FIELD OF THE INVENTION
The present invention generally relates to a method of non-destructive testing of
induction pressure welds in steam generator tubes using vibro-thermography.Fixtures
and clamps are used for holding the welded tube with weld joint in place during
inspection. Also tube is rotated at a fixed angle after each inspection process for
complete inspection of the weld circumferentially. This method is used to inspect the
induction pressure welded tubes on shop floor in sync with the welding process semi
automatically.
BACKGROUND OF THE INVENTION
Induction Pressure Welding (IPW) is a solid state butt welding process used to join
steel tubes to form coils for steam generators. In this process, tube ends to be joined
are clamped and aligned together and then heated by an electrical induction coil,
once the required temperature is attained, hot ends are brought against each other
with precise hydraulic pressure to get metallic bonding. During the entire process,
inert gas is purged around the tube ends to prevent oxide formation. This process is
completely automated which makes it highly productive. IPW process is used for
fabrication of fossil fuel steam generators components like Super Heater, Re-heater
coils, etc. A typical power plant boiler of 500 MW capacity would consist of as many
as 5000 such joints made of different materials like carbon steel, low alloy steels,
austenitic stainless steels, etc. Tubes with outer diameter ranging from 38 mm to
76.1 mm and thickness 3.6 mm to 10 mm are welded using IPW process.

The major defect types found in IPW joints are lack of bonding. Conventional
Ultrasonic Testing (UT) using Pulse Echo Contact technique is used to test IPW
joints for detection of lack of bonding. This involves manual scanning of the region
adjacent to weld using a piezoelectric angle probe of 4 MHz frequency and refracted
angle 700. Though, conventional UT is capable of detecting the lack of bonding in
these welds, it is slow, cumbersome, highly operator dependent and requires access
to the region near the weld joints, and it cannot be performed until the joint gets
sufficiently cooled.
Another known technique for non-destructive testing of induction pressure weldis dye
penetrant testing. It includes treating the welded area with a dye penetrant so that
the dye enters any crack or defects that may be present on the weld surface. The
weld is then cleaned, and the structure is treated with a powder that causes the dye
remaining in the cracks to spread into the powder making the cracks visible.An
ultraviolet (UV) light source is used to illuminate cracks in case of fluorescent dye
penetrant testing. This technique is highly inspector intensive and it can only detect
defects which are open to surface. In addition to this, destructive testing methods like
cold impact test, bend test, etc. are used to assess the integrity of these joints.
The US Patent Titled “Infrared imaging of ultrasonically excited subsurface defects in
materials” (Patent No: US 6,236,049 B1 dated 22/05/2001) discloses a technique for
infrared or thermal imaging of ultrasonically excited subsurface defects in a material.
An ultrasonic source is connected to a specimen being inspected through a coupler
that transmits the ultrasonic waves into the material with minimum attenuation. The
ultrasonic source emits a single ultrasonic pulse having constant frequency
amplitude for a predetermined period of time. A suitable thermal imaging camera is
used to

image the specimen when it is being excited by the ultrasonic source to generate
heat in the defect region due to friction in the mating surfaces of the defects. A
control unit is used to control the operation of the ultrasonic source and the camera
for timing purposes. However, this invention does not mention about using vibro
lock-in thermography for inspection of induction pressure welds.
The US patent titled “Vibro-thermographic weld inspections” (Patent No: US
2012/0288049 A1, dated 15/11/2012), discloses a method ofinspecting the quality of
J-groove welds of a nuclear reactor pressure vessel (RPV) head and the RPVbottom
mounted nozzles (BMN) using vibro-thermography. The weld to be inspected is
subjected to a transient sonic excitation while the weld area is monitored using a
remote infrared camera. The sonic excitation induces mechanical vibration, which
causes heat generation at any crack in the weld. The infrared camera detects any
temperature differentials in the weld, indicating the presence of a crack. This
invention is only applicable for quality inspection of J-groove welds and bottom
mounted nozzle in RPV.
The present invention relates to a method of inspecting the Induction Pressure
welded (IPW) jointsof steam generator tubes made of different materials usingvibro-
thermography.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to carry out the vibro-thermography scanning
of the IPW joints in steam generators.

