PREHEATING AGENCIES
[Illustration:
(_Courtesy of the Messer Mfg. Co._)
FIG. 36.--A Large Job Prepared for Welding.]
(=58=) PREHEATING, as applied to oxy-acetylene welding, means the
application of heat to the article to be welded in some manner which
is usually different than by the welding flame itself. Charcoal, coke,
kerosene, crude oil, coal and natural gas are used for this purpose.
The principal reasons for pre-heating parts to be welded are: To take
care of the effects of contraction and expansion on the confined ends;
to save time, gas, and material; and to make a better weld by making
it quicker and with less chance of burning up the metal.
[Illustration:
(_Courtesy of the Messer Mfg. Co._)
FIG. 37.--Showing how Large Work can be Covered with Asbestos Paper
when Preheating.]
(=59=) On account of the ductile qualities of steel, there is not
quite as much heat used in preheating, to take care of the contraction
and expansion, as in cast iron. On brass work a very dull red heat is
considered sufficient, or otherwise the alloys might burn out. When
preheating aluminum, there will be no change in color as the heat
is introduced, so other methods are used to determine the correct
temperature. Three methods are used for this purpose by most welders.
“Half-and-half” soldering wire will usually melt when applied to the
surface of aluminum which is preheated to the proper state; the puddle
stick when drawn smartly across the heated surface should scrape off
the oxide and leave a clear blue streak if the work is in condition
to be welded; and if a medium-sized tip is brought down so that the
neutral flame just touches the surface for a second or two, the metal
will sweat, if at the proper temperature, and small globules which have
the appearance of mercury will stand out on the surface.
(=60=) The beginner must study contraction and expansion in order that
he may know when and where to apply it in figuring out his work. Many
welding jobs have turned out to be failures through lack of knowledge
in this respect. Take, for example, a water-cooled cylinder block
of the ordinary gas engine; the water jacket may be broken when the
water is allowed to freeze in it. This problem has certainly confined
ends, but some welders have attempted to weld such jobs cold, that is,
without preheating, and possibly have succeeded in executing what they
thought was a very fine weld, but upon examination, have discovered
that the cylinder walls, which are very accurately machined, have been
warped to such an extent that the block is rendered useless. This is
strictly a “preheating” job, and the cylinder should be brought to a
dull red heat if the best results are to follow.
As has been stated elsewhere in this volume, the weld should not be
considered successful unless the piece worked upon can be returned to a
usable state.
(=61=) Several different fuels have been mentioned, all of which can
be used for preheating purposes. Charcoal is considered the best agent
for general welding, as it gives off a very steady heat which will
gradually be absorbed by the article worked upon, bringing it to the
heat desired and holding it there throughout the welding operation. It
will then permit very gradual cooling, as this sort of fire takes a
long time in dying--a desirable asset in work of this kind. On account
of the scarcity of charcoal and its high price, other agencies are
used and chief among them are torches using kerosene, crude oil, or
city gas, as a fuel. These usually heat up the work more quickly, but
care in their manipulation is necessary. A preheating torch to be used
in conjunction with city gas can be very easily constructed, if the
details of Fig. 38 are observed. This proves to be a very efficient and
cheaply constructed apparatus.
[Illustration: FIG. 38.--Preheating Torch, Constructed of Black Iron
Pipe, for Burning City Gas.]
(=62=) When work is being preheated, it is best to have it protected
from all drafts, to prevent warping. Possibly the most extensively used
material for building up temporary ovens to hold the heat and protect
the work from the air currents is fire brick and with it asbestos
paper. When setting up an ordinary casting for preheating, these bricks
are built up in builder’s fashion, about four inches away from the
piece itself, as shown in Fig. 39, and practically level with the top
of the piece. If charcoal is to be used, draft spaces are left in the
first row of bricks as shown, and the charcoal ignited through the
openings with the welding torch. The work to be welded should have the
line of weld at the top if possible and be set up from the floor, or
the surface upon which the oven is resting, on one or two fire bricks,
in order that the full benefit of the heat will be received. Asbestos
paper is then laid across the top, and the oven will appear as in Fig.
40. When starting the fire, a layer of charcoal, a matter of two or
three inches thick, is at first used, but as the chill is taken off the
piece the oven can be filled to the top, and usually this is enough to
complete the work.
[Illustration: FIG. 39.--Temporary Preheating Oven, Built of Fire
Brick.]
[Illustration: FIG. 40.--Temporary Preheating Oven of Fire Bricks
Covered with Asbestos Paper.]
(=63=) In order to protect the operator, when working over hot fires,
it is recommended that the asbestos covering be left on, and that only
a small section immediately in the vicinity of the weld be removed,
which can be accomplished by cutting a “U” in the paper as shown by
the dotted lines in Fig. 40. This can be turned back, exposing the
place which is to be welded, and at the same time protecting the
operator, to a large extent, from the unnecessary heat. When the weld
is finished, this lap can be turned back and the piece allowed to
cool. On pieces which require turning and must be welded in several
different positions, the preheating oven, as it is called, should be
built considerably larger, to provide for handling the work. It must
be remembered that during the entire operation, the piece should be
left inside the oven and should not be removed to a welding table.
Some beginners make the mistake of doing this. When welding with the
charcoal in closed rooms, during the winter months, the fumes will be
found to be very disagreeable and means should be taken to provide
indirect ventilation, otherwise the welders will be troubled with
headaches and smarting eyes.
[Illustration: FIG. 41.--Showing How Oven is Built when Preheating
Torch is to be Used. Torch is Shown at (_A_).]
(=64=) When using preheating torches, the ovens are built much closer
to the work and do not have the openings along the bottom row of
bricks. They are made as tight as possible, and in some cases it will
be found advisable to build up the walls with two layers of bricks,
with asbestos paper between them, in order to hold the heat and cause
the work to heat up in a more uniform manner. A hole is left in one
end of the oven, through which the flame of the preheating torch
is introduced as shown in Fig. 41. It is not thought best to have
the torch flame come in direct contact with the work which is being
preheated, and a baffling plate of metal or brick is placed directly
in front of the flame, in order to spread it around the oven. Judgment
will have to be used in all such work.
(=65=) The setting up of the work, when preheating, is an important
point overlooked by many welders, especially so in the case of
aluminum. Care should be taken to see that the work has a good solid
setting and is braced at a sufficient number of points, to prevent its
sagging when in a preheated condition. Many times when working on rough
surfaces, a few firebricks distributed around the bottom of the oven
with a dab of putty, clay, or retort cement, placed upon them, will
form an excellent cushion upon which the work can rest and the operator
may feel confident that no sagging will occur.