WELDING OF MALLEABLE IRON
(=134=) The welding of malleable iron, so far as the actual fusion of
the metal is concerned, is not practiced except in very few instances,
where the parts are very thin and have been completely annealed. This
is on account of its being what might be termed a heat-treated metal.
To begin with, malleable iron is cast iron, and becomes malleable only
after it has been heated to the proper condition in the presence of
material which will absorb much of its carbon content, and kept in
this state until a suitable depth of its exterior has been annealed.
It has been changed from a brittle casting to one which will bend to
some extent without breaking, and its surface, by the withdrawal of
the carbon, has been converted into steel. The interior remains cast
iron. The depth of penetration will depend entirely upon the number
of hours the work is treated. Usually it runs from one-sixty-fourth to
one-eighth of an inch, depending upon the type of work. An idea may be
gained of how a cross-section of this metal will appear, by noting Fig.
79.
[Illustration: FIG. 79.--Illustrating Cross-section of Malleable Iron.]
(=135=) A machinist would not think of destroying the temper in his
tools and then attempting to use them without retempering them. So the
welder will not attempt to melt malleable iron, for he realizes that
if he were to attempt fusing this metal that its character would be
entirely destroyed. If he should make a fusion, the weld itself and in
the vicinity thereof the metal would be very brittle and retain none of
its ductile qualities. When a weld of this kind is attempted, first,
a few steel sparks are given off from the surface of the metal, which
quickly diminish and the surface seems to recede from the flame. A
white foam appears as the steel surface is burned and many small blow
holes then make their appearance. The casting resembles a steel casting
which contains much sand and impurities. The welding of malleable
iron, in its broadest sense, is therefore not recommended, although
as it has been stated there are occasions when it can be successfully
accomplished. The best manner of bonding malleable iron is by the use
of a bronze filler-rod, and this process will hereafter be referred to,
for convenience, as welding, although it may resemble brazing in some
respects.
(=136=) The art of welding malleable iron with bronze is not very
difficult to learn. Possibly, the greatest trouble will be experienced
by the beginner in distinguishing malleable iron from other castings.
By again referring to Fig. 43 and carefully noting the various methods
outlined, this trouble should be overcome. Many times, too, if the
welder has had any mechanical experience, he can probably determine
where the casting has been used and can ofttimes satisfy himself
whether it is malleable or not. Malleable castings are very seldom
used as a wearing surface, and are generally employed where there is
strain, to replace steel castings and forgings, which are much more
expensive. If it has been determined that the metal is malleable iron,
half the battle has been won.
(=137=) In preparing malleable iron, a clean surface is necessary in
the vicinity of the weld. No “V-ing” out is necessary unless the piece
is greater than one-quarter inch in thickness, and then the surface of
the “V” should be as rough as possible. The ends are placed as close
together as possible, the same as in brazing, and a welding tip which
is one size smaller than would be used on the same thickness of cast
iron is then used, with a slightly carbonizing flame. See Fig. 23.
The work is heated, the same as in cast iron and steel. This flame is
played directly on the work in a vertical position, similar to that
used in preheating the weld in cast iron and steel, until heated to a
cherry red, back about one-half inch on each side of the weld. As soon
as this heat is obtained, the bronze filler-rod carries a quantity of
bronze flux to the weld and this further tends to clean the surface.
With the end of the filler-rod directly in contact with the work
nearest the operator, the neutral flame melts the end of the rod, which
immediately should run over the adjoining surface and through the
crack. When this occurs the flame is abruptly twisted away from that
portion of the weld to avoid burning the bronze. This is repeated along
the line of the weld until the entire surface is covered with a thin
coating of bronze. With this as a foundation more bronze is added, but
during this process the torch is turned so that the neutral flame will
not bear down directly on the bronze, which has already been added. It
should rather strike it at an angle and radiate enough heat from the
side of the neutral flame to permit a fusion between the filler-rod and
the bronze already added. Much more surface should be covered and more
of a reinforcement made than in either cast iron or steel, in order to
warrant enough strength for this class of work.
(=138=) A good bronze for welding purposes should work easily under the
influence of the oxy-acetylene flame and have sufficient alloys present
to take care of those destroyed by the action of the flame. It is not
thought advisable to work over welds of bronze, for fear of making them
porous, unless more filler-rod is added whenever the flame is brought
in contact with the weld.
(=139=) Welds of malleable iron can be made which will be even stronger
than the surrounding metal, and at times they can be reinforced by
adding small strips of steel. These can be entirely covered, to make
them inconspicuous. Contrary to custom it is recommended that plenty of
flux be used, for best results have been found when a surplus rather
than a sparing amount has been employed.
(=140=) The matter of heat in malleable iron is of considerable
importance. If not enough heat is used there will be no fusion between
the bronze and the iron, whereas on the other hand, if too much heat
is used, the bronze will not adhere, but will seem to boil on the
surface and form in small globules rather than spread over the whole
metal. In addition the character of the piece being worked on will be
changed when heated too much. This matter of heat should be given great
attention and the beginner should learn and have emphasized the fact
that the proper heat is one which will permit the bronze to run like
water over the surface, and this will form a good foundation to work
upon.
(=141=) In general, malleable iron work is seldom preheated, for this
is not necessary if the pieces have been fitted together as closely as
possible before the weld is started. Once the student has learned the
flow of metal and how to reinforce his weld, he will be in a position
to handle most any kind of malleable iron properly. It is well to
remember, however, that malleable iron is allowed to cool slowly and is
not immersed in water, as has been suggested when working on brass, for
here we have one metal in the piece itself and another in the weld, and
too great a strain would set up if they were cooled abruptly.