FIELD OF INVENTION
This invention relates to the repair welding of tube-to-tube sheets, using TIG arc
welding process with or without filler material and using a heat sink to avoid
weld contamination. More particularly the invention relates to a process of repair
welding of titanium tube-to-tube sheet to avoid weld contamination.
BACKGROUND OF INVENTION
Tube sheets are used in heat exchangers. The tubes inserted in the holes which
are drilled in the tube sheet, are welded with the tube sheet. The tube and the
tube sheets can be made from different material. Generally TIG welding is
employed in the manual or mechanized mode. After welding the weld is
subjected to dye penetrant test to ensure the integrity of the weld, which should
be free from defects. For nuclear applications, the tube sheets is made of carbon
steel and surfaced with titanium. The tubes made of titanium are inserted in the
holes on the tube sheet and welded with the titanium deposit. In case, a defect
occurs during the initial welding, the defects are removed and the area is re-
welded. During the re-welding, it is ensured that no contamination of the weld
occurs due to melting of the carbon steel substrate. The titanium deposit will be
about 5 mm in thickness. The extent of material removal for eliminating the
defect depends on the size of the defect. Normally, after the defect removal, the
repair welding is carried out. If the depth of material removal is more, there is a
possibility of the arc to penetrate the insufficient titanium deposit and melt the
steel substrate leading to unacceptable contamination of titanium by steel. This
may occur with or without filler metal addition.

US Patent No. 4028789 discloses a method of installing a sleeve in one end of a
tube. The method comprising explosively expanding sleeves into heat exchanger
tubes disposed in a tube sheet utilizing shaped charges in conjunction with
welding and brazing techniques to affectuate tube modifications and repairs. The
method further includes the step of shaping the sleeve, placing specially shaped
inserts into the sleeves, varying the explosive charges within the inserts and
varying the confinement of the explosive charge within the insert to explosively
weld the sleeve to the tube, expand the sleeve to the tube, and/or expand the
tube to engage the tube sheet and welding or brazing one end of the sleeve to
the tube to form a seal.
US Patent No. 4264801 discloses a method of welding tube to a header of heat
exchanger. According to the method a thin titanium tube is welded to a titanium
clad header of a heat exchanger, the titanium tube is inserted into a hole formed
through the header and then the body portion of a core is inserted into the hole
of the header. The core is provided with a support and a circular flange at a joint
between the body portion and the support. The flange is provided with a notch
extending radially into the flange. When the core is inserted into the hole of the
header, the flange comes to engage the header and then the front end of an
electrode is positioned in the notch to generate an electric arc, while rotating the
core or only the flange, between the front end of the electrode and a portion of
the header radially separated by a predetermined distance from the titanium
tube. Thus, the header and the titanium tube are melted and welded by the arc.
None of the prior art thus describes a method and system to avoid excessive
weld contamination during the process of repair welding of titanium tube to tube
sheet in course of TIG arc welding process.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process of repair welding
of titanium tube-to-tube sheet to avoid weld contamination which eliminates the
disadvantages of the prior art.
Another object of the invention is to propose a process of repair welding of
titanium tube-to-tube sheet to avoid weld contamination, which eliminates
contamination of the weld by the substrate material.
A still another object of the invention is to propose a process of repair welding
of titanium tube-to-tube sheet to avoid weld contamination, which restricts the
excess penetration on the weld by the welding arc.
A further object of the invention is to propose a process of repair welding of
titanium tube-to-tube sheet to avoid weld contamination, which can be
performed from different welding positions in a single or multipass welding
procedure.
Summary of invention
According, there is provided a process of repair welding of titanium tube to tube
sheet, with or with out filler wires having the same or different composition/
combination of filler wires, adapting the process of gas-tungsten are welding
(GTAW), comprising the steps of carrying-out a dye-penetrant testing of the

