(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.
CHAPTER XI
OXY-ACETYLENE CUTTING
(142) By heating a bar of wrought iron or steel to a welding heat and holding it in a stream of compressed air, or a strong blast, it will at once begin to melt and sizzle, emitting an incandescent and scintillating light. This light is dangerous to observe at close range without colored glasses. The burning of the metal can be maintained for hours, without any other source of heat except that caused by the combustion of the iron. The oxy-acetylene cutting process is based upon this principle, in that a neutral flame is applied in order to heat the part being cut to the desired temperature. Once the melting-point is reached, pure oxygen under pressure is applied to maintain oxidation and force out the burned portion.
(143) The apparatus used for cutting does not differ to any great extent from that of the welding class, except that a different torch is employed. There are combination regulators and torches manufactured, but a combination tool is always regarded by most authorities as a loss in efficiency, either on one side or another. While a low-pressure welding regulator may be used on the oxygen line for cutting, yet its use upon large work, where the pressure is high and the regulator must pass a great deal of gas very freely without freezing up, this low-pressure regulator will be a serious handicap and cause much trouble, if used.
Fig. 80.—The Cutting Torch Eats its Way through Steel of any Size with Remarkable Ease, Leaving a Clean-cut Edge. This View Shows a Cutting Torch in Operation at the Ordnance Welding School, U. S. Army.
(144) An ideal arrangement on the oxygen line for cutting is to have a double or “twin” regulator attached to the oxygen drum, one side of which will do for welding and the other, being high-pressure type, will produce a constant flow of high-pressure gas, suitable for the cutting jet. Then when cutting is done a three-hose torch should be employed. One of its oxygen connections which governs the neutral flame can be connected to the low-pressure regulator, while the oxygen jet should be controlled by the high-pressure regulator, the third connection will furnish the acetylene gas for the preheating flame. However, in place of this three-hose arrangement, most cutting is accomplished by means of a two-hose apparatus, wherein only one hose is used to convey the oxygen from a single regulator to the torch. On such apparatus much trouble is usually experienced in cutting old metals where a great deal of scale is present or in a close place where the torch is apt to get hot.
(145) Many times part of the scale or metal will pop up against the tip and cause the oxygen jet to flicker. This slight variation may cause an excessive pressure of oxygen to be introduced into the preheating flame momentarily, by backing up the oxygen in the cutting jet. This lean mixture of gas will generally flash back instantaneously and will deposit a layer of carbon on the inside of the tip, which causes much annoyance to the operator. This condition is to be found where there is but one oxygen line. In the two-hose arrangement this is entirely overcome, due to the independence of the pressure on each line.
(146) The high-pressure regulator differs from the low-pressure regulator in these respects: The diaphragm, see [Fig. 16], is much smaller in diameter, which makes it less sensitive, and of course much stronger. The diaphragm springs are usually much heavier; the nozzle contains a larger opening for passing gas freely without freezing; and to take care of the increased pressure on the line, usually a higher pressure working gauge is added to the regulator. Such a regulator is capable of passing much more gas than the low-pressure type, but as far as being as sensitive and maintaining a constant, absolute flow of gas, its design will not permit it to do so. In cutting, these requisites are not necessary. In welding, however, the delicate adjustment of the flame demands a very sensitive regulator and usually the larger the diameter of the diaphragm the more sensitive the adjustment.
