High frequency welding is an automated resistance welding process that utilizes high-frequency currents to generate localized heat for coalescing metals, developed for creating high-integrity joints in pipes and other products. The technique relies on the skin and proximity effects to optimize the welding process, with applications in various metal types and product shapes. Despite its efficiency in welding, it requires careful handling due to radiation concerns and may not be economical for smaller production volumes.

1. High Frequency Welding
Prepared by: - Darshan Shah
Smit Solanki
M.E. Welding Technology (2018-19)
Metallurgical & Materials Engineering Department,
The Maharaja Sayajirao University of Baroda

2. High Frequency Welding
• High-frequency welding is included in
a group of resistance welding process
variations that use high-frequency
welding current (1kHz to 800kHz) to
concentrate the welding heat at the
desired location.
• The heat produces the coalescence of
metals, and an upsetting force usually
is applied to produce a forged weld.
• High-frequency resistance welding is
an automated process and is not
adaptable to manual welding.
2

3. High Frequency Welding
• High-frequency resistance welding was developed during the late 1940s and
early 1950s to fill the need for high-integrity butt joints and seam welds in pipe
and tubing.
• But today the process is also used in the manufacture of products such as spiral-
fin boiler tubes, closed roll form shapes, and welded structural beams.
• A wide range of commonly used metals can be welded, including low-carbon and
alloy steels, ferritic and austenitic stainless steels, and many aluminum, copper,
titanium, and nickel alloys.
• HFW is based on two main electrical phenomena
1. Skin effect
2. Proximity effect
3

4. DC
1. Skin effect
• High-frequency current in metal conductors tends to flow at the surface of the
metal at a relatively shallow depth, which becomes shallower as the electrical
frequency of the power source is increased. This effect is called the skin effect.
• Current penetration depth in a conductor as a function of frequency.
• Due to skin effect heating is concentrated on the surface of the conductor.
Where, δ = Current penetration depth
ρ = Resistivity of the conductor
ω = Angular frequency of current
μ = Magnetic permeability of the conductor
4

5. 2. Proximity effect
• When two conductors carrying HF current are placed close to one another, the current
concentrates on the two adjacent surfaces of the conductors. This is called the proximity
effect.
• Two currents flowing in opposite directions on the same material are mutually attracted.
• Due to proximity effect the heating is concentrated on very little part of the surface of the
conductor.
5

6. Electric Resistance Welded Tubing
(Low Frequency Resistance Welding)
6

7. High Frequency Induction Welding
7

8. High Frequency Induction Welding
• The induction coil induces a circumferential current in the tube.
• The high-frequency current flows along the edge of the weld vee due to the
proximity effect, and the edges are resistance heated to a shallow depth due to the
skin effect.
• Vee length usually between 1.5 to 2 tube diameter.
• Vee angle generally is between 3° and 7°.
• The welding speed and power source level are so adjusted that the two edges are
at the welding or forge temperature when they reach the weld point.
• The forge rolls press the hot edges together, applying an upset force to complete
the weld.
• Hot metal containing impurities from the faying surfaces of the joint is squeezed
out of the weld in both directions, inside and outside the tube.
• The upset metal normally is trimmed off flush with the base metal.
• One of the advantage of using HF is that it minimize the number of turns of
induction coil to 1 – 3 turns.
8

9. Impeder
Inside the
Core
Promotes
Path ADC
Why impeder is needed?
• Due to skin effect the HF current is
flowing from the very thin layer of the
surface.
• So it distinguishes the outside layer and
inside layer of the tube as two different
conductor.
• Here middle layer of the tube acts as a
perfect insulator as no current is passing
through it due to skin effect.
• The purpose of the impeder is to increase
the impedance (inductive reactance or
effective resistance) of the current path
around the inside wall of the workpiece.
This reduces the current that would
otherwise flow around the inside of the
tube and cause an unacceptable loss of
efficiency.
• An impeder is made from a magnetic
material such as ferrite.
9

10. High Frequency Contact Welding
• The process essentially is the same as
induction welding.
• The major difference is that sliding
contacts are placed on the tube
adjacent to the unwelded edges at the
vee length.
• Generally vee length is shorter than that
used with the induction process
because the pressure rolls are not
inductively heated by the magnetic field
of the induction coil.
• So the sliding contact can be placed
within the confine space.
10

