Oxy-Acetylene Welding
Of late years oxy-acetylene welding and cutting have made great strides,
and have placed at the disposal of the metal-working trades a means of
doing many things that hitherto were impossible. Purified acetylene and
oxygen, both under pressure, are supplied to a special blowpipe or
torch. The flame in its hottest part has a temperature of about 4,000°
C. and is therefore sufficiently high to melt any metals with which it
may be brought into contact. The torch is fitted with all necessary
adjustments to vary the supply of either of the gases, and constitutes a
handy tool with which the intelligent worker soon acquires great
dexterity. In a special form of the torch there is a means of
introducing a further supply of compressed oxygen, which makes it
possible for the blowpipe flame to cut its way rapidly through thick
metal, the particles of which are actually consumed in the path of the
oxygen.
It is out of the question in a single chapter of a handbook covering
such a large scope as the present work to do more than indicate some of
the uses to which the oxy-acetylene torch or blowpipe may be put. This
chapter is obviously no attempt whatever at providing a complete working
guide to oxy-acetylene welding. All that will be here attempted is to
present a brief description of a typical outfit and some notes on
working the blowpipe, and then to give some practical instruction from
the pen of an oxy-acetylene welder on the treatment of copper, aluminium
and cast-iron. It may here be pointed out that there is now a large
number of firms specialising in the manufacture of oxy-acetylene
welders’ appliances, and most of them publish illustrated catalogues
which anyone proposing to equip himself for oxy-acetylene welding would
do well to obtain.
The source of the oxygen used in welding is now always cylinders, which
are obtainable in various sizes, either by purchase or on hire from the
gas-compressing companies, to whom they have to be returned for
recharging. The acetylene also can be had in the compressed form, but in
this case the gas is not simply compressed into steel cylinders because,
if it were, any simple shock would be likely to cause explosion. The
acetylene is therefore dissolved in liquid acetone, the cylinders
containing some porous substance such as fossil meal, which is saturated
with the acetone and the acetylene then pumped in. These also can be
bought or hired from the gas-compressing companies. A tremendous amount
of welding is done, however, with acetylene generated on the spot, and
there are on the market quite a number of approved appliances that can
be recommended, the best form of generator being that in which the
calcium carbide is dropped into the water instead of the water dripping
into the charge of calcium carbide. It is essential that the gas be
purified before use. As most people doubtless know, acetylene is one of
the hydro-carbon series of gases and is evolved by the action of water
on calcium carbide, a substance which is one of the products of the
electric furnace.
[Illustration: Fig. 72.--Diagram of Oxy-acetylene Welding Apparatus]
The particulars and instructions on pp. 136 to 139 are due to the
Acetylene Corporation, Ltd. Fig. 72 presents a diagrammatic illustration
of a complete oxy-acetylene blowpipe equipment with the exception of the
acetylene generator and holder, which apparatus may be placed in any
suitable position (preferably outside) at any reasonable distance from
the blowpipe. A is an ordinary gas tap connecting the hydraulic back
pressure valve B with the acetylene supply pipe from the acetylene
holder. The blowpipe is connected at valve C by means of a flexible tube
with the outlet tap D of the hydraulic back-pressure valve. This forms
the acetylene supply pipe to the blowpipe. The blowpipe is connected at
valve E by means of a special canvas-covered strong rubber pipe with the
outlet tap F of the oxygen pressure regulator, which is fixed, as shown,
on the oxygen cylinder. G is a pressure gauge. This pipe conveys the
oxygen supply to the blowpipe, and should be securely attached, as it is
subject to pressures varying from 5 lb. to 40 lb. per sq. in. The
hydraulic back-pressure valve should have been previously charged with
water, and the gas regulator screwed into the oxygen cylinder. The
blowpipe apparatus is now ready for use, with the taps A and D closed
and the taps C, E and F open.
