WELDING and the
CONCRETE CONTRACTOR
Part 2: Gas and Thermit Welding
Two more types of welding are
described here to complete this series
designed to broaden the builder’s
knowledge of this important field.
for the purpose, a mixture of two gases, usually oxygen
and acetylene, in correct proportion and at proper pressures. Welding is accomplished by first preheating the
two pieces to be joined, at their point of contact, with the
torch flame. When the base metal has then reached a
molten temperature, weld metal is added by using the
torch flame to melt metal from a filler rod of suitable
composition. Such gas welding is a puddling process;
that is the molten metal forming the weld is kept as a
small pool over which the flame is constantly played.
Welding then progresses largely by causing this pool to
move by melting further metal at the head of the pool
and letting the metal behind it solidify.
The main advantages of gas welding lie in the flexibility of the equipment and the small investment it involves. The technique does require considerably more
operator skill than electric welding, and operating costs
are higher. The same gases and a special cutting torch
are also used for flame cutting, a process which provides
a practical and economical means for cutting bars to
size, or for making unusual shapes from plate, prior to
welding.
THERMIT WELDING
An example of flame cutting—removing the ends of
prestressing bars. The flame-cutting torch is a
valuable accessory for the welder, notably for
preparing bar ends and plate to correct sizes and
shapes.
T
he original welding process is that of the blacksmith
shop. The steel is heated in the forge to a plastic
state and fused together by hammering on the anvil.
Obviously this is both slow and laborious and has no
place where any degree of productivity is desired.
GAS WELDING
Gas welding followed as a considerably easier and
more precise process. The technique uses a high temperature flame produced by igniting, in a torch designed
Thermit welding is basically a casting process. The
method employs the chemical reaction obtained by igniting a mixture of finely divided aluminum and iron oxide. During the reaction the oxygen leaves the iron oxide
to combine with the aluminum and the free iron is
drawn off at a very high temperature into a mold previously prepared around the parts to be welded. The reaction is rather violent and care must be taken that personnel stand well back and that any surrounding
flammable material, such as wooden formwork, is protected.
A big advantage of the method lies in its comparative
simplicity. No expensive machinery or equipment is required, the only items used being a pair of formed expendable shell molds, the correctly prepared thermit
mixture, a luting compound to seal the mold to the steel
bar, and a few minor accessories, such as tapping discs.
Kits of such materials and accessories are available commercially; they include a chemical means for igniting the
thermit to initiate the reaction and contain the precise
Components for thermit welding: molding sand; mold
halves, showing the reaction on cavity and the flow
gates to the bar; tapping discs; clamping bolts; pack of
thermit.
quantity of correctly proportioned chemicals needed to
weld a bar of a specified size. Using such a kit, it is possible for one man to assemble and complete the weld
within a few minutes.
The weld assembly procedure is as follows: First the
ends of the bars require proper preparation; they must
be square and clean edged, prepared either by the use
of oxy-acetylene torch, sawing or some other suitable
mechanical means. This is important, both to insure
Mold halves in place for thermit weld.
smooth parallel ends for welding and to remove any
fractured or chipped edges resulting from shearing. Similarly the bar ends must be clean and free from all dirt,
oxide scale, oil, grease or other foreign matter. The bars
are then aligned and held securely in place with a gap of
approximately 3/8 inch between their ends. Next step is
the fitting of the mold halves around the reinforcing bar,
the two halves being held together by means of bolts or
spring clamps. It is essential not only for the bars themselves to be properly aligned, but also for the pouring
channel of the mold to be centered directly on the gap
between the ends of the bars. This is best done by inserting an aligning rod of the requisite size and shape
through the channel and into the gap before the mold
halves are tightened in place. Finally the flares of the
mold must be luted to provide a seal between mold and
bar so that the weld metal will not escape during the actual welding process. Luting is done by packing a suitable molding compound into the flares with the fingers;
the compound should fill the flares completely and also
adequately cover and surround the area as a supporting
buttress.
Welding can now proceed. First step is to drop a metal tapping disc (or discs, according to shape) into the
bottom of the mold cavity and to tamp it lightly into
place; the purpose of the disc is to keep the molten steel
away from the bars until it reaches the correct fusion
temperature. The thermit mixture, in the form of a powder, is then poured into the mold and leveled off by
hand. This mixture must be kept dry and should be ignited immediately after placing. Ignition is achieved by
means of a small quantity of the highly flammable
chemical starting agent, placed at the top center of the
mixture. The starting agent is itself ignited with a flint
lighter. Needless to say extreme caution is essential from
the safety angle during the ignition stage. The reaction is
complete within 10 to 15 seconds after ignition to produce a white-hot, superheated steel weld metal with a
Sealing mold halves for thermit weld.
The thermit reaction.
layer of light slag floating above it. At this point the
charge melts through the tapping disc and flows into
the lower area of the mold, washing the square-cut ends
of the bars with molten metal and in so doing insuring a
homogeneously fused mass. The slag remains in the upper part of the mold cavity.
A period of up to ten minutes must elapse before all
the weld metal and slag will have solidified, although
other welds can be made during this cooling period so
no time need be lost. The mold should be left in place for
a period of at least 30 minutes after ignition. After removing the mold the projecting gates or tapping risers
can be broken off with a hammer or cut away with a
torch.
Molds are available for thermit welding in either the
vertical, horizontal or inclined positions. Research has
proved the value of preheating the steel in the area to be
welded and for many applications, particularly where
the strength factor may be critical, this is highly recommended. Preheating to 600 degrees F. promotes greater
toughness and ductility and lessens the possibility of
porosity and gas inclusion in the weld. Where moisture
may collect either in the thermit molds or on the reinforcing bars the weld area of the steel should be preheated to at least 400 degrees to get rid of all traces of moisture and the mold itself should be heated to not less than
250 degrees. This preheating can be done after the mold
is clamped in place and luted simply by applying heat to
the reinforcing bars at a distance of 2 to 4 inches from
the luted area.
Completed thermit welds.
BIBLIOGRAPHY
1 THERMIT INFORMATION SHEETS, Thermex Metallurgical,
Inc., Lakehurst, New Jersey.
2 PROCEDURE HANDBOOK OF ARC WELDING DESIGN
AND PRACTICE, The Lincoln Electric Company, Cleveland
1, Ohio.
3 MILD-STEEL ARC-WELDING ELECTRODES, ASTM Specification A 233.
4 RECOMMENDED PRACTICES FOR WELDING REINFORCING STEEL, METAL INSERTS AND CONNECTIONS IN
REINFORCED CONCRETE CONSTRUCTION, American
Welding Society, AWS D 12 1-61.
5 WELDING OF REINFORCING BAR FOR CONCRETE
CONSTRUCTION, Rudy, J. F., Suyama, F., and Schwartz, B.
H., Welding Journal (Research Supplement), Vol. 38, No. 8.
6 MODERN DESIGNING WITH STEEL, Kaiser Steel Corporation, Vol. 5, No. 2.
PUBLICATION #C620353
Copyright © 1962, The Aberdeen Group
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