Electric and Thermit Welding Briefly Considered
=Contact Welding.=--The electric welding of iron strips and sheets is
usually done by the Thomson process of “contact-welding.” In this
process the metal is brought to a welding heat by passing a very large
current through the joint to be welded, which, by virtue of its high
resistance in relation to other parts of the circuit, develops great
local heat. When the correct temperature for welding is reached, the
joint is pressed together by mechanical means, and the current
interrupted. In Fig. 71 the necessary arrangements for the welding of a
steel rim are sketched. A is the iron core of an alternating current
transformer and B the primary winding supplied with alternating current
either from a works dynamo or a public supply, and controlled by a
double-pole switch C. The transformer has a secondary winding consisting
of a single copper strip of very heavy section D, in which secondary
currents of low voltage but very large volume are induced. This winding
D terminates in two heavy metal clamps E, one fixed and the other
capable of movement by rack and pinion or screw, and the clamps must be
shaped to the contours of the work F they are intended to hold, so as to
fit well and present as little electrical resistance there as possible.
The butt ends of the wheel rim are brought into contact, current
switched on at the transformer primary, and immediately a very heavy
secondary current passes round the “winding” D, generating intense heat
at the junction of the metal rim G held in the clamps, where the
electrical resistance is comparatively high. In a few moments the joint
will arrive at welding heat and the screw feed is then operated, driving
the joint together and completing the weld, except so far as a little
hand dressing may be found necessary. Directly the weld is established
the current is switched off at the mains and the job allowed to cool
out. Nothing less than 5 kilowatts to 10 kilowatts is likely to be very
satisfactory for hoops about 3/4 in. by No. 16 gauge, and the current
must be alternating. The primary voltage and frequency is immaterial, as
the transformer can be wound to suit the circuit conditions whatever
they may be.
[Illustration: Fig. 71.--Electric Contact Welding of Steel Rim]
The method of welding by resistance, that is, by raising locally the
welding point to the temperature required by bringing the two surfaces
into contact until their high resistance produces a welding heat and
then squeezing them together, is by far the most manageable and
satisfactory commercial process of the two electrical processes. It is
adapted for “spot-welding” or producing local adhesions between metal
plates after the manner of riveting, for butt or end-on welds, for
seams, chains, rings, etc., and automatic welding machines are now made
that can deal with no less than 1,500 welds and upwards per hour with
semi-skilled labour, with the least possible percentage of failures and
a very low cost for electrical energy. Alternating current is essential
with this type of weld, and is used to energise a step-down transformer
of special construction.
=Arc Welding.=--Notwithstanding the superiority of the resistance
welding process to most commercial work, particularly that of a small
kind necessitating rapid repeat work, the “arc” method, which has been
in use for many years, and was probably the first experimented with, has
now become largely used on work where it was thought impossible to adapt
it a few years ago. The system is extensively employed in iron and steel
works, shipyards, and boiler works, and the class of work it is employed
on varies from the dismantling of iron and steel buildings, by fusing
and cutting through the structural ironwork and girders, to the filling
up of blowholes in castings. The metal to be welded is connected to one
pole, and the electrode handled by the operator forms the other pole, an
arc being struck between the two. Broken castings and forgings can be
satisfactorily repaired by running new fused metal round them.
Recently the arc system has been applied with success for making welds
on tramway rails, the resistance of the welded joint being found very
much lower than when made with the usual fishplates and bonded joints.
Continuous current gives better results than alternating for the arc
system, and a generator designed for use with this process has a
“drooping characteristic,” that is, the volts at the terminals fall
rapidly with an increase in the current output. In this way the current
is automatically limited to some extent, when the short-circuiting
effect of the operation comes into play. Successfully to weld by this
process a current of 300 to 500 amperes at 80 volts is necessary, and
every precaution has to be taken to protect the workman from the intense
glare of the arc.
=Thermit.=--Thermit is an aluminium alloy whose combustion generates so
much heat that the substance can be used for the welding of iron and
steel. It is the patented invention of Dr. Hans Goldschmidt. With
thermit as a means of melting and welding, and with the use of special
clamps and devices, a number of operations, otherwise difficult, can be
performed, and thermit has come into general use for repairing broken or
defective parts. By the use of a portable jacket and clamp, the joints
of gas, water, and steam pipes may be welded with the pipes in position;
and the advantages of such a material to an engineer far removed from
supplies and repair shop, as at sea, can hardly be enumerated. New
journals have been welded to heavy rolls, broken pump-rods have been
joined, and a number of structural parts successfully united by its
aid.
Thermit is made up as follows: Iron oxide is intimately mixed up with
about one-third as much in weight of powdered aluminium, according to
the equation Fe_{2} O_{3} 2 Al = Al_{2} O_{3} 2 Fe. The metals to be
united are placed together and surrounded by a little clay or similar
substance and the mixture is placed all around the joint. The mixture is
fired by a little magnesium, and the chemical change that follows
creates such an intense heat that iron is readily welded. When the joint
is cold and cleaned up it is hardly perceptible.
According to instructions published by Thermit, Ltd., it is of the
utmost importance that the moulds, crucibles, etc., should be kept
thoroughly dry, and they may advantageously be warmed before use to
ensure the absence of any dampness. The parts to be welded must
previously be brought to red heat, which is best effected by means of a
gas and air flame. The proper design of the mould is of the utmost
importance. It should have a runner and riser, and the metal should be
allowed to flow between as well as around the ends of the pieces to be
welded. The expense and inconvenience of making wooden patterns may be
obviated by making the model of wax, ramming the sand round this, and
subsequently melting out the wax.