First, slowly open the oxygen cylinder valve (not shown) with the key
supplied for that purpose. By means of the thumb-screw H, adjust the gas
pressure to the correct working pressure for the blowpipe used. The
approximate pressure of oxygen required for each blowpipe is as follows:
No. 2, 8 lb. per sq. in.; No. 3, 10 lb.; No. 4, 11 lb.; No. 5, 12 lb.;
No. 6, 14 lb.; No. 7, 16 lb.; No. 8, 19 lb.; No. 10, 20 lb.; No. 12, 25
lb.; No. 15, 30 lb. Then open the acetylene taps A and D, and when
acetylene is unmistakably smelt at the nozzle of the blowpipe, ignite
the gases by means of a gas jet, candle, or taper. Then by means of the
tap C slowly throttle down the acetylene until the small white cone of
flame at the nozzle of the blowpipe shows a clearly defined outline. As
some indication of the correct size of the cone, it may be mentioned
that when working with the No. 10 blowpipe this should be about 1/4 in.
diameter by 5/8 in. long. This cone in the other blowpipes is greater
or less according to the relative size.
The tap A must never be used to regulate the supply of acetylene; in
fact, after the hydraulic back-pressure valve has been charged with
water, it is best to leave this tap always on.
The working pressure for oxygen previously given should not be too
rigidly adhered to. Even in the same sizes of blowpipes the conditions
must vary slightly, and a little practical experience with each blowpipe
will soon indicate the best working conditions. If the flame is not
properly regulated it may fire back and go out. If so, the taps C and E
should be shut off at once, and a few seconds allowed to elapse before
relighting. When work is carried on for a long time at a stretch and the
burner becomes warm, it will be found necessary to slightly open the
acetylene tap C from time to time. If work is being done which involves
the nozzle of the blowpipe being held in a confined space, it is
advantageous to cool this end of the blowpipe by immersing it from time
to time in a bucket of water. While this is done the gases must be
turned off at C and E.
Welding should be done at the apex or outer extremity of the small white
cone.
If the hole in the nozzle of the blowpipe gets obstructed at any time
through beads of iron being splashed into it, or from any other cause,
it may be cleared with a piece of copper wire and cleaned with a wire
brush. No steel reamer or other sharp instrument should be used in the
hole, which otherwise will be altered in size.
On stopping work the acetylene tap C should be closed first and then the
oxygen tap E. When work is completely stopped, the oxygen cylinder
should be shut off. The oxygen cylinder valve should never be opened
until taps F and E are open, and it should then be opened slowly. In
this way sudden impact of oxygen in the regulator is obviated.
The following instructions on the methods of welding copper, cast-iron
and aluminium are contributed by a foreman welder.
=Welding Copper.=--Copper to be welded should have its edges bevelled to
enable the welding to penetrate the entire thickness of the metal.
Bevelling is not generally practised below a thickness of 3/32 in. From
3/32 in. to 3/16 in., a slight open bevel is sufficient; 3/16 in. thick
and over, the angle of the bevel should be about 90°. It is not
necessary to go beyond this even with great thickness. The bevelling
should be regular, especially at the bottom, so as not to produce holes
or excess of thickness at the bottom of the bevel.
The edges to be welded and their immediate neighbourhood should be
thoroughly cleaned. This can be done with a file, scraper, or sheets of
emery. Chemical agents such as spirits of salt or nitric acid are
sometimes employed; but it is preferable to precede their use by a
mechanical cleaning.
Before beginning the welding the parts should be carefully arranged so
that during the welding operation they remain perfectly in position.
Owing to the high conductivity of copper, a relatively larger blowpipe
tip must be used than when welding either iron or mild steel of the same
thickness. The power of a blowpipe of 225 litres with an approximate
consumption of 7·75 cub. ft. of acetylene per hour would be suitable,
with economical results, for iron or mild steel 1/8 in. thick, whereas
for copper of the same thickness the power of the blowpipe should be of
300 litres, having an approximate consumption of 10·5 cub. ft. of
acetylene per hour. Also, a blowpipe which is too strong tends to melt
the metal too rapidly. This should be as carefully avoided as that of
melting too slowly.
