OXY-ACETYLENE WELDING PRACTICE PREPARATION OF WORK

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OXY-ACETYLENE WELDING PRACTICE PREPARATION OF WORK

Preheating.--The practice of heating the metal around the weld before
applying the torch flame is a desirable one for two reasons. First, it
makes the whole process more economical; second, it avoids the danger of
breakage through expansion and contraction of the work as it is heated
and as it cools.

When it is desired to join two surfaces by welding them, it is, of
course, necessary to raise the metal from the temperature of the
surrounding air to its melting point, involving an increase in
temperatur of from one thousand to nearly three thousand degrees.

To obtain this entire increase of temperature with the torch flame is
very wasteful of fuel and of the operator's time. The total amount of
heat necessary to put into metal is increased by the conductivity of
that
metal because the heat applied at the weld is carried to other parts of
the piece being handled until the whole mass is considerably raised in
temperature. To secure this widely distributed increase the various
methods of preheating are adopted.

As to the second reason for preliminary heating. It is understood that
the metal added to the joint is molten at the time it flows into place.
All the metals used in welding contract as they cool and occupy a much
smaller space than when molten. If additional metal is run between two
adjoining surfaces which are parts of a surrounding body of cool metal,
this added metal will cool while the surfaces themselves are held
stationary in the position they originally occupied. The inevitable
result is that the metal added will crack under the strain, or, if the
weld is exceptionally strong, the main body of the work will he broken
by
the force of contraction. To overcome these difficulties is the second
and most important reason for preheating and also for slow cooling
following the completion of the weld.

There are many ways of securing this preheating. The work may be brought
to a red heat in the forge if it is cast iron or steel; it may he heated
in special ovens built for the purpose; it may be placed in a bed of
charcoal while suitably supported; it may be heated by gas or gasoline
preheating torches, and with very small work the outer flame of the
welding torch automatically provides means to this end.

The temperature of the parts heated should be gradually raised in all
cases, giving the entire mass of metal a chance to expand equally and to
adjust itself to the strains imposed by the preheating. After the region
around the weld has been brought to a proper temperature the opening to
be filled is exposed so that the torch flame can reach it, while the
remaining surfaces are still protected from cold air currents and from
cooling through natural radiation.

One of the commonest methods and one of the best for handling work of
rather large size is to place the piece to be welded on a bed of fire
brick and build a loose wall around it with other fire brick placed in
rows, one on top of the other, with air spaces left between adjacent
bricks in each row. The space between the brick retaining wall and the
work is filled with charcoal, which is lighted from below. The top
opening of the temporary oven is then covered with asbestos and the fire
kept up until the work has been uniformly raised in temperature to the
desired point.

When much work of the same general character and size is to be handled,
a
permanent oven may be constructed of fire brick, leaving a large opening
through the top and also through one side. Charcoal may be used in this
form of oven as with the temporary arrangement, or the heat may be
secured from any form of burner or torch giving a large volume of flame.

In any method employing flame to do the heating, the work itself must be
protected from the direct blast of the fire. Baffles of brick or metal
should be placed between the mouth of the torch and the nearest surface
of the work so that the flame will be deflected to either side and
around
the piece being heated.

The heat should be applied to bring the point of welding to the highest
temperature desired and, except in the smallest work, the heat should
gradually shade off from this point to the other parts of the piece. In
the case of cast iron and steel the temperature at the point to be
welded
should be great enough to produce a dull red heat. This will make the
whole operation much easier, because there will be no surrounding cool
metal to reduce the temperature of the molten material from the welding
rod below the point at which it will join the work. From this red heat
the mass of metal should grow cooler as the distance from the weld
becomes greater, so that no great strain is placed upon any one part.

With work of a very irregular shape it is always best to heat the entire
piece so that the strains will be so evenly distributed that they can
cause no distortion or breakage under any conditions.
The melting point of the work which is being preheated should be kept in
mind and care exercised not to approach it too closely. Special care is
necessary with aluminum in this respect, because of its low melting
temperature and the sudden weakening and flowing without warning.

