 
The attachment zone of the sleeve, depending on the specific method of 
attachment to the tube, produces a local stress concentration in the 
original parent tube. The zones of attachment are at increased risk of 
further primary water attack with continued operation of the generator, 
requiring additional treatments or operations to reduce susceptibility to 
degradation at the attachment zones. Sleeves thus introduce additional 
process steps, down-time, and costs for materials and equipment. 
Sleeving for a steam generator plant of this type must be made to specific 
requirements of the ASME Code. Because applications vary, the length and 
configuration of the sleeves needed varies as well, making it necessary to 
stock a large variety of sleeves to enable repairs at different areas. 
Laser welding is one means to attach a supplemental sleeve inside a tube. 
U.S. Pat. No. 4,694,136--Kasner et al discloses welding the ends of a 
sleeve to the inner walls of a heat exchanger tube in this manner. After 
placing the sleeve inside the tube and mechanically forming complementary 
annular bulges in the sleeve and the tube to fix them against 
displacement, a 500 to 700 watt laser beam is applied to the inner surface 
of the sleeve, either in successive closely spaced lines or in a helical 
continuous line. The area melted by the laser is about 0.24 inches in 
width (0.61 cm) and extends completely through the material of the sleeve, 
and about 0.025 inches (0.064 cm) deep into the tube, or about halfway 
through the tube wall. The melted material of the sleeve and tube are 
mingled and fused around the circumference at the point of attachment, 
forming a hermetic connection of the sleeve and the tube. 
According to Japanese Patent Publication 2-199,397, dated Aug. 7, 1990, it 
is known to heat-treat degraded areas of a tube along the inner surface as 
a means to reduce the later occurrence of cracking caused by tensile 
stress and corrosion. The incidence of stress-corrosion cracking in 
austenitic stainless steel is particularly increased if the steel has been 
heated to a temperature between 550.degree. and 800.degree. C. (about 
1,000.degree. to 1,500.degree. F.). The phenomenon of increased cracking 
within this temperature range, known as sensitization, occurs due to 
precipitation of carbides from solution with the iron, especially along 
interstices between fine granular bodies. Typically, in the production of 
steel, care is taken to cool the steel quickly through the sensitization 
temperature range, to minimize the degradation of structural integrity 
caused by sensitization. According to said Japanese Patent Publication 
2-199,397, heat treating is accomplished using a YAG laser beam to produce 
a temperature rise on the inner surface that is limited to below the 
temperature of sensitization. The laser beam is focused at a localized 
internal surface of the pipe and moved over a predetermined axial length 
at a sufficient rate to limit the temperature rise. A chromium or titanium 
powder can be applied to the internal surface prior to heat treating, for 
improving the stress-corrosion characteristics of the tube as a part of 
the heat treating process. The heat treatment is described as thereby 
assisting in either preventing stress-corrosion cracks or repairing 
stress-corrosion cracks after they have occurred. 
Plugging and sleeving deal with problems in the structure of a tube by 
isolating the degraded portion of the tube from the primary coolant. Heat 
treating prior to the occurrence of cracks may be helpful, but heat 
treating after cracks have occurred requires the application of additional 
material, and therefore resembles sleeving. It would be desirable to 
reconstitute the tube rather than to patch over or isolate it to deal with 
a deteriorated zone. Reconstituting the tube in situ by actually melting 
and then solidifying the tube material would preclude the need for add-on 
structures adversely affecting the flow characteristics of the tube or the 
plant. An additional material can be employed for alloying with the 
material of the original tube, or the tube can simply be reformed from its 
original metal. 