Another object of the invention is to use suitable wooden fixturestorestrict the
ultrasound energy to pass in the job only and for holding the welded tube during
inspection.
A still another object of the inventionis to use pneumatic actuator to hold the sound
transducer firmly on the tube surface during excitation.
Yet another object of the invention is to use pneumatic clamps to hold the tube
against the wooden fixtures during inspection.
A further object of the invention is to use a stepper motor to rotate the tube in 3 equal
steps to complete the weld circumference.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method of inspecting the Induction Pressure welded
(IPW) jointsof steam generator tubes made of different materials usingvibro-
thermography. In this technique, the weld to be inspected is subjected to a transient
sonic excitation and simultaneously monitored using a remote infrared camera. The
sonic excitation induces mechanical vibration, whichcauses to generate heat in the
defect region due to friction in the mating surfaces of the defectsin the weld. The
infrared camera detects the temperature gradient in and around the defect region in
the IPW joints, indicating the presence of a crack/ defect/ dis –bond/. Since the heat
source in vibro-thermography is the discontinuity itself, the identification of defects is
much simpler than the conventional ultrasonic testing method. The time taken for
inspection of a single joint is also much lesser compared to UT.Vibro-thermography

detects defects in IPW joints which are surface or sub surface defects in the 5 mm
thick tubes.
This method includes placement of the welded tube clamped to wooden support
fixtures, placingthe sonic transducer by upward and downward movements on the
tube outer surface away from the weld center using a pneumatic actuator and
maintaining sufficient contact pressure, applying sound energy of 15-25 kHz
frequency with maximum power of 2kW on the placed welded tube in a pre-defined
wave form and simultaneously recording thermal images by infrared camera of the
IP weld area and rotating the placed tube at an angle to cover complete tube
circumference in three equal steps for inspection using a special clamp with bevel
gear and suitable stepper motor.
BRIEF DESCRIPTIONS OF THE ACCOMPANYING DRAWINGS
The above brief description, as well as further objects, features and advantages, of
the present invention can be fully appreciated by reference to the following detailed
description. These features of the present invention will become more apparent upon
reference to the drawings, wherein:
Figure.1 Fixture setup for vibro-thermography of induction pressure welded (IPW)
steam generator tube
Figure.2 Section view of IPW joint showing typical de-bond in the weld
Figure.3 An angle of rotation of welded tube with one section under inspection
Figrue.4 Schematic diagram showing the rotation mechanism for welded tube

Figure.5 Schematic diagram showing the placement of sound transducer on outer
surface of the tube
Figure.6 Schematic diagram showing the retrieval position of sound transducer away
from the tube
DETAIL DESCRIPTION OF THE INVENTION
The present invention will be described in detail below with reference to an
embodiment as shown in the drawings.
Steam generator (Boiler) tubes, like platen super heater tubes, re-heater tubes,
economizer tubes are the main components in the steam generator for generating
the steam for running the turbines in fossil fuel thermal power stations. These tubes
are joined together by Induction Pressure Welding (IPW) process. During induction
pressure welding, the tubes are heated up by using induction coils and external
pressure is applied over the tubes to join together. This welding technique is partially
automated and it is used for continuous production of IPW joints in shop floor. But
sometimes due to malfunctioning of the production equipment or variation in set
parameters,the IPW joints produced may be defective. In majority of the cases, de-
bonding is the common type of defect noticed in the IPW joints. This de-bonding, if
present in a IPW joint, a physical bond is formed but does not have the sufficient
mechanical strength to withstand for steam pressure and fails before or just after
commissioning at sites. Hence, non-destructive testing of IPW tubes is a must.