A pure copper welding rod may be employed for filling in, but it is not
so effective as a welding rod made of phosphor copper. The phosphorus is
incorporated in a very small quantity, so that none remains in the weld
after its execution. A filler rod which contains too much phosphorus
lacks fluidity, and melts at a temperature much lower than that of the
copper to be welded, thus facilitating adhesion. Moreover, the welds in
which the phosphorus remains lack elongation, and therefore do not
possess the same mechanical properties as pure copper. The welding rod
after 1/16 in. of its diameter should be about equal to the thickness of
the weld, although in practice feeders about 1/4 in. in diameter are not
generally employed. Welds made on copper without a deoxidising welding
rod properly prepared have a tendency to oxidise, and therefore do not
possess the required qualities. In addition, the surface of the metal
must be covered with a carefully prepared mixture of potassium phosphate
and potassium carbonate to a depth of about 1/16 in. Upon the
application of the flame, the mixture will melt and form a glaze over
the surface of the copper, thus preventing oxidation and assuring good
work.
A flux consisting of chloride of sodium, sodium borate, and boracic acid
is also recommended. The flux should be sparingly applied by dipping the
end of the welding rod into the vessel containing the flux. The end of
the rod should be warmed in order that the flux adheres.
Before beginning the actual operation of welding, it is essential to
raise the edges of the weld and the parts in the vicinity to a high
temperature. The high conductivity of the metal necessitates this, as
any supply of molten welding rod before the edges are in a molten state
inevitably produces adhesion. The flame of the blowpipe should be
perfectly regulated and maintained without excess of either acetylene or
oxygen. In executing the weld, care must be taken to avoid contact of
the white jet of the blowpipe flame with the metal just about to be
melted. The distance of the white jet should vary according to the power
of the blowpipe, say from 3/16 in. to 3/8 in. If this distance is
increased, the gases resulting from the second phase of combustion,
carbonic acid and water vapour, influence the weld. Care must be taken
that the fusion of the metal should not be undertaken until the edges of
the weld and the parts near have been raised to a high temperature. At
this moment the welding rod and the parts to be joined should be melted
simultaneously. It is essential that the welding rod should be regularly
incorporated in the line of welding, and must not be allowed to fall in
drops. The operation should be continuous, taking care to attack
regularly the two edges of the metal. The welding is thus executed
rapidly.
It is well known that internal strains are always set up in every
process of welding, due to the expansion and contraction when a metal is
heated and cooled. Copper lacks tenacity when heated; hence contraction
of the metal, whose coefficient of expansion is also fairly high;
fractures thereby are often produced, especially in the welded part.
However, pre-heating the article to a high temperature, maintaining the
heating after the operation of welding and slow cooling, enables one in
many cases to avoid fractures due to contraction. It is also necessary
to hammer the line of welding and its vicinity. After the hammering
operation it is essential to reheat the copper, raising it to redness
(500° C. to 600° C.). Then plunge into cold water, or cool as rapidly as
possible. The structure of the weld is not quite as homogeneous as other
parts of the piece welded. This is, however, controlled largely by the
skill and workmanship of the operator, who can, at will, make the weld
more or less homogeneous.
It is impossible to enumerate in anything like detail all the work in
copper which may be executed by oxy-acetylene autogenous welding.
However, copper-smiths are advantageously making great use of the
system, thereby replacing their old methods of brazing and riveting.
=Welding Aluminium.=--In preparing aluminium to be welded, the edges
must first be thoroughly cleaned and the welding rod very pure, so as to
avoid the incorporation of impurities, which is apt to bring about rapid
disintegration in the line of welding. Bevelling the edges to be joined
is not necessary below a thickness of 1/8 in. From 1/8 in. to 3/16 in. a
slight open bevel is sufficient, 3/16 in. thick and above angle of bevel
should be about 90°. For thin sheets up to a maximum of 3/32 in.,
welding is facilitated by flanging the edges at right angles. The depth
of the flange should be slightly deeper than the thickness of the metal.
By this method no welding rod is required, the edges being simply fused.
The weld should afterwards be hammered level.