Whereas melting a metal allows the metal to flow, it would appear to be 
impossible to reform a tube in this manner without providing some form of 
mold for support. However, by melting only a small localized area at any 
one time, e.g., by laser welding, proceeding along the length of the tube 
to be repaired in a generally helical or axial scanning pattern, and/or 
along parallel lines or the like, it is possible according to the 
invention to reform the tube incrementally and to fuse over 
stress-corrosion cracks. Moreover, by melting a small area at a time using 
a focused laser, the melted metal is retained in place by the surrounding 
solid metal. As the point of welding passes, the thermal sink provided by 
the surrounding metal quickly cools the solidified metal through the 
sensitization range. 
It is an object of the invention to repair a tube having a deteriorated 
area, especially a heat exchanger tube in the heat exchanger of a nuclear 
steam generator plant, by melting a depth of the inner surface of the tube 
using welding technology, thereby fusing over cracks and rendering the 
tube material once again continuous over the zone of deterioration, and to 
a sufficient depth to return the tube to serviceable condition. 
It is a further object of the invention to repair a pressure vessel tube 
without substantially decreasing the internal diameter of the tube or 
increasing the external diameter, by melting and reforming the tube 
material at a localized small area, and scanning the area of localized 
melting to proceed over the tube surface in adjacent or overlapping lines. 
It is another object of the invention to provide a means to repair a tube 
conveniently, which improves over results obtainable either by plugging 
the tube, by adding a supplementary support/sealing sleeve, or by heat 
treating the tube in the presence of a supplemental material. In 
particular it is an object to melt and reform the tube in incremental 
lines so that cracks are fused over and the heat exchange capacity of the 
repaired tube is at least as good as that of the original tube. 
It is also an object of the invention to employ a tracking optical welding 
technique using a high powered laser, for surface welding at least a depth 
at the inside of a tube along a progressive overlapping spiral path, at a 
sufficiently slow rate of advance and a sufficiently high power level, to 
melt the tube at an isolated area that quickly cools after the welding 
point passes. 
It is another object of the invention to improve the surface of a pressure 
vessel tube by surface welding, optionally in the presence of an alloying 
material which modifies the characteristics of the original tube in the 
area of the repair. The alloying material can be a welding powder, gas or 
consumable sleeve disposed in the degraded area prior to or concurrently 
with welding. 
These and other aspects of the invention are met in a method for repairing 
a wall of a pressure vessel tube having a degraded area 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 repair 
is effected by positioning in the tube a high powered laser welding head 
with the beam energy focused at a limited area or spot on the internal 
wall of the tube. The welding head is relatively moved in the tube at a 
sufficient rate to weld along a line on the internal wall for melting the 
tube in the degraded area at least to a depth equal to a part of the 
thickness of the tube wall. The depth of melting is sufficient to restore 
the tube to serviceable condition by reforming a functionally sufficient 
thickness of tube, with regard to the particular purpose of the tube. The 
point at which the tube has been melted by the focused energy cools 
following passage of the welding head. As the point of application of the 
welding head advances along a line, the line is likewise advanced 
laterally. Localized melting and cooling of the tube material continues, 
line by adjacent or overlapping line, to encompass the degraded area. By 
successively melting, and optionally applying one or more alloying 
materials such as a powder, aerosol or insert, using materials known in 
the art, linear sections of the wall, stress-corrosion cracks and similar 
defects are fused over, repairing the degraded area. The optional 
additional welding material can be employed for improving the 
characteristics of the tube as compared to the original tube. Preferably, 
the lateral advance is less than the width of localized melting, causing 
the lines to overlap and continuously re-form the degraded surface. 
Although the entire surface of the tube can be melted and reformed in this 
manner, the localization of the melting at any one time is such that the 
metal does not substantially flow and the metal cools quickly following 
passage of the weld head, to below the temperature of sensitization. The 
advance of the welding line and the lateral displacement can be stepwise 
or continuous, and can be oriented axially or radially. 
