CHAPTER X
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].
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.