FIELD OF THE INVENTION
The present invention relates to a technique for Post weld heat treatment of
membrane water wall panel made of Tungsten enhanced Cr-Mo steel used in
supercritical boilers to avoid environment assisted cracking failures.
BACKGROUND OF THE INVENTION
Fossil fuel fired boilers are widely used for generation of power in thermal power
plants. The boiler is the chief element in thermal power stations where the
conventional fossil fuel is burnt and the heat of combustion is used to convert
water into steam which then turns the turbine leading to generation of
electricity. The heat transfer from the burning fuel to water which is later
converted into steam happens in the water walls. The water wall material has to
endure the temperature of firing and therefore are made of materials that have
good creep strength in the expected operating temperatures. With the advent of
supercritical boilers, the water wall materials had to be chosen from the family of
creep strength enhanced Cr-Mo steels. Although, these materials have good
operating characteristics, the fabricability of such materials pose some
challenges. For instance, all these materials are to welded with proper preheat
and proper Post Weld Heat Treatment (herein after referred to as PWHT) is to be
done failing which there could be chances of premature failures due to
environment assisted cracking phenomenon including the stress corrosion
cracking and the hydrogen induced cracking. Especially, tungsten enhanced 'Cr-
Mo' steels are more vulnerable to such failure mechanisms. The problem is more
pronounced in the case of boiler erection sites where the uncertainties with
regard to post weld heat treatment are quite high as this is performed locally
covering only the joint area and some portions of the base metal. Therefore, a
proper method of local PWHT along with a suitable technique is highly essential

for imparting a proper PWHT to such Tungsten enhanced materials which would
reduce the risks of environment assisted cracking.
PRIOR ART SEARCH
US patent 6676777 B2 describes a post weld heat treatment process, wherein a
welded joint made of carbon steel and low alloy steel is held within austenite
single-phase temperature range for a given time and subsequently the joint is
cooled by air-cooling or by slow cooling at a cooling rate lower than that of the
air-cooling. Whereas our invention describes a technique of local PWHT of
membrane water wall panels of supercritical boilers made of tungsten enhanced
Cr-Mo steel that is most effective in avoiding the risks of environment assisted
cracking.
US patent 5591363 describes a process of depositing layers of weld metal onto a
ferrous NiMoV low alloy steel turbine component, where during the deposition of
a first layer of weld metal, low levels of amperage are used to prevent a
dramatic increase in a level of hardness of the HAZ and during the deposition of
a second layer of weld metal, higher levels of amperage are used to temper the
heat affected zone. But, our invention relates to a technique of carrying out most
effective local PWHT for membrane water wall panels of supercritical boilers
made of tungsten enhanced Cr-Mo steel in order to eliminate the risks of
environment assisted failures.
Chinese Patent CN 102796863 A relates to a method of a local post weld heat
treatment of a steel casting, and particularly relates to a method used for a local
post weld heat treatment of a large-size steel casting after re-welding. The
method comprises the following steps of: firstly, carrying out hardness detection
on a welding zone after re-welding, wherein the hardness does not exceed a

critical upper cutoff hardness and thereafter conducting an induction heat
treatment for better quality of cast products. Whereas, our invention relates to a
novel technique for carrying out local PWHT of membrane water wall panels
made of Tungsten enhanced Cr-Mo steel used for supercritical boilers with a
specific aim to avoid the environment assisted cracking issues.
European patent EP 0034057 Bl discusses a method for post weld heat
treatment (hereinafter referred to as PWHT) of a welded portion of, for example,
a thick base metal, and more particularly to a method for appropriately judging
the time point for terminating the PWHT when sufficient of the residual diffusible
hydrogen in the welded metal has been dissipated by the after heating. But, our
invention relates to a technique of carrying out local PWHT on membrane water
wall panels of supercritical boilers that are made of tungsten enhanced Cr-Mo
steel with a view to eliminate the risks of environment assisted cracking issues.
OR1ECT OF THE INVENTION
It is therefore an object of the invention to propose a technique for Local Post
weld heat treatment of membrane water wall panel made of Tungsten enhanced
Cr-Mo steel used in supercritical boilers to avoid environment assisted cracking
failures.
SUMMARY OF THE INVENTION
According to the invention, there is provided a technique for post weld heat
treatment of welds made in membrane water wall panels of the supercritical
boilers, which are made of Tungsten enhanced 'Cr-Mo' steel. The invented
technique is capable of avoiding the risks of environment assisted failure in such
membrane water wall panels. The importance of the method lies in the fact that
tungsten enhanced 'Cr-Mo' steel are highly prone to environment assisted
cracking and in absence of preventive and innovative technique of PWHT,

