stored in a free state under
pressure greater than 15 psi can be made
to break down by heat or shock and possibly explode.
Under pressure of 29.4 psi, acetylene becomes self-explosive,
and a slight shock will cause it to explode spontaneously.
However, when dissolved in acetone, it
Figure 15-27.-Acetylene cylinder.
can be compressed into cylinders at pressures up to 250psi.
The acetylene cylinder (fig. 15-27) is filled withporous materials, such as balsa wood, charcoal, and shredded asbestos, to decrease the size of the open spaces in the cylinder. Acetone, a colorless, flammable liquid, is added until about 40 percent of the porous material is filled. The filler acts as a large sponge to absorb the acetone, which, in turn, absorbs the acetylene. In this process, the volume of the acetone increases as it absorbs the acetylene, while acetylene, being a gas, decreases in volume. The acetylene cylinders are equipped with safety plugs, which have a small hole through the center. This hole is filled with a metal alloy, which melts at approximately 212°F or releases at 500 psi. When a cylinder is overheated, the plug will melt and permit the acetylene to escape before a dangerous pressure can build up. The plug hole is too small to permit a flame to burn back into the cylinder if the escaping acetylene should become ignited.
WELDING TORCHES.-The oxyacetylene welding torch is used to mix oxygen and acetylene gas in the proper proportions, and to control the volume of these gases burned at the welding tip. The torch has two needle valves, one for adjusting the flow of acetylene and the other for adjusting the flow of oxygen. In addition, there are two tubes, one for oxygen and the other for acetylene; a mixing head; inlet nipples for the attachment of hoses; a tip; and a handle. The tubes and
Figure 15-28.-Mixing head for injector-type welding torch.
Figure 15-29.-Equal pressure welding torch.
handle are made of seamless hard brass, copper-nickel alloy, stainless steel, or other noncorrosive metals of adequate strength. There are two types of welding torches–the low-pressure or injector type and the equal-pressure type. In the low-pressure or injector type (fig. 15-28), the acetylene pressure is less than 1 psi. A jet of high-pressure oxygen is used to produce a suction effect to draw in the required amount of acetylene. This is accomplished by the design of the mixer in the torch, which operates on the injector principle. The welding tips may or may not have separate injectors designed integrally with each tip.
The equal pressure torch (fig. 15-29) is designed to operate with equal pressures for the oxygen and acetylene. The pressure ranges from 1 to 15 psi. This torch has certain advantages over the low-pressure type because the flame can be more readily adjusted, and since equal pressures are used for each gas, the torch is less susceptible to flashbacks.
The welding tips are made of hard, drawn, electrolytic copper or 95-percent copper and 5-percent tellurium. They are made in various styles and types, some having a one-piece tip either with a single orifice or a number of orifices, and others with two or more tips attached to one mixing head. The diameters of the tip orifices differ to control the quantity of heat and the type of flame. These tip sizes are designated by numbers that are arranged according to the individual manufacturer’s system. In general, the smaller the number, the smaller the tip orifice.
No matter what type or size tip you select, the tip must be kept clean. Quite often the orifice becomes clogged with slag. When this happens, the flame will not burn properly. Inspect the tip before you use it. If the passage is obstructed, you can clear it with wire tip cleaners of the proper diameter, or with soft copper wire. Tips should not be cleaned with machinists drills or other sharp instruments. These devices may enlarge or scratch the tip opening and greatly reduce the efficiency of the torch tip.
HOSE.–The hose used to make the connection between the torch and the regulators is strong, nonporous, light, and flexible to make the torch movements easy. It is made to withstand high internal pressures, and the rubber used in its manufacture is chemically treated to remove sulfur to avoid the danger of spontaneous combustion. The oxygen hose is GREEN, and the acetylene hose is RED. The hose is a rubber tube with braided or wrapped cotton or rayon reinforcements and a rubber covering. The hoses have connections at each end so they can be connected to their respective regulator outlet and torch inlet connections. To prevent a dangerous interchange of acetylene and oxygen hoses, all threaded fittings used for the acetylene hookup are left-handed threads, and all threaded fittings for oxygen hookup are right-handed threads. The hoses are obtainable as a single hose for each gas or with the hoses bonded together along their length under a common outer rubber jacket. This type prevents the hose from kinking or becoming entangled during the welding operation.
LIGHTERS.-A flint lighter is provided for igniting the torch. The lighter consist of a file-shaped piece of steel, usually recessed in a cuplike device, and a piece of flint that can be drawn across the steel, which produces the sparks required to light the torch.
Matches should never be used to ignite atorch; their length requires bringing the hand too close to the tip to ignite the gas. Accumulated gas may envelope the hand and, when ignited, cause a severe burn.
GOGGLES. -Welding goggles are fitted withcolored lenses to keep out heat and light rays and to protect the eyes from sparks and molten metal. Regardless of the shade of lens used, goggles should be protected by a clear cover glass. The welding operator should select the shade or density of color that is best suited for his/her particular work. The desired lens is the darkest shade that will show a clear definition of the work without eyestrain. Goggles should fit closely around the eyes, and should be worn at all times during welding and cutting operations. Special goggles, using standard lenses, are available for use with spectacles.
WELDING (FILLER) RODS. -The use of the proper type of filler rod is very important in oxyacetylene welding operations. This material not only adds reinforcement to the weld area, but also adds desired properties to the finished weld. By selecting the proper type of rod, either tensile strength or ductility can be secured in a weld. Similarly, rods can be selected that will help retain the desired amount of corrosion resistance. In some cases, a suitable rod with a lower melting point will eliminate possible cracks from expansion and contraction.
Welding rods are classified as ferrous and nonferrous. The ferrous rods include carbon and alloy steel rods as well as cast iron rods. Nonferrous rods include brazing and bronze rods, aluminum and aluminum alloy rods, magnesium and magnesium alloy rods, copper rods, and silver rods. The diameter of the rod used is governed by the thickness of the metals being joined. If the rod is to small, it will not conduct heat away from the puddle rapidly enough, and a burned weld will result. A rod that is to large will chill the puddle. As in selecting the proper size welding torch tip, experience will enable the welder to select the proper diameter welding rod.