Aluminium should never be welded without a flux. If welding is attempted
without a flux, globules consisting of aluminium within and a coating of
alumina (oxide of aluminium) will appear. In order to eliminate these by
the blowpipe flame it would be necessary to raise the temperature to the
melting point of the oxide of aluminium, which is nearly 3,000° C.,
whilst the melting point of metallic aluminium is only 657° C. To
produce a flux which will dissolve the oxide at the low melting point of
the metal and at the same time protect the hot metal from contact with
the air has obviously not been a simple problem to the chemist and
engineer. However, several good fluxes are now obtainable which enable
any experienced welder to effect satisfactory welds in aluminium.
A flux consisting of the following ingredients can be recommended:
sodium chloride 30 parts, potassium chloride 45 parts, lithium chloride
15 parts, potassium fluoride 7 parts, and bisulphate of potassium 3
parts.
When making fluxes for the welding of aluminium, great care is necessary
in order to completely dry the ingredients, thus avoiding their
combination with each other. On aluminium above 3/32 in. thick, the flux
is best applied by dipping the end of the welding rod into the vessel
containing the flux. The end of the rod should be first warmed in order
that the flux adheres. The welding rod after 1/16 in., its diameter
should be just about equal to the thickness of the weld, although in
practice feeders above 1/4 in. diameter are not advisable.
In executing the weld, care must be taken to avoid contact of the white
jet of the blowpipe flame with the metal just about to be melted,
because the high temperature of this part tends to produce holes which
are difficult to fill in. The distance of the white jet should vary
according to the power of the blowpipe, say from 1/4 in. to 3/4 in. The
flame should be so adjusted as to furnish an excess of acetylene. There
need be but little fear of carbonising the metal, for the reason that
the temperature of the work is comparatively low. For thin welds, up to
1/8 in. thick, it is preferable to hold the welding rod in front of the
blowpipe in the direction of the edges to be welded. As soon as the
latter begins to melt it is heated rapidly, and should be lowered to
form one molten bath with the metal of the piece. The welding is thus
done very rapidly. For great thicknesses it is preferable to obtain
fusion of the welding rod, previously heated in the molten bath of the
bevel. Directly after welding, the weld should be thoroughly washed in
clean warm water in order to remove all remaining traces of the flux,
which would otherwise continue to have a chemical action on the metal,
thereby setting up corrosion.
=Welding Cast-Iron.=--The edges of the weld should be bevelled when the
thickness exceeds 1/8 in.; this enables the welding to penetrate the
entire thickness of the metal. Both edges must be bevelled to an angle
of 45°, so as to form a right angle at the weld. The bevelling should be
regular, especially at the bottom, so as not to produce holes or excess
thickness at the bottom of the bevel. Workers who attempt to effect
welds on cast-iron above, say, 1/4 in. in thickness, without bevelling,
invariably obtain poor results, as it is impossible to get regular and
thorough penetration. The bevelling of the edges may be done by chipping
or grinding, etc. Grinding wheels made from a carbide of silicon
abrasive are very effective for cast-iron. The edges to be welded and
their immediate neighbourhood must be free from sand, dirt, and rust.
It is known that internal strains are always set up in every process of
welding, due to the expansion and contraction when a metal body is
heated and cooled. These strains are not unavoidable, but their effect
may be minimised or nullified. In the case of cast-iron, the tendency to
crack will be greatly increased if the cooling of the metal after fusion
is rapid or irregular. Consequently, the article to be welded should be
pre-heated slowly to about 700° F. to 1,000° F. Generally speaking, the
higher the temperature of pre-heating, the less the danger of cracking.
Preferably, pre-heating and subsequent slow cooling should be carried
out in a muffle, particularly where light and intricate castings have to
be dealt with.