premature failures are certain to happen. This invention serves to address such
disadvantages.
The membrane water wall panels consists of tubes welded with fins. Tubes are
fillet welded with fins. The length and width of the panels are as per the design
of the boiler. Weld joints have to be made to achieve the required length of the
membrane water wall panels at different locations and at different elevations in a
boiler. Local PWHT has to be carried out for both the tubular butt welds and the
fillet welds in the membrane water wall panels. This technique involves carrying
out the local PWHT in two stages. In the first stage, PWHT of the tube to tube
butt welds are finished. This is done by the following steps. For every tubular
butt joint, the local PWHT is carried out as per the recommendations of AWS
D10.10 standard. The portion of fins near the butt welds are removed by any
flame cutting or mechanical cutting process. The extent to which the fins are to
be cut is decided by the length of Gradient Control Band (GCB) as calculated by
using the provision of AWS D10.10 standard. Then, the Flexible Ceramic Pads
(FCP) are wound over the tubular butt welds. The length over which the FCP is
wound over the tubular butt weld is again calculated from the provisions of AWS
D10.10 standard. After winding the FCP, the insulation material is wound over
the FCP that constitutes the GCB. The FCP is connected to the power source.
Thermocouples are placed at the required locations along the length and width
of the weld to ensure the uniformity of temperature during the PWHT cycle.
PWHT is run using the required heating rate, soaking time and the cooling rate
as per the relevant material specifications or code of manufacture like Section
VIII of the American Society of Mechanical Engineers Boiler and Pressure Vessels
Code (herein after referred to as ASME BPVC).
In the second stage, the PWHT of the fillet weld between the tube and the fin is
carried out. The portion of fins that were cut previously for making way for the

PWHT of tubular butt welds are now placed in position. The fins are beveled in
the edges that abut with tubes. The weld is completed in the beveled edges and
the portion of fins removed previously are now filled. Thereafter the Finger
Element Coils (herein after referred to as FEC) are placed over these fillet welds
and the FEC is well packed by bending and compacting within the area between
the two tubes containing a single fin. Thereafter, the FEC are connected to the
heating power source. Thermocouples are placed on fillet welds at convenient
locations to monitor the temperature on the weld during PWHT. Then, the PWHT
cycle is run using the required heating rate, soaking time and the required
cooling rate as per the relevant material specifications or code of manufacture
like ASME BPVC.
BRIEF DESCRIPTION Of THE ACCOMPANYING DRAWINGS
Figure 1 - Cross sectional view of the of Membrane water wall panels of
supercritical boilers
Figure 2 - Schematic sketch of weld joints in Membrane water wall panels
Figure 3 - Sketch of Flexible Ceramic Pad (FCP)
Figure 4 - Sketch of method of carrying out PWHT of tubular butt welds in
membrane water wall panels of supercritical boilers using the FCP resistance
heating method.
Figure 5 - Schematic sketch of a small portion of membrane water wall panels
showing the details of weld joint configuration between tube and fin.
Figure 6 - Sketch of Finger Element Coil (FEC)
Figure 7 - Sketch of method of carrying out PWHT of fillet welds between tubes
and fins in membrane water wall panels of supercritical boilers using the FEC
resistance heating method.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
According to the invention, there is disclosed a technique for local post weld heat
treatment (PWHT) of welds made in membrane water wall panels of supercritical
boilers, which are made of Tungsten enhanced 'Cr-Mo' steels. Membrane water
wall panels of supercritical boiler consists of tubes and fins alternately. The tube
size, fin size and the length and width of the membrane water wall panels are
decided as per the design of the boiler. The general cross sectional view of the
membrane water wall panels is shown in figure 1. The tubes (1) are welded with
fins (2) to form the membrane water wall panel. The membrane water wall
panels are first fabricated to transportable sizes in the shop floors and later on
have to be welded at power plant erection sites. These membrane water wall
panels made of tungsten enhanced 'Cr-Mo' steel are highly prone to many
environment assisted cracking issues. Therefore the weld joints in such panels
require a proper PWHT technique that is effective enough to temper the
weldment and the HAZ and result in restoration of good ductility and toughness
to the material thereby avoiding the occurrence of any environment assisted
cracking issues. This technique is disclosed in detail in the present invention.
The weld joints in a membrane water wall panel are schematically represented in
figure 2. The weld joints (3) are made by suitable welding process. While making
the weld joints (3) in the tubes made of tungsten enhanced 'Cr-Mo' steel, the fins
are cut near the area (shown as (4) in figure 2) of the weld joint to facilitate the
welding and subsequent PWHT. The weld joints are post weld heat treated
(PWHT) using the Flexible Ceramic Pad (FCP) resistance heating method. The
schematic sketch of FCP is shown in figure 3. The FCP consists of multiple
'Nichrome' heating elements (5) stitched together to form a heating pad of the