11. High Frequency Welding
11

12. Comparison of the Induction and Contact
Welding Process
• Contact welding is a more efficient process than induction welding
because of the shorter vee length and because there are no losses in the
induction coil.
• For the seam welding of large-diameter pipe, the contact process can use as little
as half the power required by the induction process.
• The major disadvantage of the contact process is sliding contact wear.
• The service life of the contact tips decreases with increasing welding power level
so generally it is not operated above 800 kW.
• In contact welding process under some conditions, arcing between the
sliding contacts and the tube can occur. This may cause “arc marks”.
• Arc marks are required to be removed by a subsequent grinding operation.
• Generally induction welding can be used to weld the smaller-diameter
tube sizes, and contact welding can be used to weld the larger-diameters
sizes.
12

13. Welding Fins to Boiler Tube
• In Circumferential Fin Welding,
the fin is helically wound on
edge around a tube and
simultaneously welded to the
surface of the tube.
• In Longitudinal Fin Welding,
Fins can be welded
longitudinally to a tube on one
or both sides.
• This type of tube is used to
manufacture water walls in
boilers.
• Tube also can be welded to strip
or sheet metal for products
such as solar absorber plates
and freezer.
13
Weld
Weld

14. Contact Welding of Structural Beams
• Continuous high frequency
contact welding can be
adapted for the fabrication of
structural I-beams, T-beams
and H-beams.
• It is just a slight modification
to the fin welding technique
14
Weld

15. Seam Welding of Closed Roll Form Shapes
• Contact or induction processes can be
used depending on C/S.
• Lap welds are often made in roll form
components.
• lap joints must be designed with
consideration for the proximity effect,
and the two faying surfaces must be
brought together to form a vee.
• The geometry of the workpiece often
complicates the forge roll design.
15
Projecti
on Seam
Weld
Roll
Formed
Beam

16. Induction Welding of Pipe Butt Joints
• Closer the proximity conductor
develops a more confined current path.
• If a magnetic core is placed around the
proximity conductor, the current would
be further concentrated and heating
would take place directly below the
proximity conductor.
• High-frequency current in the coil
induces a circulating current
concentrated in the area of the pipe
butt joint, which is heated very rapidly.
• When the metal reaches welding
temperature, upset force is applied to
produce a forge weld.
16

17. Contact Welding of Finite-Length Plate Butt
Joints
• Same or dissimilar metals
• Can be of different thicknesses
• Low frequencies between 1 kHz and 10 kHz
generally are used.
• The current is introduced at each end of the
joint by small contacts and is confined to the
area of the joint by a proximity conductor.
• Magnetic core is used to assist in narrowing
the current path.
• When the joint reaches the welding
temperature, a forging force is applied and
the hot metal is upset.
• Welds of this type can be made at rates up to
1000 joints per hour.
17

18. 18
Welding parameters
Induction seem welding V/S Contact seem welding

19. High frequency welding parameters for
induction welding process
19

20. Welding Equipments
• Equipment for high-frequency resistance welding includes
1. Power source (usually a solid-state inverter type)
2. Induction coils
3. Contacts
4. Impeders
5. Control devices
6. Mechanical equipment for
preparing and aligning
the workpieces.
20

21. Power source
• The predominant power source in modern installations of high-
frequency welding equipment is the solid-state inverter power source.
• These units provide welding output power ranging from 50 kW to 1800
kW and operate at frequencies from 80 kHz to 800 kHz.
• Solid-state power sources are smaller in size than traditional vacuum-
tube units and typically demonstrate efficiencies over 80%, while
vacuum-tube units operate at efficiencies between 50% and 65%.
• Economically efficient operation results in a significant decrease in
power consumption and cooling-water requirements.
21