Workmen have carelessly overheated aluminum castings and, upon
uncovering
the piece to make the weld, have been astonished to find that it had
disappeared. Six hundred degrees is about the safe limit for this metal.
It is possible to gauge the exact temperature of the work with a
pyrometer, but when this instrument cannot be procured, it might be well
to secure a number of "temperature cones" from a chemical or laboratory
supply house. These cones are made from material that will soften at a
certain heat and in form they are long and pointed. Placed in position
on
the part being heated, the point may be watched, and when it bends over
it is sure that the metal itself has reached a temperature considerably
in excess of the temperature at which that particular cone was designed
to soften.

The object in preheating the metal around the weld is to cause it to
expand sufficiently to open the crack a distance equal to the
contraction
when cooling from the melting point. In the case of a crack running from
the edge of a piece into the body or of a crack wholly within the body,
it is usually satisfactory to heat the metal at each end of the opening.

This will cause the whole length of the crack to open sufficiently to
receive the molten material from the rod.
The judgment of the operator will be called upon to decide just where a
piece of metal should be heated to open the weld properly. It is often
possible to apply the preheating flame to a point some distance from the
point of work if the parts are so connected that the expansion of the
heated part will serve to draw the edges of the weld apart.

Whatever part of the work is heated to cause expansion and separation,
this part must remain hot during the entire time of welding and must
then
cool slowly at the same time as the metal in the weld cools.

An example of heating points away from the crack might be found in
welding
a lattice work with one of the bars cracked through (Figure 25). If the
strips parallel and near to the broken bar are heated gradually, the
work
will be so expanded that the edges of the break are drawn apart and the
weld can be successfully made. In this case, the parallel bars next to
the broken one would be heated highest, the next row not quite so hot
and
so on for some distance away. If only the one row were heated, the
strains set up in the next ones would be sufficient to cause a new break
to appear.

If welding is to be done near the central portion of a large piece, the
strains will be brought to bear on the parts farthest away from the
center. Should a fly wheel spoke be broken and made ready to weld, the
greatest strain will come on the rim of the wheel. In cases like this it
is often desirable to cut through at the point of greatest strain with a
saw or cutting torch, allowing free movement while the weld is made at
the original break (Figure 26). After the inside weld is completed, the
cut may be welded without danger, for the reason that it will always be
at some point at which severe strains cannot be set up by the
contraction
of the cooling metal.

In materials that will spring to some extent without breakage, that is,
in parts that are not brittle, it may be possible to force the work out
of shape with jacks or wedges (Figure 27) in the same way that it would
be distorted by heating and expanding some portion of it as described.

A careful examination will show whether this method can be followed in
such a way as to force the edges of the break to separate. If the plan
seems feasible, the wedges may be put in place and allowed to remain
while the weld is completed. As soon as the work is finished the wedges
should be removed so that the natural contraction can take place without
damage.

It should always be remembered that it is not so much the expansion of
the work when heated as it is the contraction caused by cooling that
will
do the damage. A weld may be made that, to all appearances, is perfect
and it may be perfect when completed; but if provision has not been made
to allow for the contraction that is certain to follow, there will be a
breakage at some point. It is not possible to weld the simplest shapes,
other than straight bars, without considering this difficulty and making
provision to take care of it.

The exact method to employ in preheating will always call for good
judgment on the part of the workman, and he should remember that the
success or failure of his work will depend fully as much on proper
preparation as on correct handling of the weld itself. It should be
remembered that the outer flame of the oxy-acetylene torch may be
depended on for a certain amount of preheating, as this flame gives a
very large volume of heat, but a heat that is not so intense nor so
localized as the welding flame itself. The heat of this part of the
flame
should be fully utilized during the operation of melting the metal and
it
should be so directed, when possible, that it will bring the parts next
to be joined to as high a temperature as possible.

When the work has been brought to the desired temperature, all parts
except the break and the surface immediately surrounding it on both
sides
should be covered with heavy sheet asbestos. This protecting cover
should
remain in place throughout the operation and should only be moved a
distance sufficient to allow the torch flame to travel in the path of
the
weld. The use of asbestos in this way serves a twofold purpose.