required size as dictated by the requirements of heating band specified for the
size of the tube as per the provisions contained in AWS D10.10 which specifies
requirements for local post weld heat treatment. The FCP contain lead wires (6)
to enable connection to an electric power source. The FCP is wound around the
weld joint to a prescribed distance over the weld joint in such way that the weld
joint comes at the center of the width of the FCP. This is represented in figure 4.
The total width that the FCP covers over the weld joint is taken as equal to the
Heat Band (HB) specified in AWS 10.10 specification. Prior to winding the FCP,
necessary thermocouples are placed on the weld joint to monitor the
temperature reached during the PWHT. After winding the FCP, suitable insulation
material is wound over the FCP. The width of the insulation material is taken
equal to the Gradient Control Band (GCB) specified in AWS D10.10. After packing
with insulation, the FCP is connected with the electric power source for
producing heat to PWHT to carry out PWHT of the weld joint in the tubes. The
thermocouple(s) placed on the weld are used for monitoring the rate of heating,
soaking temperature and rate of cooling of the PWHT cycle. Such a PWHT is to
be followed for all weld joints made in the tubes made of tungsten enhanced 'Cr-
Mo' steel which form a part of the membrane water wall panels of the
supercritical boilers.
After PWHT of the weld joints made in the tubes, the area that was cut open in
the fins (shown as (4) in figure 2) are to be welded again with fins. The width
and length of the fins are so chosen that the entire open area would be
completely bridged after placement of fins. The fins are to be welded with the
tubes by using a single bevel type of joint as shown in figure 5. The reason for
choosing the single bevel joint is that both welding and subsequent PWHT can
be done only from one side. The advantage that is reaped here is that this one
side welding would lead to elimination of out-of-position welding I.e. welding in
the overhead position. This would also erase welder fatigue and augment weld

quality. For PWHT of the welds between the fins and tubes, the Finger Element
Coil (FEC) is used, the schematic sketch of which is shown in figure 6. In FEC,
the heating element is a lengthy resistance heating wire of suitable diameter.
Over the heating wire, ceramic ferules are inserted for longer life of the coil. The
FEC is then packed in the area where the fins were re-welded in the areas that
were cut previously for carrying out PWHT of the weld joint in the tubes (shown
as (4) in figure 2). The figure 7 shows the schematic arrangement of placing FEC
for PWHT of the welds between tubes and fins. The FEC are then connected to a
power source along with required thermocouples for control of temperature
during PWHT of the welds between the fins and the tubes.

WE CLAIM :
1) A technique of PWHT of welds made in membrane water wall panels
made of Tungsten enhanced 'Cr-Mo' steel that are used in supercritical
boilers.
- the said technique being carried out locally over the weld joints and
under erection site conditions
- the said technique avoids environment assisted cracking issues in the
welds made in the membrane water wall panels consisting of tubes
made of Tungsten enhanced 'Cr-Mo' steel.

2) The method of PWHT of the welds made in tubes that are made of
Tungsten enhanced 'Cr-Mo' steel that is carried out by laying Flexible
Ceramic Pads over the weld joints and by cutting the fins near the tube
weld that is required for facilitating winding of FCP over the tube welds.
3) The welding of fins to tubes in the area where they were cut for
facilitating winding of FCP over the tube welds by using a single bevel
weld joint that would eliminate the need of doing PWHT on both the sides
of the fin leading to elimination of welder fatigue.

4) The method of PWHT of welds between tubes and fins using the FEC
resistance heating method wherein the FEC is packed in the area between
two adjacent tubes and the over the welds connecting the fins that were
earlier cut for facilitating winding of FCP and were re-welded after
completion of PWHT of tube welds.