Ultrasonic Testing (UT) using Pulse Echo Contact technique is used to test IPW
joints for lack of bonding. This involves manual scanning of the region adjacent to
weld using angle probe of refracted angle 700. Though, this technique is capable of
detecting the lack of bonding in induction pressure welds, it is slow, cumbersome,
highly operator dependent and requires access to the region near the weld joints.
This technique cannot be performed until the joint gets sufficiently cooled.
To overcome these problems, a novel NDT technique, Vibro-thermography, which
comprises ultrasonic/sonic energy excitation system (1)coupled to an infrared
camera (7) for capturing the thermal images, is employed.In this technique,
thermaldistinction is created in the component using sonic/ultrasonic vibrational
energy. The novelty of this invention is the method used to test the IPW tubes, which
involves placing the welded tube (6) on wooden fixtures(holders) (8)which are kept at
a distance away from the weld center. Pneumatically operated clamps (5) are used
to firmly hold the welded tube against the wooden fixtures. Thesefixtures (8)damp
vibration and keep the tube steady during the excitation which is very much essential
for performing the examination. The excitation system (1) here is a sound transducer
which operates in 15 kHz to 25 kHz range. It is connected to a horn (2) for
transmitting the energy to the tube. During testing the soundtransducer is placed
with the help of a pneumatic actuator (11) till the horn touches the tube outer surface
away from the weld center. Aluminum sheet (3) is used as a coupling material in
between the horn and the tube surface. This pneumatic actuator is attached to a
sliding bar (17) which in turn fitted to a cross head (13). This cross head is supported
by two vertical columns (12) which are bolted to a base plate (16) firmly. The infrared
camera (7) is kept away from the weld at about 2ft distance focused at the weld
area. Sound

transducer is excited using a signal generator for a given period of time and
simultaneously the infrared camera is triggered to acquire thermalimages of the weld
area with set rate of acquisition. Any dis-bond/ defect present in the weld region will
undergo clapping/rubbing action due to high power sonic/ultrasonic vibrations and
produces heat signals which is picked up by the infrared camera. These signals
indicate the presence of defect inside the weld. In the case of IPW joints, the defects
are notice at the depths of 2 to 4mm in the tube thickness.Once infrared image of the
tube is acquired, it needs to be rotated to continue the inspection on the remaining
area. This is to ensure the entire circumference of the weld is inspected. This
rotation isset in such a way that the tube circumference is covered in three equal
steps using a stepper motor (14) and a bevel gear (15).During scanning, if any
defect is found in any stage of inspection the tube is rejected and is recommended
for repair or re-weld. This method of inspection is followed for assessing soundness
of the IPW joins of the steam generators in shop floor.

WE CLAIM:
1. A method of inspection of induction pressure weld joints in steam generator
tubes using vibro-thermography, comprising steps of:
clampingan induction pressure welded tube (6) to wooden fixtures
(8) using pneumatically operated clamps (5);
placinga sonic/ ultrasonic transducer (1) on said induction pressure
welded tube (6) away from the weld centre;
applying pneumatic pressure to said sonictransducer (1) for
sufficient coupling force on said induction pressure welded tube (6);
applying sonic/ultrasonic energy of frequency 15-25 kHz with
maximum power of 2kW on the induction pressure welded tube (6) in a
pre-defined wave form;
transmitting the sonic/ultrasonic energy using a horn (2), which is
coupled to said induction pressure welded tubeusing aluminum sheet (3);
simultaneously acquiring thermal images of the weld area using an
infrared camera (7);
rotating said induction pressure welded tube (6) with the help a
bevel gear (15) and a stepper motor (14) in three equal steps to cover tube
weld circumference while inspection using said infrared camera (7).

2. The method of inspection of induction pressure weld joints as claimed in
claim 1, wherein the thickness of said induction pressure welded tube lies
within the range of 2mm-6mm.
3. The method of inspection of induction pressure weld joints as claimed in
claim 1, wherein said induction pressure welded tube is made up of steel
and is mainly used for the construction of steam generators.
4. The method of inspection of induction pressure weld joints as claimed in
claim 1, wherein said infrared camera (7) is kept at a distance of at-least 2 ft.
from the welded area.
5. The method of inspection of induction pressure weld joints as claimed in
claim 1, whereinsaid ultrasonic/sonic energy excitation system (1) is a
sound transducer operating in 15 kHz to 25 kHz range.
6. The method of inspection of induction pressure weld joints as claimed in
claim 1, wherein said ultrasonic/sonic energy excitation system (1) is held
by a pneumatic actuator.

7. The method of inspection of induction pressure weld joints as claimed in
claim 6, wherein said pneumatic actuator is attached to a sliding bar (17)
which in turn fitted to a cross head (13);
said cross head (13)is supported by two vertical columns (12) which
are bolted to a base plate (16) firmly.