It retains the heat in the work and prevents the breakage that would
follow if a draught of air were to strike the heated metal, and it also
prevents such a radiation of heat through the surrounding air as would
make it almost impossible for the operator to perform his work,
especially in the case of large and heavy castings when the amount of
heat utilized is large.
Cleaning and Champfering.--A perfect weld can never be made unless the
surfaces to be joined have been properly prepared to receive the new

metal.
All spoiled, burned, corroded and rough particles must positively be
removed with chisel and hammer and with a free application of emery
cloth
and wire brush. The metal exposed to the welding flame should be
perfectly clean and bright all over, or else the additional material
will
not unite, but will only stick at best.

Following the cleaning it is always necessary to bevel, or champfer, the
edges except in the thinnest sheet metal. To make a weld that will hold,
the metal must be made into one piece, without holes or unfilled
portions
at any point, and must be solid from inside to outside. This can only be
accomplished by starting the addition of metal at one point and
gradually
building it up until the outside, or top, is reached. With comparatively
thin plates the molten metal may be started from the side farthest from
the operator and brought through, but with thicker sections the addition
is started in the middle and brought flush with one side and then with
the other.

It will readily be seen that the molten material cannot be depended upon
to flow between the tightly closed surfaces of a crack in a way that can
be at all sure to make a true weld. It will be necessary for the
operator
to reach to the farthest side with the flame and welding rod, and to
start the new surfaces there. To allow this, the edges that are to be
joined are beveled from one side to the other (Figure 28), so that when
placed together in approximately the position they are to occupy they
will leave a grooved channel between them with its sides at an angle
with
each other sufficient in size to allow access to every point of each
surface.

With work less than one-fourth inch thick, this angle should be
forty-five degrees on each piece (Figure 29), so that when they are
placed together the extreme edges will meet at the bottom of a groove
whose sides are square, or at right angles, to each other. This beveling
should be done so that only a thin edge is left where the two parts come
together, just enough points in contact to make the alignment easy to
hold.

With work of a thickness greater than a quarter of an inch, the angle of
bevel on each piece may be sixty degrees (Figure 30), so that when
placed
together the angle included between the sloping sides will also be sixty
degrees. If the plate is less than one-eighth of an inch thick the
beveling is not necessary, as the edges may be melted all the way
through
without danger of leaving blowholes at any point.


This beveling may be done in any convenient way. A chisel is usually
most
satisfactory and also quickest. Small sections may be handled by filing,
while metal that is too hard to cut in either of these ways may be
shaped
on the emery wheel. It is not necessary that the edges be perfectly
finished and absolutely smooth, but they should be of regular outline
and
should always taper off to a thin edge so that when the flame is first
applied it can be seen issuing from the far side of the crack. If the
work is quite thick and is of a shape that will allow it to be turned
over, the bevel may be brought from both sides (Figure 31), so that
there
will be two grooves, one on each surface of the work. After completing
the weld on one side, the piece is reversed and finished on the other
side. Figure 32 shows the proper beveling for welding pipe.

Figure 33 shows how sheet metal may be flanged for welding.
Welding should not be attempted with the edges separated in place of
beveled, because it will be found impossible to build up a solid web of
new metal from one side clear through to the other by this method.

The flame cannot reach the surfaces to make them molten while receiving
new material from the rod, and if the flame does not reach them it will
only serve to cause a few drops of the metal to join and will surely
cause a weak and defective weld.

Supporting Work.--During the operation of welding it is necessary that
the work be well supported in the position it should occupy. This may be
done with fire brick placed under the pieces in the correct position,
or,
better still, with some form of clamp. The edges of the crack should
touch each other at the point where welding is to start and from there
should gradually separate at the rate of about one-fourth inch to the
foot.

This is done so that the cooling of the molten metal as it is added will
draw the edges together by its contraction.
Care must be used to see that the work is supported so that it will
maintain the same relative position between the parts as must be present
when the work is finished. In this connection it must be remembered that
the expansion of the metal when heated may be great enough to cause
serious distortion and to provide against this is one of the
difficulties
to be overcome.

Perfect alignment should be secured between the separate parts that are
to be joined and the two edges must be held up so that they will be in
the same plane while welding is carried out. If, by any chance, one
drops
below the other while molten metal is being added, the whole job may
have
to be undone and done over again. One precaution that is necessary is
that of making sure that the clamping or supporting does not in itself
pull the work out of shape while melted.

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Warmly,
Pat Mitchell
http://www.weldingsecrets.net/main.html