In all cases care should be taken in the selection of the proper size of
blowpipe tip to be used on any particular job. Therefore, the size of
tip recommended by the manufacturers should be employed. The total heat
of fusion of cast-iron being high, it is necessary to use a blowpipe
with a greater power than for the same thickness of welds on mild-steel
or wrought-iron. In the actual operation of welding, the blowpipe flame
should be played on the edges to be welded until the melting of the iron
just takes place. It is essential to avoid contact of the white cone of
the blowpipe flame with the metal just about to be melted; the point
should be kept at a distance varying from 3/16 in. to 3/4 in., according
to the thickness of the work. The two edges to be joined should melt
simultaneously. As soon as the first fusion is obtained, a little flux
or scaling powder must be added; this is usually applied by dipping the
extremity of the welding rod into the vessel containing the flux, the
rod having been previously heated. Avoid throwing the powder into the
molten metal whilst executing the weld, as the supply from the welding
rod is always sufficient.
Many kinds of fluxes for cast-iron are furnished by the manufacturers of
welding apparatus, which vary considerably in composition. The principle
of all of them is to provide some chemical which, at the high
temperature involved, will break up the oxide into its component parts.
The following combinations will perform these functions, and can be
recommended: (1) Boracic acid 80 parts, powdered chlorate of potash 20
parts, ferric carbide 15 parts. (2) Equal parts of carbonate and
bicarbonate of soda, to which is added from 10 to 15 per cent. of borax
and 5 per cent. of precipitated silica. (3) Carbonate of soda 50 per
cent. and bicarbonate of soda 50 per cent. The necessity for using a
flux may not be thoroughly appreciated; but if it is attempted to weld
cast-iron without it difficulty will at once be experienced.
Do not add any metal from the welding rod until the bottom of the [V] is
filled from the sides. It is found that by employing silicon in the
welding rod, in the form of ferro-silicon, the iron combines with the
silicon in preference to the carbon, allowing the carbon to take the
form of graphite, and thus facilitate the formation of grey iron. The
welding rod should contain about 4 per cent. of silicon and as low as
possible in manganese. The purchase of such a welding rod is not at all
difficult, and may be obtained from the same manufacturers as the flux,
from 1/8 in. to 1/2 in. in diameter.
One criticism of cast-iron welding has been directed against the
hardness of the weld. This hardness may be due to a number of causes,
such as inefficiency of the operator, unsatisfactory fluxes and welding
apparatus, rapid cooling, etc. Therefore, as stated previously, in order
to get good workable welds, there must be slow cooling after the welding
is complete; and there is no reason why the worker who carefully follows
the instructions given, and applies himself diligently to the task,
should not be able to weld cast-iron of any thickness in an efficient
and workmanlike manner.
This method of welding cast-iron successfully solves an unlimited
variety of manufacturing and repair problems in the engineering
industry, and can be relied on to make homogeneous welds on cast-iron.
It is impossible to enumerate in anything like detail all the work in
cast-iron which may be executed by oxy-acetylene welding; but the
following are some of the applications for which it has already been
advantageously employed: For repairing broken machine parts, gear boxes,
motor cylinders, crank cases, tanks, manifolds, flywheels, etc., filling
blowholes and defects in castings. Castings impossible or difficult to
mould can be made in parts and united. Teeth broken from gear wheels can
be renewed, and adding metal in any desired quantity to worn parts of
cast-iron articles. As a concrete example of its economical and positive
aid to the engineering industry, the following may be of interest. A
cast-iron belt-wheel would have gone on the scrap heap, a total loss,
with four of the six spokes broken, three entirely out. It was 5 ft. in
diameter, and weighed about 500 lb., but was not worth much as scrap
metal. Scrapping it meant the purchase of a new wheel, and perhaps a
long delay in getting one cast. But with the oxy-acetylene process the
three spokes that were fractured were welded into place; the fourth
spoke broken near the hub was also welded. There were seven welds, each
about 1-1/2 in. by 4 in.; the job was done profitably at a cost of £5,
ready for delivery in two days, and was considerably better than buying
a new wheel, and waiting two weeks or two months for delivery. The
process is particularly suitable for this class of work, and cannot fail
to give satisfaction if performed by an experienced welder. The cost of
welding a given job depends not only on its thickness, but on the skill
of the workman. For example, the same class of job may vary as much as
50 per cent. if executed by different operators.