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 11⁄2 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.
CHAPTER XVII
Lead Burning
Lead-burning or flaming is the autogenous welding of lead by means of either an aero-hydrogen or oxy-coal-gas blowpipe flame. In the past the apparatus required included a hydrogen-gas generating chamber (called the “lead-burning machine”) and a blower or air chamber. The hydrogen was made by the action of dilute sulphuric acid on zinc. That system is now, or should be, obsolete, having been superseded by the cleanly and altogether more convenient process of employing two cylinders, one of compressed coal-gas and the other of compressed oxygen, in conjunction with an injector-pattern blowpipe. Gauges and regulators are required as in the oxy-acetylene process.
The oxy-acetylene process may be successfully applied to lead-burning in spite of the great heat of such a flame. The consumption of acetylene, according to Mr. D. Richardson’s translation of Granjon and Rosenberg’s French work, is only 1 to 2 cubic feet per hour for lead 1⁄16 in. to 3⁄16 in. thick, and the process is stated to have “considerable advantages over all other methods of autogenous soldering.”
In lead-burning it is customary to employ a triangular stick of refined lead for filling up the seams. By being burnt or joined together in this way, the lead becomes homogeneous, and the various parts of it equally withstand the same chemical action and heat. For this reason it is used for joining the seams of chemical and acid tanks, and for the joints of pipes used for the conveyance of such chemicals. Solder being an alloy, the acid would have a solvent action on it, eating it away and rendering it useless, and it would also give rise to electrical action, practically impossible when only one metal is exclusively employed. Lead-burning is also often used on external or roof work.
The seams burnt on sheet-lead are of two kinds: one forming a butted joint, the other a lapped joint.
In burning a butted seam, the two edges of the lead to be joined are butted together, and shaved about 1⁄4 in. to 3⁄8 in., or slightly less, on each side. The gas and oxygen are turned on and adjusted so as to produce a flame from about 5 in. to 6 in. long, and tapering to a fine point. The hottest part of the flame is the centre of the thickest portion, about 1 in. or 11⁄2 in. from the jet. Hold the jet in the right hand, and a strip of lead in the left, and allow the flame to play on the end of the strip, which is held just above the seam. As the strip melts, the jet is diverted on to the seam so as to fuse the edges together, the additional lead forming a thickened portion. The strip is again melted, and joined to the edges, and also to the thickest part; and so on along the length. Care should be taken to burn the lead through, but not for the metal to flow beneath the seam. After a little practice, the operator will know exactly when to apply and when to remove the jet.
[Fig. 73] shows a flat butted joint partly burnt. The stick of lead is just nipped with the flame, and a bead of lead dropped on the seam. The flame is then directed on to this bead until it is fused with the seam. When bead and seam are melted together, the flame is immediately raised. The next bead of lead is then dropped on the seam so as to half cover the previous bead, as shown at M ([Fig. 73]). The flame is then directed on the second bead, the flame being immediately raised after these are fused together, and this operation is repeated until the whole of the seam is burnt.
A flat lapped joint, partly burnt, is shown by [Fig. 74]. In burning this joint, the stick of lead is only required to fill up any irregularities in the burning, and is not required to form the seam in the same way as it is in a butted joint, because in lapped burning the overcloak is burnt down on to the undercloak, as shown in [Fig. 75]. In horizontal and vertical burning, lapped joints only should be used.
[Fig. 75] shows a specimen of horizontal or side burning, and [Fig. 76] one of vertical or upright burning. In burning both of these, the stick of lead is not required at all, the overcloak being in each case burnt down on to the undercloak. Care must be taken that both the overcloak and undercloak of a lapped joint are well shaved.
Fig. 73.--Butted Seam Partly Burnt
Fig. 75.--Horizontal or Side Burning
Fig. 74.--Lapped Seam Partly Burnt
Fig. 76.--Vertical or Upright Burning
Fig. 77.--Burning Upright Joint
Fig. 78.--Branch Joint Ready for Burning
The seams should not be soiled or greased, and care must be taken not to tarnish them in any way. If the lead is not shaved quite clean, or it becomes tarnished after it is shaved, it will be found difficult to burn it together successfully. No tallow or smudge is necessary. The operator will soon detect the presence of any foreign substance or dirt on the lead, and the shavehook should be kept handy to remove it.
In burning a vertical lapped seam, starting at the bottom, the lapping lead is melted, and as it runs is turned on to the back portion and fused into it. A slight projection is formed, which holds the next melting, and so on, each layer forming a base for the next, and adding to the height until the top is reached.
In practising either horizontal or vertical burning, the student should first place his work at an easy angle—say, at about 25° or 30°—gradually raising it as he becomes proficient until the seam is in a horizontal or vertical position as desired. Two surfaces can be burned together in any position—horizontal, vertical, or even overhead, where soldering would be impossible.
Pipe joints can also be made by burning. First one pipe is opened to form a socket like a slip joint. The male part, which must enter at least 3⁄4 in., must be well shaved and made to fit tight. [Fig. 77] shows an upright joint prepared and partly burnt. [Fig. 78] shows a section of a branch joint as prepared for burning. Care must be taken to work up a good thick shoulder for the socket N.