ESL-IE-83-04-111
ENERGY SAVING THROUGH HIGH FREQUENCY ELECTRIC RESISTANCE WELDING
Humfrey N. Udall
THERMATOOL CORP.
Stamford, Connecticut
ABSTRACT
High-Frequency electric resistance heating
systems have been widely used for many years for
both welding and heat treating. In the past the
major reason for using High-Frequency heating pro­
cesses has been because of their very much higher
speeds compared to most other methods. This has led
to lower manufacturing costs through higher produc­
tivity. In addition to the higher productivity,
however, the High-Frequency processes typically pro­
vide considerable saVings of energy. In these days
of escalating energy costs, these savings can also
lead to significant reductions in manufacturing
cos ts.
INTRODUCTION
In order to appreciate the reasons why high­
frequency electric resistance heating can lead to
significant energy savings it is first necessary to
understand the principles underlying this process.
There is a tendency when alternating current
flows in a conductor for the current density to be
greater at the surface than at the center. At low
frequencies such as 60 Hz this effect is negligible
except in the case of very large conductors; how­
ever, as the frequency increases so the "skin ef­
fect" takes on greater importance. The high-fre­
quency welding and heating processes which are dis­
cussed in this paper use frequencies typically be­
tween 300 and 400 KHz and at these frequencies the
Fig. 1. Skin Effect
of HF current as
seen on an isolated
conductor
Fig. 2. Proximity Effect
of HF current as seen in a
pair of conductors
effective depth of current flow is very shallow. The
skin effect in an isolated single conductor carr~ing
400 KHz high-frequency current is shown in Figur 1.
The current flows along the entire surface of th
material. The effective depth in steel above itl
Curie point is about .030" (0.76 mm). In additi n
to the "skin effect" there is also a "proximity f­
fect" Which is seen when a pair of "go" and "ret rn"
conductors are close together. This is shown in
Figure 2. The skin effect is still present but he
current concentrates at the two conductor surfac s
closest to each other.
HIGH-FREQUENCY WELDING PROCESSES
The most wi dely used practical appl ication
the prOXimity effect is in the high-frequency we
ing of tube and pipe as shown in Figure 3. In t
process a continuous steel strip is formed into
open seam cylinder and the edges are brought to­
gether in a "vee" with an angle of approximately
to 7 degrees. The high-frequency current is int
duced into the tube edges either through sliding
metal contacts upstream of the vee apex or in so
cases with an induction coil encircling the tube
Fig. 3. High-Frequency Longitudinal Butt Seam
Welding of Tube and Pipe
Proceedings from the Fifth Industrial Energy Technology Conference Volume II, Houston, TX, April 17-20, 1983
701
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ESL-IE-83-04-111
th i s poi nt. Here the "go" and "return" conductors
are the tube edges themselves. This is an almost
ideal example of the skin effect and the proximity
effect which together confine the heating currents
to the edges of the tube to be welded allowing them
to be heated to welding temperature at a high rate
of speed and forge welded together at the weld point
by the squeeze roll assembly. The resultant weld
has an extremely narrow heat affected zone and hence
requires a very small heat input compared either to
low frequency electric resistance welding or to arc
welding processes. Figure 4 is a graph which shows
the power cost savings for high-frequency welding
compared to low frequency electric resistance weld­
ing. Table 1 gives a comparison between typical
speeds and power inputs for longitudinal seam weld­
ing by both the high-frequency resistance process
and conventional arc welding processes.
Power Cost Savings
HF Contact vs. ERW Welding
(Based on $.04/KWH Rate)
-:- 30
THERMATOOL
Welders
~ VT-80
• VT-160
a:
J:
en 25
a:
:5-I 20
o
e, 15
en
Tubes are welded on production mills at diame­
ters which may range from 0.4" (10 mm) on small tube
mills to 48" (1220 mm) on large pipe mills. Wall
thickness can range from 0.003" (0.08 mm) to 1.0"
(25 mm). Typical welding speeds are from 500 ft/min
(150 m/m) to 50 ft/min (15 m/m) and the capacity of
the welders used is from 30 KW to 600 KW. In addi­
tion to the power cost savings considerably higher
productivity is realized because of the higher weld­
ing speeds which can be obtained using the high-fre­
quency process.
~
10
en
5
~
.080" .120" .160" .200"
2.0mm 3.0mm 4.0mm 5.1mm
The advantages of high-frequency welding are
not limited to the longitudinal seam welding of tube
and pipe. Many other industrial products are pro­
duced using modifications of the basic process.