The realization of these objects will be appreciated from the following 
discussion of particular exemplary embodiments of the invention. However 
it should also be appreciated that the invention is capable of variation 
from the examples, in accordance with the scope of the invention as 
claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, for repairing a wall 22 of a pressure vessel tube 24 
having a deteriorated zone, a welding head 32 is placed into the tube 24 
at the deteriorated zone. The welding head is activated and moved 
progressively relative to the tube so as to melt a localized area at a 
point along a line 42 on a surface of the wall. As the welding head is 
advanced, 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 sufficient 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. 
Any defects which were present in the degraded zone of the tube become 
fused due to the melting of the tube material. The weld melts the material 
of the tube at least to a depth equal to a part of a thickness of the 
wall. It is possible to melt entirely through the depth of the tube wall, 
because the melted volume is conical or cup-shaped in cross section, with 
the width of the melted portion being greatest at the radial inside of the 
tube, and less proceeding away from the weld head. The melted material 
cools upon passage of the weld head, whereupon a repair has been effected 
without the necessity of adding to the wall thickness, plugging the tube 
or otherwise adversely affecting the flow and thermal characteristics of 
the tube. 
Continuously during melting along a first line, or stepwise after the weld 
line has passed over a predetermined length, 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-overlapping 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 FIGS. 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 FIG. 3. The rotation can be stepwise, 
continuous or oscillating. 
Preferably, the welding process uses laser welding, although other means 
for isolated local melting of a point on the tube are also possible. For 
laser welding 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 employed. An example of an appropriate 
laser welding device for use according to the invention is disclosed in 
U.S. Pat. No. 4,694,136--Kasner et al, which is hereby fully incorporated 
here in. 
Referring to FIGS. 1 and 2, a drive means 72 is operable to rotate and 
axially translate a stem 74 comprising 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 application to the tube wall. Lens 67 has a focal length 
substantially 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 
displacement 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. 
FIGS. 1 and 2 illustrate an embodiment arranged to produce a helical 
pattern 48 of weld lines. In FIG. 3 an axial pattern is produced, using an 
axially oscillating drive means that moves the weld head up and down in 
the tube. A motor 96 can be provided for this purpose as shown. As in the 
previous embodiment, lenses focus the light emitted at the end 82 of the 
fiber optic cable 68. 
The welding head 32 is advanced axially and rotationally to cover the 
entire deteriorated area 26, in a series of passes. Parallel axial weld 
lines as shown in FIG. 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. 
In order to guide each weld line so as to evenly overlap the previous weld 
line, it is possible to vary the rate of advance (and perhaps focusing) of 
the laser beam on the workpiece. Preferably, 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. 
FIG. 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 specific 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 temperature of melting of course varies with the material of 
the tube. For Inconel 600 stainless steel (ASME Alloy 600), as 
advantageously employed for steam generator heat exchanger tubes, the 
melting temperature is about 1,350.degree. to 1,410.degree. C. (or 
2,470.degree. to 2,575.degree. F.). The typical thickness of the tube wall 
of a nuclear steam generator is about 0.050 to 0,055 inches (1.3 to 1.4 
mm). Preferably the weld depth extends through 80 to 100% of the wall 
thickness. Of course it is possible to apply the invention to thicker or 
thinner tubes or to materials other than stainless steel, by 
correspondingly changing the power level, the rate of advance of the beam, 
etc. The dimensions, power levels and the like are exemplary only. 
FIG. 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. As shown in FIG. 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. This is possible because the bead tends to 
taper in cross section, having a typically conical shape as shown in FIG. 
7. Although 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 (shown in FIGS. 2 and 4) 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 application 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 FIG. 6. 
The invention is particularly applicable to correcting degradation of the 
heat transfer tubes of a nuclear steam generator plant. Typically, 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 FIG. 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. 
The invention having been disclosed, a number of variations and 
alternatives will now be apparent to persons skilled in the art. The 
invention is not limited to the examples disclosed above and includes a 
reasonable extent of variation in accordance with the appended claims, to 
which reference should be made in assessing the scope of the invention in 
which exclusive rights are claimed.