Welding of Steel Specially Considered.
—Some steels will “stand the fire” better than others, which means that they will stand more heat before they reach the point when they begin to burn.
The different kinds of steel used in a general way may be summed up as blister, spring, shear, double shear, and cast steel. Blister steel will, as a rule, stand the most heating before beginning to burn, and the others follow in the order given. The difference in heating will vary from nearly a white heat on blister to the yellow heat of cast steel.
A simple method of ascertaining what heat a steel will stand before beginning to burn is as follows: Heat the steel to its burning point, and at various stages, beginning at the yellow stage, lay it on the anvil and give it a few blows with the hand hammer. Repeat the process until a heat is reached that will cause the steel, when struck with the hammer, to give off sparks like small fireworks. When this stage is reached it will show that the burning point is arrived at.
Careful observations of these points will enable the smith to know just when to begin to use the flux for welding, which, by the way, must be just before the steel reaches the burning point.
Another important point is the formation of the scarfs. These should not be fullered down so thin or left in the same form as the scarfs as when joining iron, and should be shorter (compare A and B, [Fig. 66]).
Fig. 66.—Scarfs for Iron and Steel
In bringing the steel up to its welding point, care must be taken to have it at a uniform heat throughout at the part for welding; and, to get this, the blast should not be forced at the start, but used gently. In some cases the blast should be stopped occasionally, to allow the steel to soak. Then restart the blast, and gradually force it when beginning to use the flux, continuing so until the welding point is reached. Only light blows should be given at the start, just to cause the two parts to stick together; but when stuck, the harder the blow the better the weld.
An idea seems prevalent that the flux has a certain influence on the steel, and converts it into a form that makes it more weldable. The real use of a flux in this case is simply to retard the heat, and form a coat or shell on the steel, and so counteract and prevent the burning action which takes place when heating steel in an ordinary blast fire.
The reason why different fluxes are required for different brands of steel is no doubt due to the differences in manufacture. As a rule, the greater the heat the steel will stand before burning, the less it requires a flux to protect it, so that in a great many cases a flux consisting of some clean, sharp sand is all that is required; but the steels that will burn at a lower heat require something more than sand to protect them. Hence arises the necessity of adding burnt borax, crushed glass, powdered marble, etc.
The method of welding steel is as follows; but before proceeding to get the heat make a point of having a shallow tin on the forge large enough to hold a sufficient quantity of the flux, so that it will cover the scarfed end when being dipped in same. Have a clean fire and plenty of firing on the hearth, so that the heat can be well covered. Start with gentle blast until the heat is nearly up to burning point. If necessary, stop the blast and let the heat soak for a few seconds so as to ensure a uniform heat. Gradually force the blast, and keep withdrawing the “heats” and roll them well in the flux, and so continue until it is thought that the heat is plastic enough to unite. Place the scarfs in position, give a few light blows until the parts stick together, then hammer well home and move smartly so as to ensure the proper joining together whilst in a plastic state. When welded, do not continue the hammering or tooling at too low a heat; but if further hammering is necessary, re-heat the work.
These hints are not applicable to every make of steel. With the special steels manufacturers issue particular instructions.
A flux for welding cast steel consists of 2 oz. each of powdered chalk, soda and burnt borax, mixed with 1 lb. of silver sand.
A firm of steel manufacturers recommend a mixture of 21 oz. of sand and 7 oz. of salt, moistened; the steel is to be treated in a fire of sulphurless coal.
CHAPTER XII
Making Blowpipes
How to Make a Bench Gas Blowpipe.—The blowpipe illustrated by [Fig. 67] gives a powerful flame. It can be clamped to the edge of the workbench by means of a winged nut, a hole being made near the edge of the bench to accommodate the bolt. A piece of hard wood A, 5 in. by 2 in. by 1 in. thick, has a strong iron bolt B passed through at one end. A 2-in. cube C, which should also be of good hard wood, is screwed firmly to the other end of A, the combined block being perforated through the centre to take a length of gas tube D, which carries a gas-bracket with flange, elbow joint, and tap E. The flange should be screwed down to the top of the block. The elbow joint allows the direction of the flame to be adjusted within a wide range movement. The arm of the bracket is removed, and a shorter tube F, 3 in. long, is substituted. This carries a 1⁄2-in. iron T-piece G. The tube H, which is 3 in. long, should be of brass, threaded at one end to fit into the T.
Fig. 67.—Bench Gas Blowpipe
The air is conveyed through an 8-in. brass tube J 1⁄4 in. in diameter, which should be smooth inside. This latter point is of some importance, and, if preferred, a glass tube may be used instead of brass, the current of cold air having a sufficient cooling effect to prevent undue heating. The end should be cut off sharp with a file in the ordinary way and left in that condition. Smoothing the edge by fusion in a flame will not improve matters, but rather the reverse. Of course, the other end, which comes outside, must be smoothed to prevent injury to the indiarubber tube used for making connection with the bellows. The air tube must be held firmly in the centre of the gas tube, while capable of being moved in or out for the purpose of adjusting the flame. This can be done quite satisfactorily by means of a short brass tube or nipple K, threaded to screw into the T (see [Fig. 67]). A sound cork should be driven into this short tube so as to entirely fill it, a hole being made with a cork-borer to admit the air tube. This hole must be exactly central, and the cork must grip rather tightly.
A foot-bellows is generally used for supplying the air, the bellows being connected with the air jet J by means of an indiarubber tube. The tube D, which should extend an inch or so below the bench, is to be connected with the gas supply.