22. Basic Circuit of a Solid-State Inverter Power Source
22

23. Solid-State Inverter Switching Sequence
23

24. Induction Coils
• It is generally fabricated from copper tubing, hollow copper bar or copper sheet.
• Normally water cooled
• Highest efficiency is obtained when the induction coil completely surrounds the
workpiece.
• Coil may have one or more turns.
• The strength of the magnetic field reduces rapidly as the distance between the
coil and the workpiece is increased.
• Spacing between the coil and the workpiece ranges from 3 mm for small-
diameter products to up to 25 mm for large-diameter products
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25. AWS Welding Handbook
Contacts
• Made of copper alloy or may be
composed of hard metallic or ceramic
particles in a copper or silver matrix.
• Contact tips are hold via mounts.
• Replaceable due to wear.
• Internal and external cooling is required
for the contact tip and mounts.
• Area of contact tip = 0.25 - 1 in2
• Welding current = 500 - 5000 Amps
• Force of the contact tip against the
workpiece ranges between 20 – 200 N
• Service Life
• 1000 feet under very severe conditions
• 3,00,000 feet if circumstances are
optimal
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26. Impeders
26
• The primary function of an impeder is to increase
the impedance around the inside circumference
of the tube, thus diverting more energy into the
weld “vee”.
• An impeder is made from a magnetic material
such as ferrite.
• The impeder must be cooled to prevent its
temperature from rising above its Curie
temperature, where it becomes nonmagnetic.
(For ferrite, it is between 170°C and 340°C.)
• The impeder is positioned so that it extends
about 1.5 mm to 3 mm beyond the apex of the
vee and the equivalent of 1 to 2 workpiece
diameters upstream of the induction coil.

27. Control Devices
• Control Devices maintain proper welding conditions at different mill
speeds.
• Especially to minimize scrap material resulting when the mill is started
and stopped, the weld power can be automatically adjusted as a
function of mill speed by control Devices.
• So it can virtually eliminate any unwelded seam when the mill is stopped
and restarted.
• The system also will reduce scrap.
• Weld temperature control system reads the output of a pyrometer or
analyzes the image obtained by an infrared camera aimed at the weld
vee, and automatically adjusts the welding power to maintain a
constant, preset temperature.
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28. Advantages of High-Frequency Welding
• Produce welds with very narrow heat-affected zones (HAZ) so often
postweld heat treatment is eliminated.
• High welding speed and low-power consumption
• Able to weld very thin wall tubes
• A wide range of commonly used metals can be welded.
• Minimize oxidation and discoloration as well as distortion of workpiece
• Flux is almost never used but inert gas shielding is can be used for joining
highly reactive metals such as titanium
• High efficiency
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29. Limitations of High-Frequency Welding
• As the equipment operates in the radio frequency range, special care
must be taken to avoid radiation interference in the plant’s vicinity
• Special precautions must be taken to protect the operator and plant
personnel from the hazards of high-frequency current.
• Uneconomical for products required in small quantities
• Need of proper fit-up for the surfaces to be joined
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30. Some Products of High-Frequency Welding
30[Reference: Welding Handbook, Volume 2, p.665, AWS]

31. References
1. AWS Handbook, Volume 3 - Welding Processes, Part 2
2. John Wright , “Principles of high frequency induction tube welding”
3. Ilona Iatcheva, Georgi Gigov, Georgi Kunov, Rumena Stancheva, “Analysis of induction
heating system for high frequency welding,” Facta Universitatis, Ser: Elec. Energ. Vol. 25,
No 3, December 2012, pp. 183 - 191 DOI: 10.2298/FUEE1203183I
4. John Wright , “Optimizing Efficiency in HF Tube Welding Processes”
5. H. HAGA, K. AOKI AND T. SATO, “Welding Phenomena and Welding Mechanisms in High
Frequency Electric Resistance Welding—1st Report,” AWS 60th Annual Meeting held in
Detroit, Michigan, during April 2-6, 1979
6. Yan Pei, “High Frequency Induction Welding & Post-Welding Heat Treatment of Steel
Pipes,” University of Cambridge, June 2011
7. A. SPAHIU, “Experimental study of the induction heating in the manufacturing of
metallic tubes by longitudinal welding process,” U.P.B. Sci. Bull., Series C, Vol. 69, No. 2,
2007
8. OKABE Takatoshi, IIZUKA Yukinori, IGI Satoshi, “High Reliability Technology of the Weld
Zone of High-Frequency Electric Resistance Welding Linepipes,” FE GIHO No. 34 (Aug.
2014), p. 77–83
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32. Thank You