Among these are fins spirally welded to tubes for
use in heat exchangers'and the economizer sections
of steam boilers, fins longitudinallY welded to tube
for the manufacture of boiler water walls, tubes
longitudinally welded to sheets for freezer liners
and solar absorber plates, spirally welded corru­
gated culvert and spirally welded piling, longitu­
dinally welded roll formed sections for the trans­
portation industry, longitudinally welded "I" and
"H" beams for truck trail er frames, mobil'e home
frames and general light structual uses and cable
she~thing which is continuously welded around the
cable core. Some of these applications are illus­
trated in Figure 5.
Fig. 4.
Although the majority of high-frequency weld­
ing applications are for carbon, alloy and stainless
steels, a wide variety of non-ferrous materials are
also welded. These include copper, brass, cupro­
nickel and other copper alloys, aluminum and alumi­
num alloys, titanium and zirconium. Dissimilar met­
als can also be high-frequency welded such as carbon
steel to stainless steel for automotive trim and
copper tubes to aluminum strips for solar absorber
plates. Dual wall tubes can also be longitudinally
seam welded from two separate strips of material for
automotive and transportation applications. The two
walls may both be of carbon steel to provide a sound
deadening effect or one wall m~ be of stainless
steel and the other of carbon steel to provide in­
creased corrosion resistance at minimum cost.
WALL THICKNESS
TABLE I
POWER SAVINGS H.F. WELDING VS. ARC WELDING
(CARBON STEEL)
PROCESS THICKNESS ARC
INCHES VOLTS
ARC POWER
AMPS KW
FCAW
0.125
325
HFRW
0.125
FCAW
0.25
28
475
SAW
0.25
33
900
HFRW
0.25
SAW
0.50
HFRW
0.50
28
200
4.6
13,200
2.1
13.3
125
10.6
29.7
130
22.8
11 ,820
4.7
90
41.6
8,400
13.2
9.1
275*
555*
34
1100
SPEED POWER
FT /HR KWH PER
100 FT
37.4
1100*
Data for arc welding is from Welding Handbook
Seventh Edition, Volume 2 pages 177, 179, and 210.
Data for H. F. Welding is from THERMATOOL CORP.
NOTE:- Input power requirements for 160 KW, 300 KW
and 600 KW welder output capacities respectively.
Proceedings from the Fifth Industrial Energy Technology Conference Volume II, Houston, TX, April 17-20, 1983
702
ESL-IE-83-04-111
Fig. 5. Examples of some of the varied products made using the High-Frequency
Resistance or Induction welding processes.
HIGH-FREQUENCY SELECTIVE SURFACE HARDENING
The same basic high-frequency principles of
"skin effect" and "proximity effect" can be applied
to the selective surface hardening of steels. This
process also has significant potential for saving
energy since in many cases a workpiece will be com­
pletely hardened when only a small hardened area is
actually needed to resist local wear. The basic
arrangement of such a system is shown in Figure 6.
In this case the "go" current runs between two con­
tacts on the workpiece and the "return" current runs
in a proximity conductor which is postioned close to
the workpiece and whose shape defines the shape of
the current path in the workpiece. Figure 7 is a
cross section showing the relationship of the HF
current pattern to the proximity conductor. In ad­
dition to saving energy this process also leads to
very high productivity since in all cases the hard­
ening cycle is completed in less than one second.
Heating times of 0.2 to 0.5 second are typical and
the process is applicable to most grades of carbon
steel. hardenable cast irons, and many powder metal
compositions. Lines up to 40" (l020 mml long x 0.1"
(2.5 mml wide have been hardened as well as lines up
to 0.75" (19 mml wide x 4" (l00 mml long. Figure 8
CURRENT--I7~__~2
HF
A
CONTACT
Fig. 6. High Frequency Selective Surface Hardenlng
- Gasic Arrangement
PROXIMITY CONDUCTOR
HF CURRENT PATTERN
SECTION A-A
Fig. 7.
Cross Sectional View of Fig. 6.
Proceedings from the Fifth Industrial Energy Technology Conference Volume II, Houston, TX, April 17-20, 1983
703
ESL-IE-83-04-111
Fig. 8. Pipe wrench jaw with cross section showing
HF selective hardening on tooth tips.
Fig. 9. Cross section cut from an automotive part
with a higher magnification photograph of one of the
six HF selective surface hardened wear faces.
shows the jaw of a pipe wrench and a cross section
of the serrations which have been selectively hard­
ened at the tips. Figure 9 shows a cross section of
an automotive component which has been internally
hardened in six places together with a higher magni­
fication photograph of one of the selectively hard­
ened areas. The process is applicable to many parts
used in the automotive, appliance and consumer
products industries.
CONCLUSIONS
In summary high frequency welding and selec­
tive surface heating can be used in many high pro­
duction processes both to improve productivity and
to save energy, with a resulting decrease in manu­
facturing costs.
Proceedings from the Fifth Industrial Energy Technology Conference Volume II, Houston, TX, April 17-20, 1983
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