original weld between a carbon steel substrate having a top surface , and the
tubes preparing the edges of the overlaid material for repair welding after
removing the defects developed during the original welding, selecting a heat sink
corresponding to the shape, joint configuration, and thickness of the proposed
joint, and placing the heat sink inside the tubes, making a weld with adequate
penetration, the excess heat input during the repair welding being absorbed by
the heat sink , and removing the heat sink after completion of the repair
welding.
Description of the accompanying figure
Figure 1 indicates the details of the arrangement for the tube-to-tube sheet
repair welding according to the invention.
As shown in figure 1, a tube sheet comprises a carbon steel substrate (1). The
top surface of the substrate (1) is overlaid with a material (2) such as titanium,
inconel, Monel, stainless steel, to a required thickness. Precise holes drilled in
the tube sheet to accommodate the tubes (3). Edges (4) of the overlaid material
(2) is prepared for welding after removing the defects. A heat sink (5) is located
in the tube (3). A weld (6) is made using a known welding process. The method
of welding is suitable for carrying out repair welding of in components/joints
requiring control of heat input during welding. This method of repair welding is
aimed at avoiding contamination of the weld by the substrate material (1) with a
different chemical composition. The method involves the use of the heat sink (5)

to avoid excessive penetration of the substrate metal by the arc. After removing
the defects and based on the available material, the edges (4) of the overlaid
material (2) is prepared. The heat sink (5) is selected to match the shape fitting,
joint configuration and thickness, and is placed inside the tube (3) so as to
effectively absorb the excess heat input to the joint. The heat sink (5) may be a
ring, disc, or insert, and may or may not form a part of the filler material. This
ensures that the weld (6) to be made without excessive heat input leading to
adequately deep penetration avoiding contamination of the weld (6) by the
substrate material (1). The repair welding is carried out with or without the
addition of filler material. This ensures that the weld (6) to be made without
excessive heat input leading to adequately deep penetration avoiding
contamination of the weld (6). The heat sink is removed after repair welding by
employing methods such as drilling, grinding, re-rolling etc. This method can also
be used for blocking/sealing the passage wherever required.

We Claim:
1. A process of repair welding of titanium tube to tube sheet, with or with
out filler wires having the same or different composition/ combination of
filler wires, adapting the process of gas-tungsten are welding (GTAW),
comprising the steps of:
- carrying-out a dye-penetrant testing of the original weld between
a carbon steel substrate (1) having a top surface (2), and the
tubes (3);
- preparing the edges (4) of the overlaid material (2) for repair
welding after removing the defects developed during the original
welding;
- selecting a heat sink (5) corresponding to the shape, joint
configuration, and thickness of the proposed joint, and placing the
heat sink (5) inside the tubes (3);
- making a weld (6) with adequate penetration, the excess heat
input during the repair welding being absorbed by the heat sink
(5); and
- removing the heat sink (5) after completion of the repair welding.

2. The process as claimed in claim 1, wherein the top surface of the
substrate comprises a layer of material selected from a group of titanium,
inconel, monel, stainless steel.
3. The process as claimed in claim 1, wherein the repair welding is optionally
carried-out with the addition of filler materials.
4. The process as claimed in any of the preceding claims, wherein the heat
sink (5) comprises one of a ring, a disc, and insert, and wherein the heat
sink (5) optionally forms a part of the filler material.
5. A process of repair welding of titanium tube to tube sheet, with or with
out filler wires having the same or different composition/ combination of
filler wires, adapting the process of gas-tungsten are welding (GTAW), as
substantially described and illustrated herein with reference to the
accompanying drawings.

The invention relates to a process of repair welding of titanium tube to tube
sheet, with or with out filler wires having the same or different composition/
combination of filler wires, adapting the process of gas-tungsten are welding
(GTAW), comprising the steps of carrying-out a dye-penetrant testing of the
original weld between a carbon steel substrate (1) having a top surface (2),
and the tubes (3), preparing the edges (4) of the overlaid material (2) for repair
welding after removing the defects developed during the original welding,
selecting a heat sink (5) corresponding to the shape, joint configuration, and
thickness of the proposed joint, and placing the heat sink (5) inside the tubes
(3), making a weld (6) with adequate penetration, the excess heat input during
the repair welding being absorbed by the heat sink (5); and removing the heat
sink (5) after completion of the repair welding.